AcknowledgementsFunding for this handbookwas made by possibleby a grant from theU.S. EnvironmentalProtection Agency PesticideEnvironmental StewardshipProgram. Financial supportalso was provided by OregonHop Commission, OregonState University, Universityof Idaho, U.S. Departmentof Agriculture AgriculturalResearch Service, WashingtonHop Commission,and Washington StateUniversity. The editorsgratefully acknowledgethe many reviewers andauthors who contributedto this publication. Wealso recognize the U.S. hopindustry for its continuedsupport of research,extension, integratedpest management, andenvironmental stewardship.Reference in this publicationto a trademark, proprietaryproduct, or company nameby personnel of the U.S.Department of Agricultureor anyone else is intendedfor explicit description onlyand does not imply approvalor recommendation to theexclusion of others that maybe suitable.All rights reserved. Noportion of this bookmay be reproduced inany form, includingphotocopy, microfilm,information storage andretrieval system, computerdatabase, or software, orby any means, includingelectronic or mechanical,without written permissionfrom the Washington HopCommission.Copyright is not claimedin any portion of this workwritten by U.S. governmentemployees as a part of theirofficial duties.© 2009 Washington HopCommissionContributorsAmerican Phytopathological Society, St. Paul, MinnesotaC. Baird, Southwest Idaho Research and Extension Center, University of Idaho, ParmaDez J. Barbara, Horticulture Research International, Warwick, United KingdomJames D. Barbour, Southwest Idaho Research and Extension Center,University of Idaho, ParmaRon A. Beatson, HortResearch, Motueka, New ZealandJohn C. Bienapfl, University of California, DavisCenter for Invasive Species and Ecosystem Health (formerly Bugwood Network),University of Georgia, TiftonAmy J. Dreves, Oregon State University, CorvallisKen C. Eastwell, Irrigated Agriculture Research and Extension Center,Washington State University, ProsserBernhard Engelhard, Bavarian State Research Center for Agriculture, Wolnzach, GermanyGlenn C. Fisher, Oregon State University, CorvallisDavid H. Gent, U.S. Department of Agriculture - Agricultural Research Service,Oregon State University, CorvallisKen Gray Image Collection, Oregon State University, CorvallisGary G. Grove, Irrigated Agriculture Research and Extension Center,Washington State University, ProsserFrank S. Hay, University of Tasmania, Burnie, Tasmania, AustraliaDavid G. James, Irrigated Agriculture Research and Extension Center,Washington State University, ProsserDennis A. Johnson, Washington State University, PullmanWalter F. Mahaffee, U.S. Department of Agriculture, Agricultural Research Service,Corvallis, OregonTrish McGee, Sustainability Victoria, Melbourne, Victoria, AustraliaMark E. Nelson, Irrigated Agriculture Research and Extension Center,Washington State University, ProsserCynthia M. Ocamb, Oregon State University, CorvallisRobert Parker, Irrigated Agriculture Research and Extension Center,Washington State University, ProsserWilson S. Peng, Irrigated Agriculture Research and Extension Center,Washington State University, ProsserSarah J. Pethybridge, Botanical Resources Australia, Ulverstone, Tasmania, AustraliaJohann Portner, Bavarian State Research Center for Agriculture, Wolnzach, GermanyCal B. Skotland, Irrigated Agriculture Research and Extension Center,Washington State University, ProsserDarrell R. Smith, Busch Agricultural Resources, Inc., Yakima, WashingtonT. J. Smith, HortResearch, Motueka, New ZealandDouglas B. Walsh, Irrigated Agriculture Research and Extension Center,Washington State University, ProsserFlorian Weihrauch, Bavarian State Research Center for Agriculture, Wolnzach, GermanyJoanna L. Woods, Oregon State University, CorvallisLarry C. Wright, Irrigated Agriculture Research and Extension Center,Washington State University, Prosser
Table of ContentsIntroductionPest Management, Crop Loss, and IPM..............................1Principles of Integrated Pest ManagementSystems-level Management.................................................1Pest and Natural Enemy Identification................................2Pest and Natural Enemy Biology and Life History..............2Economic Injury Levels andEconomic (Action) Thresholds..................................2Monitoring for Pests, Damage, and Treatment Success........3Multi-tactic Management Approaches................................3Pesticide Toxicology and SelectivityPesticide Toxicity Ratings....................................................4Pesticide Resistance Management........................................6Disease ManagementFungal and Bacterial DiseasesAlternaria Cone Disorder..........................................8Black Root Rot.........................................................9Downy Mildew.......................................................10Fusarium Canker....................................................15Fusarium Cone Tip Blight......................................16Gray Mold..............................................................17Powdery Mildew.....................................................18Red Crown Rot.......................................................22Sclerotinia Wilt (White Mold)................................23Sooty Mold.............................................................24Verticillium Wilt.....................................................25Diseases of Minor Importance.................................27Virus and Viroid DiseasesCarlavirus Complex: American hop latent virus,Hop latent virus, and Hop mosaic virus...............28Apple mosaic virus....................................................29Hop stunt viroid.......................................................30Other Viruses, Viroids, and Virus-like Agents.........32NematodesHop Cyst Nematode...............................................34Abiotic DiseasesHeptachlor Wilt......................................................35Arthropod and Slug Pest ManagementCalifornia Prionus Beetle.....................................................36Hop Aphid..........................................................................38Garden Symphylan..............................................................40Hop Looper and Bertha Armyworm ..................................42Root Weevils.......................................................................44Twospotted Spider Mite......................................................46Minor Arthropod and Slug Pests ........................................49Beneficial ArthropodsPredatory Mites................................................................52Predatory Lady BeetlesAphid Feeders.........................................................54Mite Feeders...........................................................57Predatory BugsMinute Pirate Bug...................................................58Big-Eyed Bug..........................................................59Predatory Mirid......................................................59Assassin Bugs..........................................................60Damsel Bugs...........................................................60Chart of Seasonal Development for Key Groups ofPredatory Athropods...............................................61Parasitic Wasps (Parasitoids)..............................................62Predatory Thrips...............................................................63Predatory and Parasitic Flies .............................................64Other Beneficial Arthropods and Pathogens......................66Weed Management...............................................69Planning a Weed Management Program...........................70Prevention........................................................................70Weed Seedling Identification............................................71Cultural Tactics.................................................................72Herbicides........................................................................72Table of Efficacy Ratings for Weed ManagementTools in Hops.........................................................75Calculating Treated Acres versus Sprayed Acres.................76Common Symptoms of HerbicideInjury on Hop........................................................77Nutrient Management and Imbalances.......79Index............................................................................81
Use pesticides with care. Apply them only to plants, animals, or siteslisted on the label. When mixing and applying pesticides, follow all labelprecautions to protect yourself and others around you. It is a violation of thelaw to disregard label directions. If pesticides are spilled on skin or clothing,remove clothing and wash skin thoroughly. Store pesticides in their originalcontainers and keep them out of the reach of children, pets, and livestock.
IntroductionPest Management, Crop Loss, and IPMDavid H. GentProduction of high quality hopsrequires careful attention to numerousarthropod, disease, and weed pests, aswell as horticultural practices that mayexacerbate or suppress these pests. Multipleplant pathogens and arthropods have beendocumented as pests of hop in the PacificNorthwestern United States, and manyplants common in the region can becomeweeds in hop yards in certain circumstances.The damage these organisms may causeranges from insignificant to complete lossdue to direct reduction in quantity of yieldor diminished yield quality that can renderhops unsalable.The goal of the Field Guide forIntegrated Pest Management in Hops is toprovide growers, consultants, extensionpersonnel, and other pest managers withcurrent, science-based information onidentification and management of arthropodpests, beneficial organisms, diseases,and weeds affecting hops in the PacificNorthwest. Principles of IPM, farm IPMplanning, pesticide toxicology, and nutrientmanagement are presented so that thegrower or pest manager can better utilizethis information in the context of an entirefarming system. Correct identification of pestproblems is the first step in IPM, and colorimages have been included as diagnostic aidswherever possible. Information is presentedon the life cycle and biology of the primarypests of hops in the Pacific Northwest toprovide key concepts underlying managementrecommendations.Information on current pesticideregistrations for hops is available in the PacificNorthwest pest management handbooks (seesidebar), which are revised annually.The editors also acknowledge thesignificant contributions of the generalreferences at right that provided thefoundation and scaffolding for this book.Principles of Integrated Pest ManagementJim D. BarbourIntegrated pest management (IPM)is a pest management strategy formallydeveloped in the 1950s by entomologistsand other researchers in response towidespread development in agriculturalsettings of pesticide resistance in insectsand mites, outbreaks of secondary andinduced insect and mite pests resulting frompesticide use, and transfer and magnificationof pesticides in the environment. Initiallyfocusing on biological control of insectsand mites in agricultural systems, IPMover the last 60 years has assumed abroader role and meaning, encompassingmanagement of diseases and weeds as wellas insects and mites (and other arthropods)in agricultural, horticultural, and urbansettings. Broadly speaking, IPM emphasizesselecting, integrating, and implementingcomplimentary pest management tactics tomaintain pests at economically acceptablelevels while minimizing negative ecologicaland social impacts of pest managementactivities. Although the details of IPMprograms vary to meet the needs ofindividual cropping situations, all are basedon several related principles.Systems-level ManagementModern IPM emphasizes themanagement of agricultural systems, ratherthan individual pests, so as to prevent orreduce the number and severity of pestoutbreaks. This is also referred to as agroecosystemplanning or whole-farm planning.A focus on whole-farm planning is alsoa focus on prevention, which expandsmanagement efforts in time and space.In agricultural crops, this includes usingcultural methods such as crop rotations andfallow periods, tillage, and variety selection(i.e., use of pest-resistant or tolerant varietiesand pest-free rootstock), and legal methodssuch as quarantines. Of these, crop rotationmay be the most difficult to implement inhop because the perennial nature of the cropand the trellis system limit the productionof alternative crops in hop yards. Includedin prevention is the conscious selection ofagronomic procedures such as irrigation andfertilizer management that optimize plantproduction and reduce plant susceptibilityto pests. Prevention can be very effective andcost-efficient and presents little or no risk topeople or the environment.PacificNorthwestPestManagementHandbooksPacific NorthwestPlant DiseaseManagementHandbook, http://plant-disease.ippc.orst.edu/Pacific NorthwestInsect ManagementHandbook, http://pnwpest.org/pnw/insectsPacific NorthwestWeed ManagementHandbook, http://pnwpest/pnw/weedsGeneralReferencesBurgess, A. H.1964. Hops: Botany,Cultivation andUtilization. WorldCrop Books,IntersciencePublication, NY.Mahaffee, W. F.,Pethybridge, S. J.,and Gent, D. H., eds.2009. Compendiumof Hop Diseases andPests. AmericanPhytopathologicalSociety Press, St. Paul,MN.Neve, R. A. 1991.Hops. Chapman andHall, London.1
Monitoring for Pests, Damage, and Treatment SuccessThe concepts of acceptable pest levels, economicinjury levels, and economic thresholds implya need to monitor for levels of pests or pest damagein relation to these levels. Monitoring is fundamentalto IPM because it is used to objectivelydetermine the need for control and also to assessthe effectiveness of control after action has beentaken. Sampling and monitoring requires the abilityto identify pests, pest damage, and key naturalenemies of pests, as well as knowledge of pest andnatural enemy biology and life history. In monitoring,the grower or a scout takes representative samplesto assess the growth status and general healthof the crop, the presence and intensity of currentpest infestations or infections, and the potential fordevelopment of future pest problems. Monitoringmay take many forms such as presence/absence orcounts of pests from visual inspection of plants orplant parts or traps placed in or around fields (e.g.,sticky traps, pheromone traps, spore traps). Samplingshould be conducted to provide a representativeassessment of the pest population in all areas tobe similarly treated, such as part of a field, a singlefield, or adjacent fields. Various sampling schemeshave been developed to assist in monitoring efforts.Monitoring an area for environmentalconditions (especially temperature and relativehumidity) that are favorable or unfavorable for pestdevelopment is also important. This includes theuse of models (e.g., the powdery mildew risk index,degree-day for downy mildew spike emergenceand spider mites) to forecast conditions conduciveto disease or pest development, and surveying thearea for the presence of alternate hosts of hop pests(e.g., agricultural or ornamental varieties of prunethat might harbor overwintering hop aphids) andnatural enemies (e.g., flowering weeds that providehabitat for natural enemies).Monitoring, when conducted routinely—at least weekly during the growing season—andin combination with good record keeping andawareness of model forecasts, can help determinetrends in pest and natural enemy populationgrowth over time. This assists in planning forpest management decisions and assessing theeffectiveness of control actions.Check theAgWeatherNetwebsite at URLhttp://weather.wsu.edu/ foravailable diseaseand pest models.Consult withlocal experts forinformation on usesand limitationsof pest forecastmodels in IPM.3Multi-tactic Management ApproachesWhen prevention is not effective or possibleand monitoring indicates that a pest population hasreached or exceeded an action threshold, interventionis required to lower pest numbers to acceptablelevels. For any given pest situation, pest/crop managerswill need to choose one or more appropriateand compatible management tactics. The basictypes of controls are mechanical, biological, andchemical.Mechanical controls include simple handpicking,erecting barriers, using traps, vacuuming,and tillage to disrupt pest growth and reproduction.Tillage is commonly used to manage weeds inhop, and can be important in managing arthropodpests such as the garden symphylan.Biological controls are beneficial organismsthat prey on or parasitize pests, or organisms thatdo not damage crops but compete with pests forhabitat and displace pests (e.g., Bacillus pumilus forpowdery mildew management). Some biologicalcontrol agents are commercially available for releaseinto cropping systems (i.e., fields, greenhouses) innumbers that can overwhelm pests or that supplementexisting natural enemy populations. Addingagents to the ecosystem is referred to as augmentativebiocontrol; an example would be the releaseof predatory mites Galendromus occidentalis and/orNeoseiulus fallacis, which can be purchased and releasedfor management of twospotted spider mites.Natural enemy populations also can be augmentedusing commercially available chemical attractants,such as methyl salicylate. Biological control also canbe implemented by managing crops to conserveexisting natural enemies (conservation biologicalcontrol) through preserving habitat (includingalternative hosts and prey) necessary for normalnatural enemy growth and reproduction, or by usingmanagement tactics (e.g., selective pesticides orpesticide uses) that have minimal negative impacton natural enemies. In hop, biological control ismost widely practiced in the form of conservationbiological control through the use of selective pesticidesand modified cultural practices.Chemical controls include synthetic andnatural pesticides used to reduce pest populations.Many newer synthetic pesticides are much lessdisruptive to non-target organisms than older,broad-spectrum chemistries (e.g., organophosphate,carbamate, and pyrethroid insecticides). Insecticidesderived from naturally occurring microorganismssuch as Bacillus thuringiensis, entomopathogenicfungi and entomopathogenic nematodes, andnatural insecticides such as nicotine, pyrethrin,and spynosins are important tools in many organicfarming operations, and are playing larger roles innon-organic crop production. Selective pesticidesshould be chosen over non-selective pesticidesto preserve natural enemies and allow biologicalcontrol to play a greater role in suppressing pestoutbreaks. However, broad-spectrum pesticidesremain useful and necessary components of IPMprograms as measures of last resort when othermanagement tactics fail to maintain pests atacceptable levels.Photos Above: A. J. Dreves,D. H. Gent, D. H. Gent
4Pesticide Toxicology and SelectivityPhotos: D. H. Gent,W. S. PengJ. D. Barbour,D. G. JamesPesticide Toxicity RatingsDouglas B. WalshPesticides are essential tools inIPM when other management tacticsfail to control pests at acceptable levels.Approximately 250 to 300 pesticideactive ingredients are used in the PacificNorthwestern United States, and inevitablypesticide use involves some degree ofexposure and risk to humans, non-targetorganisms, and the environment. Table 1 isprovided as a guide to the relative impactof specific pesticides registered for use onhop on non-target beneficial arthropods.The pesticide “signal word” (column 2 oftable) indicates the potential hazard thesepesticides could pose to a mixer or applicator.The signal word “Danger” identifies aproduct as being a Category 1 pesticide,and includes products such as 2,4-D,ethoprop, and folpet. These products have atoxicological profile that could cause injuryor irritation to individuals exposed to lowconcentrations. The signal word “Warning”identifies a product as a Category 2 pesticide,and includes products such as clethodim,cymoxanil, and beta-cyfluthrin. These arematerials that will typically require theuse of fairly extensive personal protectiveequipment, but exposure levels required tocause injury or irritation are substantiallygreater than Category 1 pesticides. Thesignal word “Caution” identifies a Category3 pesticide, and includes products such asthe biocontrol bacterium Bacillus pumilus,carfentrazone, and various Bt formulations(e.g., Bacillus thuringiensis subsp. kurstaki).A Category 3 pesticide is a product that cancause injury or irritation at a relatively highexposure rate. Personal protective equipmentis required, typically including safety glasses,pants, rubber boots, gloves, and long-sleevedshirts. No signal word is required for aCategory 4 pesticide. Simple safety rulesshould be followed with these products toavoid exposure.Pesticide impacts on humans do notnecessarily mirror the impacts those samepesticides would have on beneficial hopyard arthropods. Human physiology differsfrom arthropod physiology, and substantialdifferences exist among arthropods as well.Differences in both susceptibility andresilience factor into a pesticide’s impacton a population of beneficial arthropods.Large predatory insects, for example, maybe able to survive greater doses (i.e., be lesssusceptible) than smaller predatory insectsand mites. However, larger insects typicallywill complete only one or a few generationsover the course of a growing season in thePacific Northwest, whereas a smaller insectmight complete more generations and havea greater chance of recovering its populationlevel (i.e., be more resilient). If a populationis depressed due to pesticide exposure itmay not recover in a hop yard unless thereis an immigration of new individuals fromoutside of the yard.To standardize topical mortalitystudies, the International Organization forBiological Control (IOBC) has categorizedpesticides using a ranking of 1 to 4.Category 1 pesticides in the IOBC ratingsystem are rated as “harmless” to a candidatepopulation of beneficial arthropods if lessthan 30% of a populations dies followinga direct exposure. A Category 2 pesticidein the IOBC rating system is defined as“slightly harmful” to the beneficial. Directexposure to the pesticide will result inmortality levels between 30 and 79%. ACategory 3 pesticide in the IOBC ratingsystem is defined as “moderately harmful”to the beneficial arthropod. Direct exposureto the pesticide will result in mortalitylevels between 79 and 99%. A Category4 pesticide in the IOBC rating systemis defined as “harmful” to the beneficial.Direct exposure to the pesticide will resultin mortality levels greater than 99%. (IOBCcategories 1-4 should not be confusedwith the categories 1-4 relating to humanexposure and indicated by signal words“Danger,” “Warning,” and “Caution” asdescribed in the first column of this section.)Table 1 provides information on three keybeneficial arthropods that occur on hop:predatory mites, lady beetles, and lacewinglarvae. The rankings are summarized froman amalgam of research projects that havebeen conducted on these organisms in thePacific Northwes on crops including treefruit, hop, mint, and grape.
Active IngredientTable 1. Signal Words and Relative Impact of Pesticides Registeredfor Use on Hop on Representative Non-target Beneficial ArthropodsSignalWordTrade NameBeneficial Arthropod IOBC Ranking aPredatoryMitesLadyBeetles5LacewingLarvaeFungicidesBacillus pumilus Caution Sonata 1 ND NDBoscalid Caution Pristine 1 ND NDCopper Caution Various formulations 1 ND NDCymoxanil Warning Curzate 60DF ND ND NDDimethomorph Caution Acrobat ND ND NDFamoxadone & cymoxanil Caution Tanos ND ND NDFolpet Danger Folpan 80WDG ND ND NDFosetyl-Al Caution Aliette WDG ND ND NDKaolin Caution Surround 3 ND NDMefenoxam Caution Ridomil ND ND NDMetalaxyl Warning MetaStar ND ND NDMineral oil/petroleum distillate Caution Various formulations 2 ND NDMyclobutanil Warning Rally 40W 2 1 NDPhosphorous acid Caution Fosphite and other formulations ND ND NDPyraclostrobin Caution Pristine ND ND NDQuinoxyfen Caution Quintec 1 ND NDSodium borate Warning Prev-Am 2 ND NDSpiroxamine Caution Accrue ND ND NDSulfur Caution Various formulations 2 ND NDTebuconazole Caution Folicur 3.6F 1 ND NDTrifloxystrobin Caution Flint 1 ND NDHerbicides2,4-D Danger Weedar 64 and other formulations ND ND NDCarfentrazone Caution Aim EC 1 ND NDClethodim Warning Select Max 1 ND NDClopyralid Caution Stinger 1 ND NDGlyphosate Caution Roundup and other formulations 1 ND NDNorflurazon Caution Solicam ND ND NDParaquat Danger Gramoxone and other formulations 1 ND NDPelargonic acid Warning Scythe ND ND NDTrifluralin Caution Treflan and other formulations 2 ND NDInsecticides/MiticidesAbamectin Warning Agri-Mek and other formulations 3 3 NDB. thuringiensis subsp. aizawai Caution Xentari and other formulations 1 2 NDB. thuringiensis subsp. kurstaki Caution Dipel and other formulations 1 2 NDBeta-cyfluthrin Warning Baythroid XL 4 4 4Bifenazate Caution Acramite-50WS 1 2 NDBifenthrin Warning Brigade and other formulations 4 4 4Cyfluthrin Danger Baythroid 2E 4 4 4Dicofol Caution Dicofol 1 1 NDEthoprop Danger Mocap 4 4 NDFenpyroximate Warning Fujimite 1 3 NDHexythiazox Caution Savey 50DF 1 1 NDImidacloprid Caution Provado and other formulations 1 3 3Malathion Warning Various formulations 2 4 3Naled Danger Dibrom 2 4 3Pymetrozine Caution Fulfill 1 1 1Pyrethrin Caution Pyganic and other formulations 2 2 2Spinosad Caution Success and other formulations 2 2 1Spirodiclofen Caution Envidor 2 2 1Spirotetramat Caution Movento 1 1 1Thiamethoxam Caution Platinum Insecticide 1 1 NDaInternational Organization for Biological Control (IOBC) has categorized pesticides using a ranking of 1 to 4. Rankings represent relative toxicity basedon data from studies conducted with tree fruit, hop, mint, and grape. 1 = less than 30% mortality following direct exposure to the pesticide; 2 = 30 to 79%mortality; 3 = 79 to 99% mortality; and 4 = greater than 99% mortality. ND = not determined.
6Strategiesto MinimizeDevelopmentof PesticideResistance◆◆Utilize culturalpractices toreduce pathogen,weed, and pestpopulationswheneverpossible. Forexample, removingoverwintering flagshoots or basalspikes and basalsucker growthby mechanical orchemical methodshelps reduce theinoculum level ofpowdery mildewand downy mildew.◆◆Limit the numberof applications ofresistance-pronepesticides asdirected by thelabel.◆◆Apply pesticidesat rates specifiedon the label; do notreduce rates.◆◆Adjustapplication volumeper acre basedon the size andvolume of the cropto attain excellentspray coverage.◆◆Alternate or tankmix products withdiverse modes ofaction within andbetween seasons.Pesticide Resistance ManagementMark E. Nelson, Robert Parker, and David H. GentMany of the most widely usedpesticides pose an inherent risk of resistancedevelopment. Pesticide resistance is aconsequence of repeated use of an herbicide,fungicide, or insecticide/miticide withthe same mode of action, resulting in alack of efficacy for a particular pesticideagainst a particular pest. Resistance hasbeen documented among numerous peststhat may affect hop. Examples includeherbicide resistance in kochia and pigweed,organophosphate resistance in hop aphidand twospotted spider mite, and Ridomilresistance in the downy mildew pathogen.Resistance develops in a pestpopulation and not in individuals. It occurswhen a pesticide is applied repeatedlyand susceptible pests are controlled butnaturally resistant individuals of the samespecies reproduce and increase in absenceof competition. Resistant strains of thepest become prevalent in a population overtime due to this selection pressure. Forexample, studies have shown that kochiais a genetically diverse weed species andin a kochia population a small number ofplants (i.e., 1 in 1,000,000 plants) may benaturally resistant to a particular herbicide.Repeatedly exposing kochia populationsto the same herbicide may result in arapid buildup of resistant weeds. Resistantweeds will then dominate over time dueto this selection pressure and previouslyeffective herbicides will fail to control thepopulation.Resistance can be quantitative orqualitative. Quantitative resistance manifestsas a gradual loss of control that occurs as apest population becomes more tolerant to apesticide. In these situations, a product mayperform brilliantly when first used and thenover a period of years slowly deteriorate inefficacy. As a result, the compound mustbe applied at higher rates and/or shorterintervals in order to maintain control. Anexample of this quantitative resistance isfosetyl-Al (Aliette WDG) against the downymildew pathogen. The registered labelrate for Aliette has been 2.5 lbs. per acre(and remains so in most hop productionareas), but this rate is no longer effectivefor control of the downy mildew pathogenin Oregon, where a Section 24c “SpecialPigweed. (H. F. Schwartz, Colorado StateUniversity, Bugwood.org)Local Needs” registration was sought andreceived for the higher rate of 5 lbs. per acre.Alternatively, qualitative resistance is “all ornone,” where a pesticide performs brilliantlyfor a period of time but provides no controlafter resistance develops. A good example ofqualitative resistance is metalaxyl (Ridomil)against the downy mildew pathogen. Onceuseful, this fungicide now provides nocontrol in yards where resistance is present.Note that persistence of resistance in apest population varies among pesticides andpests. For instance, resistance to metalaxylcan still be detected in the downy mildewpathogen in hop yards that have not beentreated with this fungicide in over 10 years.Conversely, resistance to abamectin (Agri-Hop aphids on leaf. (D. G. James)
8At-A-Glance:AlternariaCone Disorder◆◆Symptomseasily confusedwith powdery and/or downy mildew.◆◆Promote aircirculation in thecanopy.◆◆Time irrigationsto reduce periodsof wetness oncones.◆◆Some powderyand downy mildewfungicides likelyprovide somesuppression ofAlternaria conedisorder whenapplied later in theseason.◆◆Confirmcone browningis caused byAlternaria conedisorder beforeimplementing anycontrol measures.Disease ManagementFungal & Bacterial DiseasesAlternaria Cone DisorderDavid H. GentAlternaria cone disorder is causedby the fungus Alternaria alternata, whichis widespread in hop yards and otheragricultural systems worldwide. Strains ofAlternaria fungus are known to attack morethan 100 other plants, including crops suchas apple, potato, sunflower, and wheat.While the presence of the fungus iswidespread, the disease is not known tobe associated with direct yield losses in theU.K. and Australia and is thought to be ofminor importance in the United States. Thedisease can occasionally damage cones andreduce crop quality. It is reported to occurmost commonly on late-maturing varietiesexposed to wind injury, humid conditions,and extended periods of wetness oncones. Cone browning caused by powderymildew and downy mildew is commonlymisdiagnosed as Alternaria cone disorder.SymptomsAlternaria cone disorder symptomsvary depending on the degree of mechanicalinjury to cones; they may be limited toone or a few bracts and bracteoles or insevere cases entire cones may becomediscolored. Symptoms appear first onthe tips of bracteoles as a light, reddishbrowndiscoloration (Fig. 1). Bracts mayremain green, which gives cones a stripedappearance. When cones have been damagedby wind, disease symptoms may appearon both bracteoles and bracts as a moregeneralized browning that can cover entirecones (Fig. 2). The disease can progressrapidly; the killed tissue becomes dark brownand is easily confused with damage caused bypowdery or downy mildew. Affected bractsand bracteoles may display a slight distortionor shriveling of the diseased tissues.Disease CycleAlternaria alternata generally is aweak pathogen that invades wounds createdby insect feeding, mechanical injury, orlesions created by other pathogens. Otherstrains of the fungus may survive as a decayorganism on textiles, dead plants, leather, orother organic materials. On hops, AlternariaFigure 1. Reddish-brown discoloration of thetips of bracts and bracteoles of a cone affectedby Alternaria cone disorder. (D. H. Gent)Figure 2. Further discoloration of cones affectedby Alternaria cone disorder. (S. J. Pethybridge)cone disorder is primarily a disease of conesdamaged by mechanical injury. Severeoutbreaks often are associated with windinjury accompanied with high humidityor extended periods of dew. The pathogensurvives between seasons on decaying plantmaterial, organic matter, and/or as a weakpathogen on other plants.ManagementManagement of Alternaria conedisorder requires accurate diagnosis ofthe disease, which is confounded by itssymptomatic resemblance to powderymildew or downy mildew. Simply recoveringthe fungus from discolored cones does notnecessarily indicate that it was the causeof the browning since the pathogen alsois found on healthy cones. The diseasecan be minimized by reducing damage toburrs and cones caused by strong winds,pesticide applications, and other pests andpathogens; promoting air circulation in thecanopy; and timing irrigations to reduceperiods of wetness on cones. No fungicidesare registered for control of Alternaria conedisorder. However, certain fungicides (e.g.,Flint and Pristine) applied for control ofpowdery and downy mildew likely providesome suppression of Alternaria cone disorderwhen applied later in the season.
Black Root RotFrank Hay and David H. GentThe fungus-like organismPhytophthora citricola causes a crown androot rot of hop referred to as black rootrot. The disease tends to be most damagingto hop plants in poorly drained soils andareas with high water tables. CertainCluster varieties such as Cluster types E-2and L-8 are particularly susceptible. Thepathogen has a relatively broad host rangethat includes cherry, fir trees, raspberry,strawberry, and walnut.SymptomsInfected roots and crowns have acharacteristic water-soaked and blackenedappearance with a distinct boundary betweendiseased and healthy tissue (Fig. 3).Infection can spread from the crown for severalinches up the base of the bine. In severecases, leaves become yellow and bines wiltrapidly during warm weather or when plantsbecome moisture-stressed. Young plantsirrigated heavily to encourage productionin the first year can wilt later in the seasonas a result of black root rot. As the diseaseprogresses, leaves turn black and remain attachedto the bine. Severely infected plantsare weakened and may die during winteror the following spring. Affected plantsoften are found in areas of hop yards withpoor drainage. Wilting symptoms causedby black root rot can be mistaken for Verticilliumwilt, Fusarium canker, or damagecaused by California prionus beetle.Disease CycleThe black root rot pathogen survivesin soil as dormant sexual spores (oospores),which can survive 18 months or more. Inthe presence of free water and host roots,oospores or the asexual spores (sporangia)germinate and infect the plant directly ormay release motile spores (zoospores) thatare attracted to compounds released fromhost roots (e.g., ethanol and certain aminoacids and sugars). The motile zoospores settleon roots and later produce mycelia thatinfect and grow through the host tissues.ManagementGrowers should avoid establishinghop yards in areas with poor water drainage,especially with highly susceptible varietiessuch as Cluster types E-2 and L-8. ClusterL-1 and Galena are considered partiallyresistant, while Brewers Gold, Bullion,Cascade, Columbia, Comet, Eroica, Fuggle,Hallertauer, Nugget, Olympic, Tettnanger,and Willamette reportedly are highly resistantto black root rot. Reducing cultivation andavoiding injury to crowns and roots canprovide some reduction in disease sinceinfection is favored by wounds. Certainphosphorous acid fungicides are registeredfor control of black root rot, but theirefficacy has not been reported. Phenylamidefungicides (i.e., various formulations ofRidomil) applied for control of downymildew may provide some control, althoughthese products are not registered specificallyfor control of black root rot.At-A-Glance:BlackRoot Rot◆◆Plant resistantvarieties whenpossible.◆◆Avoid poorlydrained fieldsand excessiveirrigation.◆◆Avoid damagingroots duringcultivation.◆◆Phosphorousacid fungicidesand variousRidomilformulations mayprovide somecontrol.9Figure 3. Extensive black discoloration caused by black root rot. Notice the distinctmargin between healthy tissue and the black, diseased tissue. (R. A. Beatson)See the PacificNorthwest Plant DiseaseManagement Handbookat http://plant-disease.ippc.orst.edu/ for acurrent list of registeredherbicides.
10At-A-Glance:DownyMildew◆◆Select the mostresistant varietythat is availablefor the intendedmarket.◆◆Establish hopyard with diseasefreeplantingmaterials.◆◆Thoroughlyremove all basalfoliage duringspring pruning.◆◆Prune yards aslate as possiblewithout adverselyaffecting yield.◆◆Strip leavesfrom binesafter trainingand removebasal foliagewith chemicaldesiccants.◆◆Applyappropriatefungicides duringthe first year ofproduction andwhen weather isfavorable to thedisease.◆◆Rotateand tank-mixfungicides to delaydevelopment ofresistance.Downy MildewDavid H. Gent and Dennis A. JohnsonDowny mildew is caused by thefungus-like organism Pseudoperonosporahumuli. It is one of the most importantdiseases of hop in the Pacific Northwestand worldwide. Yield and quality lossesfrom downy mildew vary depending onsusceptibility of the variety and timingof infection, and may range from nondetectableto 100% crop loss if significantcone infection or plant death from crownrot occurs.Figure 4. Basal spikes: Hop shootssystemically infected with the downymildew pathogen. (D. H. Gent)Figure 5. Profuse sporulation on theunderside of a hop leaf appears darkpurple to black. (D. H. Gent)Figure 6. Infection of shoots after training.Notice the yellowing, stunting, and downcurlingof the leaves. (D. H. Gent)Figure 7. Stunted lateral branches resultingfrom downy mildew. Production from thesebranches will be lost. (D. H. Gent)See the Pacific Northwest PlantDisease Management Handbookat http://plant-disease.ippc.orst.edu/ for a current list ofregistered herbicides for downymildew and other diseases.
12VarietyTable 2. Disease Susceptibility and ChemicalCharacteristics of the Primary PublicHop Varieties Grown in the U.S.UsagePowderyMildewDisease Susceptibility aDownyMildewVerticilliumWiltBrewers Gold Bittering S MR MRBullion Bittering S MR RCascade Aroma MR MR MRCentennial Bittering MR S UChinook Bittering MS MR RColumbia Aroma MS MR SComet Bittering R S RCrystal Aroma R S REast Kent Golding Aroma S S MRFirst Gold Bittering R S MRFuggle Aroma MS R SGalena Bittering S S RGlacier Aroma S S UHall. Gold Aroma MS R SHall. Magnum Bittering S R MRHall. Mittelfrüh Aroma MS S SHall. Tradition Aroma MR R MRHorizon Bittering MS S MRLate Cluster Aroma S S RLiberty Aroma MR MR UMt. Hood Aroma MS S SNewport Bittering R R UNorthern Brewer Bittering S S RNugget Bittering R S SOlympic Bittering S MS RPerle Aroma S R MRPioneer Bittering MR MR USaazer Aroma S MS SSaazer 36 Aroma S MS SSpalter Aroma S R MRSterling Aroma MS MR UTeamaker Aroma MR MR STettnanger Aroma MS MS STolhurst Aroma S S UU.S. Tettnanger Aroma MS MS SVanguard Aroma S S UWillamette Aroma MS MR SaDisease susceptibility ratings are based on greenhouse and field observations in experimentalplots and commercial yards in the Pacific Northwest as of 2009. Disease reactions may varydepending on the strain of the pathogen present in some locations, environmental conditions,and other factors, and should be considered approximate. S = susceptible; MS = moderatelysusceptible; MR = moderately resistant; R = resistant; U= unknownDisease CycleThe downy mildew pathogenoverwinters in infected dormant buds andcrowns (Fig. 12). It spreads into developingbuds during the winter and early spring, andsome (but not all) infected buds give riseto basal spikes when shoots emerge in thespring. The pathogen sporulates profuselyon the undersides of leaves of spikes whennighttime temperatures are greater than43 °F and humidity is greater than 90% inthe hop yard. Sporangia are released in midmorningto early afternoon, and germinateindirectly to produce swimming zoosporeswhen the temperature is favorable and freewater is present on leaves, shoot tips, orcones. Zoospores enter hop tissues throughopen stomata, and consequently the mostsevere infections occur when wetness occurson plant surfaces during daylight. Infectionis favored by mild to warm temperatures (60to 70 °F) when free moisture is present forat least 1.5 hours, although leaf infectioncan occur at temperatures as low as 41 °Fwhen wetness persists for 24 hours or longer.Infection of shoots can becomesystemic, producing secondary spikes andadditional sporangia that perpetuate thedisease cycle. When shoots near the crown(approximately 6 inches in height or less)become infected, mycelia can progressthrough the shoot and invade the crown.Carbohydrate reserves are reduced insystemically infected rhizomes and theplants become weakened over time, resultingin reduced yield or plant death.ManagementNo single management tactic providessatisfactory control of downy mildew.Careful attention to cultural practices,judicious irrigation management, andtimely fungicide applications are needed tomanage the disease successfully. Varietiesvary widely in their susceptibility to downymildew (Table 2), although no varieties arecompletely immune. When possible, selectthe most resistant variety that is availablefor the intended market and plant themost resistant varieties in areas with knowndowny mildew pressure (e.g., next to riversor in low-lying areas with cool air pooling).Cascade, Fuggle, Magnum, Newport, andPerle are among the most resistant to downymildew. Cluster is notably susceptible.
13sporangiophores emergewith sporangia onundersideof leafzoospores are releasedfrom mature sporangiumzoospores infect leaves,cones and shootscycle of sporulation/infectionrepeats throughout the seasonoosporeantheridiumoogoniummycelia grow systemicallythroughout the plant,infecting the crown and budsinfected shoots emergein springmycelia overwinter in buds and crownsABOVE: Figure 12. The life cycle of Pseudoperonospora humuli on hop. (Prepared by V. Brewster) BELOW: Figure 13. Hop plants pruned thoroughlymechanically (A) or chemically by a desiccant (C) in early spring. Notice in A and C that all shoots on the sides of the hills have been removed.Incomplete mechanical (B) or chemical pruning (D) can result in more severe outbreaks of both downy mildew and powdery mildew. (D. H. Gent)
14DOWNY MILDEW INCIDENCEFigure 14. Association of spring pruning quality to the incidence ofplants with downy mildew in 97 commercial hop yards in Oregonduring 2005 to 2008. Excellent = No foliage or green stemsremaining after pruning, Moderate = Foliage or green stems onsome hills after pruning, and Poor = No pruning was conducted orfoliage and green stems were present on all hills after pruning.DOWNY MILDEW INCIDENCEDOWNY MILDEW INCIDENCE0.40.30.20.10Figure 15. Association of spring pruning timing to theincidence of plants with downy mildew in 6 commercial yardsof Willamette in Oregon. Hop yards that received the delayedpruning treatment were chemically pruned 10 to 14 dayslater than the growers’ standard pruning timing.10.80.60.40.2010.80.60.40.20NTPruning QualityExcellent Moderate PoorDelayed PruningStandard Pruning2007 2008Moderate DiseasePressureHigh DiseasePressureAliette Flint NT Aliette FlintFigure 16. Efficacy of Aliette WDG and Flint under moderate andhigh disease pressure in Washington. NT = Non-treated.Non-infected rhizomes or softwood cuttings shouldbe selected when establishing new hop yards since plantingmaterial may harbor the pathogen. Thoroughly removeall basal foliage during spring pruning (Figs. 13 and 14).Pruning yards as late as possible, provided all green tissueis removed, generally reduces the severity of downy mildew(Fig. 15). However, optimum timing for pruning must bedetermined carefully for each variety since pruning too latecan reduce yield.In high disease pressure situations, strip leaves frombines after training and remove basal foliage with chemicaldesiccants to reduce disease spread higher into the canopy.Decisions on stripping and the intensity of basal foliageremoval also depend on the severity of downy mildew,presence of powdery mildew, and consideration of thenegative impacts on beneficial insects and mites. In situationswhere downy mildew is threatening late in the season, earlyharvest of yards can minimize cone infection. However, yieldand alpha acid content is reduced when plants are harvestedtoo early and this practice also needs to be considered carefully.Timely fungicide applications often are needed tomanage the disease when weather is favorable to the pathogen.Fungicide applications during the first season a yard is plantedmay be beneficial to help minimize crown infection anddisease levels in ensuing seasons. Under high disease pressurein western Oregon, a fungicide applied just after the first spikeemerges and before spring pruning significantly enhancescontrol of downy mildew later in the season. Later fungicideapplications should be timed to coincide with major infectionevents. See the Pacific Northwest Plant Disease ManagementHandbook at http://plant-disease.ippc.orst.edu/ for a currentlist of registered herbicides.The downy mildew pathogen has a high potential fordeveloping resistance to certain fungicides. Strict adherenceto resistance management tactics is essential to delay thedevelopment of resistance. Resistance to phenylamidefungicides (e.g., various Ridomil formulations) and fosetyl-Al (Aliette WDG) is common in the Pacific Northwest.Phenylamide fungicides should not be used where resistantpopulations have been detected, since resistance to this classof fungicides appears to persist for many years (>15 years)in the pathogen population. Where phosphonate fungicidessuch as fosetyl-Al have been used extensively, resistanceto low rates (e.g., 2.5 pounds Aliette WDG per acre) ofthese products is likely to occur. High rates of phosphonatefungicides are needed for disease control where this resistanceis present. Strobilurin fungicides (e.g., Flint and Pristine)applied for management of powdery mildew can providesuppression of downy mildew. The activity of strobilurinfungicides against both downy mildew and powdery mildewcan be exploited on varieties susceptible to both diseases,bearing in mind that strobilurins have a high risk of incitingresistance development in both the downy mildew andpowdery mildew pathogens. Efficacy of Aliette WDG andFlint under both moderate and high disease pressure is showngraphically in Figure 16.
Fusarium CankerDavid H. GentFusarium canker is caused by thefungus Fusarium sambucinum. The diseaseis often present at a low incidence in hopyards, although in some circumstances ahigh incidence of plants may be affected.Symptoms of the disease are conspicuousand diseased plants are easily identified.Yield losses from Fusarium canker have notbeen quantified rigorously.SymptomsThe base of an affected bine is swollenand tapers near the point of attachment atthe crown (Fig. 17). Affected bines can bedetached from the crown with a gentle tug.Older leaves on the lower part of the binemay become yellow. Disease symptomsoften are not recognized until affectedbines wilt suddenly (Fig. 18) at floweringor in response to high temperatures andmoisture stress. Leaves on wilted binesremain attached. Bine wilting is often mostevident after mechanical injury to binesfrom cultivation, pesticide applications withan air blast sprayer, or high winds, sincebines break off from crowns at these times.Severely affected plants may be killed duringthe winter, particularly when the diseaseoccurs on young plants.Disease CycleThe disease cycle of Fusarium cankerhas not been investigated thoroughly. Thefungus that causes the disease is widespreadin soil and also can be found in associationFigure 17. Swollen basal portion of a bineaffected with Fusarium canker. (D. H. Gent)Figure 18. Wilted bine due to Fusariumcanker. Notice that wilted leaves remainattached to the bine. (D. H. Gent)with plant debris, diseased crowns, andapparently healthy planting materials. Itis thought that the pathogen infects hopplants primarily through wounds createdby mechanical damage (e.g., wind, tractors)at or below the soil line. Insect feeding alsomay create wounds that allow the pathogento gain entry into the hosts.ManagementGrowers should remove diseasedtissue from affected hills, if practical, andavoid propagation from diseased hills.Hilling up soil around the base of binespromotes growth of healthy roots andcan reduce the incidence of bine wilting.Reducing free moisture near the crowndue to irrigation can help. Application oflime to increase pH near the crown andavoiding use of acidifying ammoniumnitrogen fertilizers can help to reducedisease incidence. Minimizing injury tobines during field operations, arching (i.e.,tying bines and strings together to facilitateequipment passage), and preventing damageto bines from arthropod pests can all helpto reduce wounds that allow the fungus togain entry into the plant. No fungicides areregistered for control of Fusarium canker.At-A-Glance:FusariumCanker◆◆Avoidpropagation fromdiseased hills.◆◆Mound soilaround thebase of bines topromote growthof healthy rootsand reducewilting.◆◆Reduce freemoisture near thecrown.◆◆Add lime toincrease pHnear the crownand avoid useof ammoniumnitrogenfertilizers.◆◆Minimize injuryto bines duringfield operationsand from pests.◆◆Arching stringsmay help toreduce bineinjury.15
16 Fusarium Cone Tip BlightDavid H. GentCone tip blight generally is aAt-A-Glance:FusariumCone TipBlight◆◆Time irrigationsto reduce periodsof wetness oncones.◆◆Fungicideapplications donot appear to beeffective.◆◆This sporadicdisease does notwarrant specificcontrol measuresin most yards.disease of minor importance in the PacificNorthwestern United States, although insome instances up to 30% of cones can beaffected. The disease has been attributedto several Fusarium species, includingFusarium crookwellense, F. sambucinum, andF. avenaceum.SymptomsAffected bracts and bracteoles at thetip of the cone become a medium to darkbrown as the cone matures (Fig. 19). Thebrowning may be limited to a small portionof the tip of the cone or in severe casesencompass as much as 60% of the cone.A characteristic symptom of the disease isthat all bracts and bracteoles in the whorl ofthe cone tip tend to be affected. Browningand death of the tip of the strig (central axisthat bears the nodes) generally are apparentwhen the affected bracts and bracteoles areremoved (Fig. 20).Disease CycleLittle is known about the diseasecycle. The pathogens may survive in soil,plant debris, and/or in association withhop crowns. The cone tip blight pathogens,as well as other Fusarium species, may berecovered from apparently healthy burrs,bracts, strigs, and stigma. Anecdotal reportssuggest that the disease is favored by highhumidity during cone development.ManagementControl measures have not beendeveloped for cone tip blight, but the diseaseoccurs sporadically enough that specificcontrol measures are not needed in mostyards. Limited evaluations of fungicidesindicate Fusarium spp. are recovered at alower rate from burrs and cones treated withstrobilurin fungicides, but these treatmentshave not been successful for management ofcone tip blight.Figure 19. Medium browndiscoloration of bracts andbracteoles on a cone with conetip blight. (S. J. Pethybridge)Figure 20. Discoloration of strigs due to cone tip blight. (Courtesy J. C. Bienapfl. Reproducedwith permission from Compendium of Hop Diseases and Pests, 2009, W. Mahaffee,S. Pethybridge, and D. H. Gent, eds., American Phytopathological Society, St. Paul, MN.)
Gray MoldDavid H. GentGray mold generally is a disease ofminor importance in hops of the PacificNorthwestern United States. The disease isfavored by prolonged wet, humid conditions,and can result in cone discoloration andpoor cone quality. The disease is caused bythe fungus Botrytis cinerea, a widespread andcommon pathogen found on numerous cropsincluding bean, raspberry, strawberry, andtree fruit.SymptomsAffected cones have light to darkbrown spots on the tips of bracts andbracteoles, which can enlarge with timeand cause discoloration of entire cones.Bracteoles are more susceptible to damagethan bracts, and diseased cones can developa striped appearance. Gray mold symptomsare similar to Alternaria cone disorder butcan be distinguished by the presence ofgray, fuzzy fungal growth that begins at thetip of the cone (Figs. 21 and 22). Signs ofthe pathogen may not be present in dryweather.Disease CycleThe gray mold fungus may survive asa decay organism on organic materials, inand on leaves, and in the soil as dormantresting structures known as sclerotia.The pathogen is active over a range oftemperatures when free moisture isavailable, with an approximate temperatureof 68 °F being optimal. The fungus canremain dormant in or on plant tissuesduring unfavorable conditions and becomeactive when weather or host factors arefavorable. Infection on cones is favoredby wet weather and injury caused by fieldoperations, insect feeding, or other diseases.ManagementFungicide applications can reducegray mold damage to hops. (See the PacificNorthwest Plant Disease ManagementHandbook at http://plant-disease.ippc.orst.edu/ for a current list of registeredherbicides.) However, in most years thedisease causes minimal damage to hops inthe Pacific Northwest and special controlmeasures have not been necessary. Culturalpractices such as increasing row and plantspacing and management of overheadirrigation to reduce the duration of wetnesson cones help to reduce the incidenceof gray mold. Damage to cones frominsect feeding can exacerbate gray mold,and efforts should be made to managearthropods at economic thresholds.At-A-GlanceGray Mold◆◆Minimaldamage to PNWhops.◆◆Controlmeasuresgenerally notneeded.◆◆Manageirrigation andpromote airmovement toreduce wetnesson cones.◆◆Managearthropod pestsat economicthresholds toprevent injury tocones.◆◆Fungicideapplications canreduce gray molddamage to hopcones during wetweather.17Figures 21 and 22. Medium brown discoloration and fungal growthon the tip of a cone due to gray mold. (S. Radisek)
18 Powdery MildewAt-A-GlancePowderyMildew◆◆Select earlymaturingorresistant varietieswhen possible.◆◆Applyadequate butnot excessiveirrigation andfertilizer.◆◆Remove allgreen tissuesduring springpruning.◆◆Applyappropriatefungicides assoon as possibleto protectregrowth afterpruning andthroughoutseason.David H. Gent and Mark E. NelsonPowdery mildew is caused by thefungus Podosphaera macularis, and is one ofthe most important diseases of hop in thePacific Northwest. The disease can causesevere crop damage, in some cases resultingin complete loss of marketable yield due tolost production and reduced cone quality.SymptomsDisease symptoms appear as powderywhite colonies on leaves, buds, stems, andcones (Fig. 23-25). During periods of rapidplant growth, raised blisters often are visiblebefore sporulation can be observed. Infectionof burrs and young cones causes abortion orsevere distortion of the cone as it develops.Affected cones may develop a characteristicwhite powdery fungal growth, althoughin some cases fungal growth is visible onlyunder bracts and bracteoles and only withmagnification. Affected cones become reddishbrownas tissues are killed (Fig. 25), or mayturn a medium brown after kiln drying.Disease CycleIn the Pacific Northwest, thepathogen is known to overwinter onlyin infected buds. Where sexual matingoccurs there is potential for overwinteringstructures (called chasmothecia orcleistothecia) to form and survive in and oncrop debris and soil. However, the sexualstage of the fungus has not been confirmedin the Pacific Northwestern United States.Shoots that emerge from an infected budoften are rapidly covered with fungalgrowth, and are termed “flag shoots” (Fig.26). Flag shoots occur on a small percentageof hills, on average approximately 0.7% inWashington and 0.02% of hills in Oregon,and provide the initial spores to beginoutbreaks each spring. Flag shoots oftenare not detected until they become heavilycovered with powdery mildew, althoughinfected shoots can be found at a low levelas soon as shoots emerge in spring.As the plant develops, the pathogenspreads and infects young leaves, moving upthe bine in sync with plant growth. Leavesbecome increasingly resistant to infection asthey age, especially when they are producedduring hot weather (> 85 °F). Diseasedevelopment is favored by rapid plantgrowth, mild temperatures (47 to 82 °F),high humidity, and cloudy weather. Underideal conditions at 65 °F, the fungus cancomplete its life cycle in as little at 5 days.Burrs and young cones are very susceptible◆◆Eliminatebasal growthwith chemicaldesiccants toremove diseasedtissue.◆◆Apply highlyeffectivefungicides toprotect burrs andyoung cones.◆◆Harvest timelyto minimize croplosses in the fieldwhen powderymildew occurs oncones.Figure 23. Powdery, white colonies on a leaf severely affected by powdery mildew. (D. H. Gent)
17 19to infection, and their development isarrested by infection, resulting in reducedcrop yield and quality. Infections occurringlater in the season are thought to leadto browning and an apparent prematureripening. Extremely cold weather during theoverwintering period is thought to reduce,but not eliminate, survival of the fungus ininfected buds (Fig. 27).PHOTOS THIS PAGE,CLOCKWISE FROMFAR LEFTFigure 24.Leaves and stems extensivelycolonized by the powderymildew fungus surrounding andoriginating from a flag shoot.(D. H. Gent)Figure 25. Cone with severebrowning caused by lateseasoninfection by thepowdery mildew fungus. Notewhite fungal growth (mycelium)on bracts. (D. H. Gent)Figure 26.A young shoot with severepowdery mildew (“flag shoot”)resulting from bud infection andoverwintering. (D. H. Gent)Figure 27.Life cycle of Podosphaeramacularis on hop. The sexualstage of P. macularis (shownby arrows on the bottom andleft side of the figure) is notknown to occur in thePacific Northwestern U.S.(Prepared by V. Brewster)conidiophoreconidiumflag shootconidia reinfect leaves,cones, buds and shootscrownbudascosporedischarge inspring, aftera wettingevent, infects leavesclose to the groundchasmotheciamycelioid appendagesoverwintering myceliain and on crown budsoverwintering chasmotheciachasmothecia oncone and leaf littercross section of budwith internal chasmothecia
20See the PacificNorthwestPlant DiseaseManagementHandbook athttp://plantdisease.ippc.orst.edu/ for a currentlist of registeredherbicides.ManagementControl of powdery mildew requiresintegration of crop sanitation practices,adequate but not excessive fertilization andirrigation, and timely fungicide applicationsto keep disease pressure as low as possibleduring the season and up to harvest. Althoughgrowers often have little ability toselect resistant varieties because of marketfactors, many resistant varieties are available(Table 2, page 12). Newport, Nugget, andseveral proprietary varieties are resistant topowdery mildew, while Cascade and Libertyhave useful levels of tolerance. Selection ofearly-maturing varieties (e.g., Fuggle) canhelp to escape late-season powdery mildew.Management of powdery mildewshould begin in early spring by thoroughlyremoving all green tissues during springpruning, including shoots on the sides ofhills (Fig. 13, page 13, and Fig. 28). Thetiming of the first fungicide applicationafter spring pruning is critical in affectingthe severity of disease later in the season.This application should be made as soon aspossible after shoot growth resumes.Regular fungicide applications areessential for economic production of mostsusceptible varieties. However, fungicideapplications alone are not sufficient tomanage the disease economically. Underhigh disease pressure, removal of basalgrowth with chemical desiccants is essentialto remove diseased tissue and delay diseasedevelopment. Desiccants should be appliedonce bines have grown far enough up thestring so that the growing tip will not bedamaged. Achieving adequate cover of densebasal growth during fungicide applicationsis difficult, and removal of basal foliage iscritical for reducing later infection of leavesand cones. Results of a field trial usingdesiccants alone are shown in Figure 29.Several factors influence thedevelopment and severity of powderymildew on cones, including disease severityon leaves, temperature and rain duringcone development, late-season fungicideapplications, and harvest date. Highlyeffective fungicides, such as Quintec,applied to young, developing cones cansignificantly reduce incidence of powderymildew on cones at harvest (Fig. 30). Theefficacy of any fungicide, however, can varygreatly depending upon the severity ofthe disease pressure present (Fig. 31). Theincidence of cone infection is also correlatedwith timing of the last fungicide application,and applications should continue untilthe pre-harvest interval as specified by thelabel. The powdery mildew pathogen hasan extremely high ability to reproduce,therefore careful attention to fungicideresistance management guidelines is criticalto delay the development of resistance.When powdery mildew is presenton cones near harvest, timely picking willminimize crop losses in the field. Earlyharvest also can help to reduce damage tocones, although yield can be reduced.20Pruning QualityFigure 28.Association of spring pruningquality to the incidence ofcones with powdery mildewin 50 commercial hop yardsin Oregon and Washingtonduring 2000, 2005, and2006. Excellent = No foliageor green stems remainingafter pruning, Moderate =Foliage or green stems onsome hills after pruning,and Poor = No pruningconducted or foliage andgreen stems present on allhills after pruning.PERCENT DISEASED CONES151050Excellent Moderate Poor
PERCENT DISEASED LEAVES100806040200Non-treatedGramoxoneAim19 Jun 25 Jun 3 Jul 10 Jul 17 Jul 24 Jul 30 Jul 13 Aug 22 Aug 28 Aug17 21Figure 29.Incidence of hop leaves withpowdery mildew in relationto herbicide treatments toremove basal leaf growthin Washington. Applicationsof Aim EW were applied 6July, 3 Aug, and 20 August.Applications of GramoxoneMax + Desicate II wereapplied 6 July and 3 August.No fungicides were appliedin this trial.PERCENT DISEASED CONES100806040200Quintec 18 July & 1 Aug Quintec 15 Aug & 30 Aug Other NTFigure 30.Effect of Quintec timingon incidence of coneswith powdery mildew inWashington in 2008 underextremely high diseasepressure. NT = Non-treated.Other = Another fungicideapplied during 18 July to 30August.806040200PERCENT DISEASED CONES100Moderate Disease PressureQuintecFlint/AccrueFlint/FolicurAccrue/FolicurRallyNon-treatedHigh Disease PressureQuintecFlint/AccrueFlint/FolicurAccrue/FolicurRallyNon-treatedFigure 31.Efficacy of powderymildew fungicides undermoderate and high diseasepressure in Washington.Notice that most fungicidesprovide acceptable controlwhen disease pressure ismoderate.
22At-A-GlanceRed Crown Rot◆◆Select and plantonly high-qualityplanting materials.◆◆Avoid woundingcrowns duringspring pruning.◆◆Maintain plantvigor by avoidingearly harvests,maintaining basalfoliage, andavoiding soilappliedherbicidesthat reduce rootdevelopment.◆◆Avoid replantingin the hole leftby removing adiseased plant.◆◆Fumigation canimprove plant vigorand yield, but hasnot been adoptedwidely in Australiawhere this diseaseprimarily occurs.Figure 32. Reddish-browndecay and dry rot of acrown affected by redcrown rot. (D. R. Smith)Red Crown RotDavid H. GentRed crown rot has been described onhop plants in Australia and Oregon. In Australiathe disease was attributed to a fungusdescribed as a Phacidiopycnis sp. The namingof this fungus was in flux, and the propername of this organism is now thought to bePhomopsis tuberivora. Data from Australiaindicate affected plants may suffer yield lossesof up to 20%. In Oregon, plants have beenkilled by red crown rot and yield losses appearto be higher than 20% in some instances.SymptomsThe pith tissue of affected roots andcrowns is orange to red, which developsinto a dry rot of the root (Figs. 32-33) witha distinct boundary between diseased andhealthy tissue (Fig. 33). Roots and crowns ofapparently healthy plants also may have thisappearance, but the degree and severity of rotis more pronounced in diseased plants. In theadvanced stages of the disease, entire crownsare destroyed, leading to weak, uneven shootgrowth and yellowing of lower leaves (Fig.34). Bines on severely affected plants oftenfail to reach the top wire and have limiteddevelopment of lateral branches. Severelyaffected plants can be killed. Affected plantstend to be aggregated in roughly circularpatches, although in some young hop yardsdiseased plants may be more generallyscattered across a yard.Disease CycleThe only data available on the diseasecycle of red crown rot are from researchconducted in Victoria, Australia. In thatenvironment, the disease was thought tobe associated with planting poor qualityFigure 33. Reddish-brown rot of a younghop root caused by red crown rot. Noticethe distinct margin between diseased andhealthy pith tissues. (D. H. Gent)Figure 34. Weak growth of bines andplant death caused by severe red crownrot. Notice that affected plants areaggregated in this yard. (D. R. Smith)rootstock, injury to crowns during springmowing of shoots (slashing), and culturalpractices that reduced plant vigor, such asearly harvest and leaving insufficient foliageon plants after harvest. The causal organismcan be recovered from soil, plant debris,and healthy crowns. The host range of thepathogen also includes alfalfa, beet, potato,and several trees and woody ornamentals.The fungus is a weak pathogen and diseasesymptoms rarely develop on these hosts.ManagementControl measures for red crown rothave not been investigated or validated inthe Pacific Northwest; the disease currentlyappears to cause economic damage inrelatively few yards. Red crown rot hasbeen managed successfully in Australiathrough a combination of careful selectionof high quality, disease-free plantingmaterials, avoidance of crown woundingduring spring pruning, and culturalpractices that maintain plant vigor. Othermanagement recommendations promotedin Australia include removing diseasedplants and avoiding replanting in the holeleft by removing a diseased plant. Effortsshould be made to improve plant vigorby avoiding early harvests, maintaining asmuch foliage as possible after harvest tohelp plants increase carbohydrate reserves,and avoiding soil-applied herbicides thatreduce root development. Boron deficiencyhas been implicated in red crown rot inVictoria, although conclusive evidence ofa link between boron deficiency and thedisease is lacking. In Victoria, fumigationwith dazomet provided an approximate60% increase in yield in year one and 14%in year two. However, this practice has notbeen adopted in Australia due to the highcost of fumigation.
Sclerotinia Wilt (White Mold)David H. GentSclerotinia wilt, also referred to aswhite mold, affects nearly 400 weed andcrop plant species, including importantcrops in the Pacific Northwest such asnumerous legumes (e.g., green bean andpea), canola, carrot, lettuce, potato, andsquash. The disease is caused by a fungus,Sclerotinia sclerotiorum, and is an occasionalproblem on hop in wet, cool climates suchas those found in the hop productionregions in New Zealand or western Oregon.However, the disease occurs rarely on hop inthe Pacific Northwest. Sclerotinia wilt cancause damage when soil and plants remaincontinuously wet and temperatures are mild.SymptomsDisease symptoms generally appear inlate spring or early summer as soft, watersoakedlesions on bines just below or nearthe soil surface at the crown. The infectedtissue collapses, creating a light brown tograyish lesion approximately 1 to 4 incheslong. During wet weather, fluffy whitegrowth of the fungus may form on theinfected tissue (Fig. 35). Small, hardenedblack overwintering structures (sclerotia)form on and in diseased bines. As thedisease progresses, the lesions expand andmay girdle the bine, causing a wilt. Leavesgenerally remain green until the bine isgirdled completely. Disease symptoms mayappear similar to those caused by Fusariumcanker or Verticillium wilt. However, thepresence of fluffy white mycelia and sclerotiaare diagnostic for Sclerotinia wilt.Disease CycleThe pathogen overwinters as longlivedresting structures (sclerotia) in infestedcrop debris and in the soil. Sclerotiacan germinate directly and infect roots,or, if conditioned by exposure to moistconditions and cool temperatures, cangerminate to produce one or numeroussmall mushroom-like structures calledapothecia (Fig. 36). The soil surface mustremain wet for several days or longer forapothecia to form, and with hops thisgenerally occurs when plants produceabundant, lush foliage that shades the soilnear the crown. A sclerotium may produceone or numerous apothecia, and eachapothecium may produce produce severalmillion airborne spores called ascospores.Ascospores require a nutrient source uponwhich to grow before invading a host,and often this nutrient source is senescentleaves or other plant tissues near the crown.Severe epidemics of Sclerotinia wilt on hopreportedly are associated with hilling soilinfested with sclerotia onto crowns and withfrost injury of developing basal buds. Newsclerotia are formed in and on infected binesand are returned to the soil, where they maysurvive five years or longer and perpetuatethe disease cycle. The pathogen also maysurvive on numerous broadleaf weeds in andaround hop yards.ManagementControl measures for Sclerotinia wiltof hop usually are not needed in the PacificNorthwest. Avoiding varieties reported to beespecially susceptible (e.g., Fuggle, Bramling)might be useful in wet, mild areas. Culturalpractices that reduce the duration of wetnesson plants and the soil surface can reducedisease incidence. These cultural practicesmay include limiting nitrogen fertilization,removing excess basal shootsand leaves, stripping leavesfrom lower bines, and timingirrigations to allow the toptwo inches of the soil to drycompletely between irrigations.Formulations of the parasiticfungus Coniothyrium minitans(marketed under the trade nameContans WG) are available forbiological control of Sclerotiniasclerotiorum. The efficacy of thisproduct for Sclerotinia wilt inhop has not been investigated.At-A-GlanceSclerotiniaWilt or WhiteMold◆◆Controlmeasures usuallyare not neededin the PacificNorthwest.◆◆Utilize lesssusceptiblevarieties wherepossible.◆◆Limit excessivebasal growth andtime irrigations toreduce wetness onplants and soil.23◆◆Commercialformulations of abiological controlagent are available.ABOVE: Figure 35. White fungalmycelia and sclerotia (small blacksurvival structures) on hop binesaffected by Sclerotinia wilt. (T. J. Smith)AT LEFT: Figure 36. Sclerotium ofSclerotinia sclerotiorum that hasgerminated to produce an apothecium.Numerous apothecia can be producedfrom a single sclerotium. (D. H. Gent)
24At-A-GlanceSooty Mold◆◆Sooty moldis controlled bycontrolling hopaphids.◆◆Naturalenemies ofhop aphidcan providesignificant levelsof control whennot disrupted bybroad-spectruminsecticides.Sooty MoldDavid H. GentSooty mold is not a disease, but rathera complex of common fungi that growsuperficially on insect excretions depositedon leaves and cones. The appearance ofsooty mold is due to the presence anddevelopment of phloem-feeding insects,most importantly the hop aphid. Hopaphids probe the phloem strands of hopplants, ingesting more plant fluids thancan be processed by their digestive systems.Aphids expel the excess plant fluids as adilute solution known as “honeydew,”comprised of sugars, amino acids, and othersubstances, which provides a food sourcethat supports the growth of dark-pigmentedfungi that grow on the surface of leaves andcones, reducing the quality of cones.SymptomsOnce aphids colonize and commencefeeding, plant tissues become coveredwith sticky honeydew and develop a shinyappearance before sooty mold becomesevident. Signs and symptoms of sootymold soon develop on this honeydew as aflattened, black mass of fungal growth thatresembles a fine layer of soot (Fig. 37). Burrsand developing cones later may becomecovered with honeydew, quickly becomingblack and sooty in appearance. Entire bracts,bracteoles, and lupulin glands may becomeFigure 38. Black sooty mold on a cone. Noticethe white aphid castings present under thebracts and bracteoles. (D. H. Gent)black and sticky, but sooty mold tends to bemost prevalent on the undersides of bractsand bracteoles and on leaves shaded fromthe sun (Fig. 38).ManagementSooty mold is managed by controllinghop aphids (Fig. 39) when populations exceedeconomic thresholds. Natural enemiesof hop aphid can provide significant levels ofcontrol when not disrupted by insecticides,therefore when possible broad-spectrum insecticidesshould be avoided.Figure 37. Black sooty mold on hop leaves.(D. H. Gent)Figure 39: Hop aphids are a major contributingfactor in sooty mold. This is the winged formof the hop aphid. For aphid photos and controlinformation, see the arthropod pest controlsection of this handbook. (L. C. Wright)
Verticillium WiltDavid H. Gent and Mark E. NelsonVerticillium wilt is a potentiallydamaging disease of hop and numerousother crops including alfalfa, cherry, maple,mint, potato, as well several herbaceousplants, woody ornamentals, and commonweeds. On hop, Verticillium wilt may becaused by two related fungi, Verticilliumalbo-atrum and V. dahliae. The host rangeand severity of disease caused by thesepathogens varies. Several strains of V. alboatrumhave been described. Some may causerelatively minor wilting symptoms (nonlethalor fluctuating strains) while others cancause severe symptoms (lethal or progressivestrains) that rapidly can kill susceptiblevarieties. Non-lethal strains of V. albo-atrumare common in the Pacific Northwest andhave been reported on hop. Lethal strainsof Verticillium albo-atrum have not beenreported from the United States. Verticilliumdahliae causes a relatively minor wilt diseaseon hop. This pathogen has a broader hostrange than V. albo-atrum, and occurscommonly on hop in the United States.SymptomsDisease symptoms vary dependingon the aggressiveness of the Verticilliumpathogen that is attacking the plant. Withnon-lethal strains of V. albo-atrum, diseaseFigure 40. Upward curling and wiltingof leaves associated with Verticilliumwilt caused by a non-lethal strain ofVerticillium albo-atrum. (D. H. Gent)Figure 41. Swollen bine with wilted leavesresulting from infection by a non-lethalstrain of Verticillium albo-atrum, one of theVerticillium wilt pathogens. (D. H. Gent)symptoms often appear initially on lowerleaves as yellowing and death of tissuebetween major veins and upward curlingof leaves (Fig. 40). Affected bines becomenoticeably swollen (Fig. 41) and when thesestems are cut open the vascular tissue isdiscolored a medium to dark brown (Fig.42). These symptoms generally are firstrecognized near flowering or when plantsbecome moisture stressed. Eventually, oneor all of the bines on a hill harboring theinfection completely wilt (Fig. 43). Lethalstrains of V. albo-atrum can cause rapiddeath of leaves, side arms, and plant death.Bine swelling is less apparent with lethalstrains of V. albo-atrum, but the degree ofvascular browning is more severe than thatassociated with non-lethal strains of thepathogen. Verticillium albo-atrum has beenreported on hop more frequently in Oregonthan Idaho or Washington.Symptoms of Verticillium wiltcaused by Verticillium dahliae may varydepending on environment and variety.In some cases, such as with the varietyWillamette, plants may be infected butthe only noticeable symptom is swelling ofthe bines and a general yellowing of lowerleaves near the main bines. Some degree ofbrowning often is present when these binesare cut open. Verticillium dahliae tends tocause more severe symptoms on hop plantsin Washington than Oregon.At-A-GlanceVerticilliumWilt◆◆Plant resistantvarieties whenpossible.◆◆Clean equipmentbetween yards tominimize spreadingthe pathogen.◆◆Plant onlydisease-freerhizomes orcuttings.◆◆Do not returntrash or compostfrom yards withVerticillium wilt tohop yards.◆◆Control weedswith herbicidesand reducecultivation wherepossible.◆◆Reduce nitrogenfertilization asmuch as possible.25
Diseases of Minor Importance 27Armillaria Root Rot(Shoestring Root Rot)Armillaria root rot, also known asshoestring root rot, is a common disease ofnumerous forest and orchard trees, shrubs,and vines caused by species of the fungusArmillaria. On hop, disease symptomsappear initially as wilting of plants. Plasterwhitesheets of the pathogen grow under thebark of infected bines near the soil surface.As the disease progresses, the crown maydisplay a powdery rot. The disease generallyis a minor concern for hop. However, newyards should not be planted after susceptibletree crops. If a hop yard must be establishedfollowing a tree crop, all roots and stumpsshould be removed and destroyed if thedisease was present.Black MoldBlack mold is caused by anunidentified species of the fungusCladosporium. The disease can cause abrown discoloration of bracts that givesaffected cones a striped appearancesomewhat similar to Alternaria conedisorder. In the case of black mold, thebracts become brown and the bracteolesremain green. The darkly pigmentedspores of the fungus are easily observed onaffected bracts under low magnification. Thediscoloration is most prominent on conesprotected from direct sunlight, such as thoseon low lateral branches. The disease causesnegligible damage, but black mold is easilyconfused with downy mildew or Alternariacone disorder and misdiagnosis may lead tothe unnecessary application of fungicides.Crown GallCrown gall, caused by the bacteriumAgrobacterium tumefaciens, is the onlybacterial disease of hop reported in theUnited States. The disease results in fleshyto hard tumors (galls) on bines at or nearthe soil surface close to the crown, resultingin weak bine growth, wilting of affectedbines, and, in severe cases, plant death. Thedisease appears to be most damaging innurseries and on young plants; older plantscan be affected without obvious symptomsor damage. Generally, no special diseasemanagement strategies are needed for crowngall. Softwood cuttings and rhizomes shouldbe harvested only from plants free of thecrown gall bacterium.Rhizoctonia solaniRhizoctonia solani has been reportedin very rare instances to cause lesions onyoung shoots of ‘Brewers Gold’ in BritishColumbia. Lesions are sunken and brickred to black in color. Affected shoots arestunted and may collapse if girdled by alesion near the crown. The occurrence of thedisease in British Columbia was attributedto hilling soil on top of plants immediatelyafter spring crowning. This practice isuncommon, and should continue to beavoided.At-A-GlanceMinor Diseases◆◆Avoid plantinghops followingtrees susceptibleto Armillaria rootrot.◆◆Black moldsymptoms areeasily confusedwith those ofdowny mildew orAlternaria conedisorder.◆◆Crown gallcan impactyoung plants;take care toharvest cuttingsand rhizomesfrom uninfectedplants.◆◆While rare,Rhizoctoniasolani may befavored by hillingplants afterspring crowning.
28At-A-GlanceCarlavirusComplex◆◆Use onlycertified virus-freeplanting stockwhen establishingnew yards.◆◆Insecticide usefor aphid controlis inefficientfor limiting theintroduction ofviruses, but canreduce the rate ofspread within ayard.Virus and Viroid DiseasesCarlavirus Complex: American hop latent virus,Hop latent virus, and Hop mosaic virusKenneth C. Eastwell and Dez J. BarbaraThree carlaviruses are known to infecthop plants: Hop mosaic virus, Hop latentvirus and American hop latent virus. All areknown to occur in mixed infections andall but American hop latent virus are foundworldwide. American hop latent virus isfound primarily in North America.SymptomsHop latent virus and American hoplatent virus do not cause visually obvioussymptoms on any commercial hop varieties.Of the three carlaviruses, Hop mosaic virus isthe most likely to cause both symptoms andcrop loss. On sensitive varieties, chloroticmosaic mottling can develop between majorleaf veins (Fig. 44). Severely affected plantsmay establish poorly when planted, haveweak bine growth, and often fail to attachto the string. The varieties that developthese symptoms typically are those of theGolding type or those that have Goldingparentage. However, some strains of Hopmosaic virus appear to cause infections thatmay be almost symptomless on Goldinghops. The three carlaviruses reduce growth,which is particularly critical in establishingnew plantings. Yield can be reduced byapproximately 15%, but varieties sensitiveto Hop mosaic virus can suffer losses as greatas 62% as a result of infection. Changes inbrewing characteristics induced by theseviruses are minor and appear to be analogousto over maturity of the hop cones at harvest.Disease CycleCarlaviruses are transmittedmechanically and in a non-persistentmanner by aphids.All three are transmitted by thehop aphid, Phorodon humuli, and Hopmosaic virus and Hop latent virus arealso transmitted by the potato aphid,Macrosiphum euphorbiae, and green peachaphid, Myzus persicae. Transmission byaphids is thought to be quite inefficient,however. Propagation and distribution ofvirus-infected plants is the primary modethrough which carlaviruses are spread longdistances. Root grafting and mechanicaltransmission are thought to contributeto localized spread. Carlaviruses typicallyhave narrow host ranges and for practicalpurposes the only hosts for these pathogenslikely to be near hop yards are other hopplants. Over the life of a hop planting, ahigh percentage of plants in a particular hopyard may become infected.Figure 44. Yellow mosaic pattern on Chinookdue to Hop mosaic virus. (K. C. Eastwell)ManagementSince vegetative propagation of virusinfectedplants is the principal factor invirus spread, the use of certified virus-freeplanting stock is the most practical methodof limiting any virus disease, particularlyduring the early stages of plant growth anddevelopment. Application of insecticidesto control aphids is inefficient for limitingthe introduction of virus since the viruswill be transmitted before the viruliferousaphids are killed. However, reducing aphidpopulations can reduce the rate of secondarytransmission within a hop yard.
Apple mosaic virusKenneth C. Eastwell and Dez J. BarbaraApple mosaic virus is considered themost important virus disease of hop aroundthe world. Originally, it was believed thatthe disease was caused by either Applemosaic virus or the closely related virusPrunus necrotic ringspot virus. Recent dataindicate that all natural infections of hopare by Apple mosaic virus and that previouslydescribed isolates of Prunus necrotic ringspotvirus in hop plants were genetic variants ofApple mosaic virus.SymptomsApple mosaic virus induces chloroticrings or arcs that can become necrotic.Frequently, these merge to create oak-leafline patterns on leaves (Figs. 45-47). Theseverity of symptoms is dramatically affectedby environmental conditions. Symptoms areusually most severe when a period of coolweather with temperatures below 80° F isfollowed by higher temperatures. Plants canbe infected for several seasons without diseaseexpression until appropriate environmentalconditions occur. Under conditions wheresevere symptoms are expressed, cone andFigure 45. Necrotic ringspots andoak-leaf line pattern on Nugget due toApple mosaic virus. (D. H. Gent)Figure 46. Oak-leaf line pattern caused byApple mosaic virus, without the development ofringspot symptoms. (D. H. Gent)Figure 47. Necrotic ringspot due to Applemosaic virus. Development of this symptom istemperature dependent; necrotic ringspots maynot be apparent in all seasons. (D. H. Gent)alpha acid yield can be reduced up to 50%.A mixed infection of Apple mosaic virus andHop mosaic virus may result in enhanceddisease severity and crop loss.Disease CyclePropagation of Apple mosaic virusinfectedplants is the primary modeof transmission, although mechanicaltransmission in the hop yard and rootgrafting appear to be significant factors inthe local spread of the virus. Since Applemosaic virus is not expressed every growingseason, infected plants may be selectedinadvertently for propagation and spreadthe virus to other hop yards.Apple mosaic virus belongs to a genusof viruses that includes some pollen- and/or seed-transmitted viruses, but these routesof spread do not appear to be significantfor Apple mosaic virus. The rate of spreadis dependent on hop variety, climaticconditions, and farm management practices.No known insect or mite vectors transmitApple mosaic virus. Apple mosaic virus has ahost range that bridges several major plantgroups that include apple, pear, and rosebut there is no evidence to suggest that thevirus is naturally transmitted from one hostspecies to another.ManagementSelection and propagation of plantingmaterials free of Apple mosaic virus areessential for disease management. The use ofcontact herbicides rather than mechanicalpruning to control basal growth may reducemechanical transmission of Apple mosaicvirus to adjacent plants.At-A-GlanceApple mosaicvirus◆◆Use onlycertified virusfreeplantingstock whenestablishing newyards.◆◆Use contactherbicides ratherthan mechanicalpruning tocontrol basalgrowth to reducemechanicaltransmission ofApple mosaicvirus to adjacentplants.29
30At-A-GlanceHop stuntviroid◆◆Use onlycertified viroidfreeplanting stockwhen establishingnew yards.◆◆If a smallnumber of plantsare infected,promptly removeto minimizespread.◆◆Thoroughly killall volunteer plantswhen replantinghop yards.◆◆Use contactherbicides ratherthan mechanicalpruning to controlbasal growth toreduce mechanicaltransmission toadjacent plants.◆◆Thoroughlywash farmequipment toremove plantresidue and sap.◆◆Disinfectingknives andcutting tools withan appropriatedisinfectantsolution for10 minutesmay reducetransmission.Hop stunt viroidKenneth C. EastwellHop stunt viroid is a sub-viralpathogen that causes a serious disease ofcultivated hop. It spread throughout Japanin the 1950s and 1960s. Presence of theviroid in North American-grown hopswas confirmed in 2004. The disease hasnot been widely reported in hop growingregions of the world other than Japanand North America. Hop stunt viroid canreduce alpha acid yield by as much as 60%to 80% per acre.SymptomsThe severity of symptoms causedby Hop stunt viroid is dependent on thehop variety and the weather. Visiblesymptoms of infection may take threeto five growing seasons to appear afterinitial infection of mature plants. Thislong latent period before the appearanceof discernible symptoms frequently leadsto the propagation and distribution ofinfected root pieces. Early-season growthof infected bines is delayed and foliage isgenerally pale relative to healthy bines (Fig.48). During active growth, the length of theinternodes of infected bines is reduced byas much as two-thirds compared to healthybines (Fig. 49). The degree of stunting istemperature-dependent, with more severestunting occurring in warmer growingregions or seasons. As bines mature, thedevelopment of lateral branches is inhibited(Fig. 49). The cones borne on the sparseand shortened lateral branches are smallerand development is delayed compared tocones on healthy plants. The developmentof yellow-green foliage continues to appearat the base of infected bines throughout theseason. The response of different varietiesto infection is not well known but on somesensitive varieties yellow speckling appearsalong the major leaf veins (Fig. 50). Thismay be the result of a mixed infection ofHop stunt viroid and a carlavirus.Disease CycleThe only known mechanisms oftransmission are through propagationof infected plants and mechanicaltransmission. There is no evidence that Hopstunt viroid is transmitted through hop seedsor via an insect or mite vector. Hop stuntFigure 48. Pale green and yellow leaveson Willamette associated with Hop stuntviroid. (K. C. Eastwell)viroid has a greater tendency to move alongrows rather than across rows, suggesting thattransmission by bines rubbing together on awire is inefficient. Observation suggests thatagricultural operations are the primary modeof viroid transmission once an infection hasbecome established in a planting. Hop stuntviroid is readily transmitted mechanicallyby workers, cutting tools, and equipmentduring cultural activities such as pruning,thinning, and mechanical leaf stripping.Mechanical transmission is most likelyto occur when sap pressure is high andabundant contaminated sap is forced fromcut or wounded surfaces, contaminatingwound sites on other plants. Hop stuntviroid can remain infectious in dry plantdebris in the field for three months, but it isunknown if this contributes significantly totransmission of the viroid in the field.ManagementSince propagation is the majorroute of Hop stunt viroid spread, the useof planting material certified free of thispathogen is the best means of limiting itsdistribution. Hop stunt viroid spreads bymechanical means and presumably also byroot grafting. If a small number of plantsare infected, they should be removedpromptly, with care to remove as muchroot tissue as possible. Because of the latentperiod, removal of only symptomatic plants
may allow nearby infected plants to remainin the hop yard. Several plants adjacentto symptomatic plants should also beremoved. If possible, plants to be removedshould be treated in late summer with asystemic herbicide, such as glyphosate, tokill roots. Sites should be allowed to layfallow for one season so that remainingliving roots will produce shoots that canbe treated with herbicide. Soil fumigationmay also be helpful in killing infected rootpieces that remain after roguing if largerareas are affected.Precautions should be employed tolimit spread within a hop yard and betweenyards. The use of contact herbicide forspring pruning is preferable to the use ofmechanical mowers that may transmit theviroid. Similarly, removing basal vegetationlater in the season by chemical rather thanmechanical means also reduces the riskof transmission. Thorough washing offarm equipment to remove plant residueand sap may help reduce the likelihoodof transmission to new fields. Treatingknives and cutting tools with a disinfectantsolution for 10 minutes may reducetransmission. Many products includingbleach (sodium hypochlorite), calciumhypochlorite, and hydrogen peroxide havebeen suggested but results are inconsistent.31ABOVE RIGHT: Figure 49. Reducedgrowth and sidearm development ofWillamette due to Hop stunt viroid.(D. H. Gent)AT RIGHT: Figure 50. Prominentyellow speckling along and betweenleaf veins associated with infection byHop stunt viroid. (D. H. Gent)
32Other Viruses, Viroids, and Virus-like AgentsKenneth C. Eastwell and Dez J. BarbaraAt-A-GlanceOther Viruses,Viroids, andVirus-likeAgents◆◆These virusesand viroids donot merit controlat this time, butgrowers shouldbe aware ofsymptoms.◆◆Use of virusandviroid-freeplanting stockis a first line ofdefense.◆◆Some of theseviruses areproblematic inEurope and/orother countries,but are notcurrently an issuein the U.S.Several virus and viroids are knownto occur in hops that are not addressedby current management practices in thewestern United States. However, growersshould continue to be vigilant for theappearance of symptoms that may indicatethe presence of one of these agents.Hop latent viroidThe group of sub-viral hop pathogensthat contains Hop stunt viroid also includesHop latent viroid. The presence of Hoplatent viroid has been confirmed in mosthop-producing regions of the worldincluding the United States; wherever it isknown to occur, it is widely distributed.Hop latent viroid has a very limited naturalhost range so the primary source of newinfections is the use of infected propagationmaterial or mechanical transmission fromother hop plants. Infection by Hop latentviroid does not cause overt symptoms onmost varieties, but it can reduce alphaacid production up to 20% in the limitednumber of symptomless varieties that havebeen studied. The variety Omega is sensitiveto Hop latent viroid (Fig. 51) and infectedplants of this variety express obvioussymptoms including general chlorosis, poorgrowth, and retarded development of lateralbranches. Total alpha acid production ininfected Omega plants can be reduced by50 to 60%. The epidemiology of Hop latentviroid is still not totally clear but controlmeasures adopted elsewhere have centeredon producing viroid-free hops and plantingaway from sources of infection such as olderplantings.Apple fruit crinkle viroidAnother sub-virus pathogen, Applefruit crinkle viroid (AFCVd) was firstreported to occur in hops in Japan in 2004.This viroid is not known to occur in NorthAmerica in either its hop or fruit tree hosts.Very little additional information is availableabout this viroid in hops. Symptoms arereported to be very similar to those inducedby Hop stunt viroid and appropriate controlmeasures are similar (see Hop stunt viroid,preceding two pages).Figure 51. Yellowing of leaves and weakgrowth of Omega variety caused by Hoplatent viroid. The pathogen is widespreadin hop yards in the U.S. but symptomsare rarely produced on varieties currentlygrown in the U.S. (D. Barbara)Arabis mosaic virusReports of the hop strain of Arabismosaic virus appeared in early literature ofthe U.S. hop industry. However, recentattempts to identify infected plants failedto detect the presence of this virus incontemporary hop production in the UnitedStates. Arabis mosaic virus is transmitted bya nematode, Xiphinema diversicaudatum,which has a very limited distribution in theUnited States. The absence of the nematodevector and the adoption of new varietiesbred in the United States have contributedto the apparent elimination of Arabismosaic virus from current U.S. productionareas. In the United Kingdom, where thenematode vector is indigenous, infectionby Arabis mosaic virus is reported to reduceyield by 23% to 50%. Arabis mosaic virusis also transmitted by introducing sap frominfected plants into mechanical wounds,but this is thought to be an insignificantroute of virus spread. Plants infected withArabis mosaic virus can display a diversity ofsymptoms depending on variety, weather,
and season. Early-season symptomsinclude short, erect shoots with shortenedinternodes that fail to climb or cling tostrings (Fig. 52). The sparse appearanceof bines early in the growing season(“bare-bine disease”) is the most commonsymptom (Fig. 53). Leaves may roll upwardand develop outgrowths occasionally onthe underside. With the onset of warmweather, symptoms are absent on newlyformed shoots. In other cases, infectedplants develop “nettlehead disease,” or severedistortion of leaves with deep divisionsbetween lobes and short internodesleading to a rosette appearance (Fig. 54).Development and maturation of cones issignificantly delayed on affected bines. Thelimited presence of the vector for Arabismosaic virus in North America suggestsadequate control can be achieved by the useof virus-free plants for propagation.Strawberry latentringspot virusStrawberry latent ringspot virus infectshop plants in Eastern Europe, but no clearsymptoms have been described. This virusis related to Arabis mosaic virus and istransmitted by the same nematode vectorthat has a very limited distribution in NorthAmerica. The impact on hop production isunknown.Tobacco necrosis virusTobacco necrosis virus is transmittedby the soil-borne fungus Olipidiumbrassicae, which infects a wide range ofplant species. Sporadic infection of hop hasbeen reported in Europe, but no specificsymptoms or reduction in yields have beenascribed to this virus. Tobacco necrosis virusis occasionally associated with field cropsnear major hop production areas in NorthAmerica but infection of hop has not beenconfirmed on this continent.Humulus japonicuslatent virusHumulus japonicus latent virus wasfirst isolated from Humulus japonicus(Japanese hop) seedlings grown fromseed imported into the United Kingdomfrom China. The infected plants weredestroyed and the virus was not detected bysubsequent testing conducted in the U.K.or by limited testing in North America. Thisvirus seems to have been common in bothwild H. japonicus and commercial hops inChina but is little studied and its currentstatus is unknown. No symptoms havebeen described on current commercial hopplants experimentally inoculated with thisvirus, and the virus did not move beyondthe inoculated leaves. In China, the viruswas widely spread within plants that werenaturally infected. Symptomless infectionof commercial hop plants is of concernbecause production losses from this virusare unknown. No control measures arerequired at this time beyond enforcementof quarantine measures to prevent theintroduction of foreign plant material.Other Viruses and aPhytoplasma of MinorImportanceSeveral different viruses have beenassociated with mottling and chloroticrings on infected hop plants. Alfalfa mosaicvirus and Cucumber mosaic virus have widehost ranges and are transmitted by severalaphid species, mechanical inoculation,and seed. These viruses occur frequentlyin field crops grown in North America,but confirmed reports of infection of hopplants are absent. Most reports of diseasecaused by these viruses have originated inEastern Europe. The impact of infectionbeyond the appearance of foliar symptomsis unknown. In addition to producing leafchlorosis and mottling, Petunia asteroidmosaic virus induces leaves that are deformedand rugose (i.e., rough, wrinkled). There areno known natural vectors for Petunia asteroidmosaic virus. It is likely transmitted throughmechanical means although details of themechanism of natural spread remain unclear.In 2004, a phytoplasma was reportedto naturally infect hops in Poland; someof the infected hop plants exhibited severeshoot proliferation accompanied by severedwarfing. Further characterization of DNAsequences obtained from the infected plantsindicated that the phytoplasma is similarto Aster yellows phytoplasma (CandidatusPhytoplasma asteris). Aster yellows andrelated phytoplasmas are frequently detectedin hop production regions of NorthAmerica but no natural infections of hopplants have been reported on this continent.33Figure 52. Stunted shootsand leaf curling caused byArabis mosaic virus. (A. Eppler,Justus-Liebig Universität,Bugwood.org)Figure 53. Severe stuntingof plants caused by Arabismosaic virus. (A. Eppler,Justus-Liebig Universität,Bugwood.org)Figure 54. “Nettlehead” diseasecaused by Arabis mosaic virusresulting in severe distortion.(A. Eppler, Justus-LiebigUniversität, Bugwood.org)
34 NematodesAt-A-GlanceHop CystNematode◆◆In most casesthe effect of hopcyst nematodesis not sufficient towarrant controlmeasures.◆◆Nematicideis unlikely tobe economic oreffective.Hop Cyst NematodeFrank S. HaySeveral species of nematodes (a.k.a.“eelworms”) feed on hop roots but aregenerally considered of minor importanceto hop production. The perennial natureof hop, the size of its root system, and itsrapid growth rate during spring suggest thathop plants have a great capacity to toleratenematode feeding. The most commonspecies associated with hop is the hop cystnematode, Heterodera humuli.SymptomsThe symptoms of nematodefeeding injury on hop have not been welldocumented. Symptoms are likely to besimilar to water stress and/or nutritionaldeficiencies, and could include a generalreduction in growth. Where such symptomscannot be attributed to other factors, thennematodes might need to be considered as apossible cause.Hop cyst nematodes are visible inspring; the cream-colored, pear-shapedfemales are approximately 1/50-inch longand they appear on the roots of hop plants.As they mature, the females harden anddarken to form egg-containing cysts. Cystscan be found attached to the root surface orin the soil.Disease CycleHop cyst nematodes survive as eggswithin cysts. Eggs hatch into microscopic,worm-like juveniles as hop plants emergefrom dormancy in spring. Juvenilespenetrate the root and form a feedingsite. Females mature on the surface of theroot. Up to several hundred eggs are laidinternally within the female, which darkens,hardens, and dies, forming a protective cystaround the egg mass. Hop cyst nematodeundergoes one to two generations peryear. Hop cyst nematode is also known tointeract synergistically with the soil-bornefungus Verticillium albo-atrum (Verticilliumwilt) to reduce hop growth and increase theseverity of wilt symptoms.ManagementIn most cases the effect of hop cystnematode is not sufficient to warrant controlmeasures. One study in Australia suggestedsome 38% loss in yield between plants withhigh numbers (5040 per 200 ml soil) andthose with lower numbers (924 per 200ml soil) in spring. Despite this, control ofnematodes with nematicide is unlikely to beeconomic or effective due to the perennialnature of the hop, rapid multiplication rateof H. humuli, and difficulty of applying aneffective dose of a nematicide to the depthsthat hop roots and nematodes can penetrate.At present little is known about thedifferences in the resistance to or toleranceof hop varieties to nematodes.
Abiotic DiseasesHeptachlor WiltMark E. Nelson and David H. GentHeptachlor is an insecticide thatwas used on several crops in the PacificNorthwest, including potato, strawberry,and sugar beet. It was used extensively in1955 and 1956 for control of strawberryroot weevil on hop and this led to severedie-out in treated hop yards. Heptachlorwas removed from the U.S market in 1972,but residues of the pesticide are extremelypersistent and still can cause injury to hopplants planted in soil with levels belowcurrent detection thresholds (i.e., 1 to 10ng/g soil). Fields treated with chlordanecan also lead to wilting since this closelyrelated pesticide also contained heptachlor.Chlordane was banned in 1983.SymptomsYoung hop plants initially grownormally, but often cannot establish a rootsystem and wilt and die during the summeror following season. Affected plants havea rough and corky bark that cracks andbleeds sap (Fig. 55). The bases of bines mayswell and become brittle, causing them toFigure 55. Rough and corky bark on a stem ofa plant with heptachlor wilt. (M. E. Nelson)Figure 56. Wilting of young hop plantsdue to heptachlor wilt. (D. H. Gent)break off from the crown. Leaves becomeyellow and die as bines begin to wilt (Fig.56). Stems of affected plants develop acharacteristic brown spotting that developsinto a rot. Eventually entire crowns mayrot, leading to plant death. The patternof affected plants is influenced by whereheptachlor was applied in the past, andoften there is a distinct boundary betweenhealthy and affected plants. Heptachlorresidues also may increase the susceptibilityof hop plants to Verticillium wilt.ManagementEconomic production of hop often isimpossible in fields that were treated withheptachlor. Varieties vary in their sensitivityto heptachlor, although specific informationon variety sensitivity is limited. Willametteis sensitive to heptachlor, while Late Clusterand some super alpha varieties appear lesssensitive. Hops should not be planted tofields with a history of heptachlor wilt.Although soil tests can be used todetect heptachlor residues, some varietiesare susceptible to heptachlor damageat levels below current detection limits.Therefore, a negative soil test may not be areliable indicator of the risk of heptachlorwilt. In suspect fields, plants of the desiredvariety should be planted and observed forheptachlor wilt symptoms for at least oneyear before planting the entire yard.At-A-GlanceHeptachlorWilt◆◆Do not establishhop yards whereheptachlor hasbeen applied inthe past.◆◆Avoid plantinghighly susceptiblevarieties suchas Willamette infields that maycontain heptachlorresidues.◆◆Soil tests forheptachlor areavailable, butsome varietiesare susceptibleto heptachlordamage at levelsbelow currentdetection limits.◆◆A negative soiltest may not be areliable indicatorof the risk ofheptachlor wilt.17 35
36 Arthropod and Slug Pest ManagementAt-A-GlanceCaliforniaPrionusBeetle◆◆Identify,remove anddestroy crownsof infestedplants.◆◆Fumigate orfallow fieldstwo to threeyears beforereplanting.◆◆Treatpost-harvestwith labeledsoil-appliedinsecticides.California Prionus BeetleJim D. BarbourPest Descriptionand Crop DamageAdult California prionus (Prionuscalifornicus) are large red-brown to blackbeetles 1 to 2 inches in length with longantennae characteristic of the longhornedbeetle family, to which this beetle belongs(Fig. 57). The larvae are cream-colored,legless, from 1/8 to 3 inches long (Fig.58), and have strong, dark mandibles thatare used to chew plant roots. Californiaprionus larvae do not curl into a c-shapewhen disturbed as do the larvae (grubs) ofother soil-inhabiting beetles such black vineweevils and June beetles. Adults do not feed,but larva feed on plant roots, resulting indecreased nutrient uptake, water stress, andreduced plant growth. Severe infestationscan completely destroy crowns and killplants (Fig. 59). Less severe infestationscause wilting, yellowing, and death of oneor more bines (Fig. 60). Feeding damage islikely to be associated with the occurrence ofsecondary pathogens that can rot crowns.Biology and Life HistoryAdults in the Pacific NorthwesternUnited States emerge from pupation sitesin the soil in late June and early July. Adultsare active at night and not frequentlyencountered during the day. Males locatefemales for mating using a pheromonereleased by females. Eggs are laid on or inthe soil near the base of plants. A singlefemale can lay 150 to 200 eggs in her twotothree-week lifetime. Larvae hatching fromeggs move to plant roots, where they feedfor three to five years. Mature larvae pupateduring the early spring in cells constructedfrom soil and lined with root material.Monitoring and ThresholdsLarvae can be quantified only bydestructively sampling the crown and rootsof plants suspected of being infested. Adultsfly to light traps, but light trapping isexpensive. Light traps capture largely malesand there is no information indicating thatcapture of adults at light traps is correlatedto the severity of infestation of hop crownsand roots. Economic thresholds basedon economic injury levels have not beenestablished.Figure 57. Adult California prionus beetles (left, female; right, male). Adultbeetles are 1 to 2 inches long with prominent antennae. (D. H. Gent)
ManagementManagement of California prionusconsists of identifying, removing, anddestroying (e.g., burning) roots andcrowns of infested hop plants. It may benecessary to dig up and remove all plantsin severely infested fields. If all plants havebeen removed and destroyed, the fieldcan be fumigated and replanted to hop,or planted to a non-host crop for two tothree years to further reduce Californiaprionus populations prior to replanting.The potential for use of the volatile matingpheromone produced by females formanaging California prionus via matingdisruption or adult trapping techniquesis currently being investigated. Ethoprop(Mocap EC) is labeled for control ofCalifornia prionus in hop. The long preharvestinterval of this pesticide (90 days)combined with summer emergence of adultsmay limit use of ethoprop for Californiaprionus management to post-harvestapplications. See the Pacific NorthwestInsect Management Handbook at http://pnwpest.org/pnw/insects for a current list ofregistered insecticides.17 37PHOTOS AT RIGHT,FROM TOP:Figure 58. Cream-colored,legless larvae of the Californiaprionus beetle. Larvae range insize from 1/8 to 3 inches long.(D. H. Gent)Figure 59. California prionuslarva feeding in a hop crown.Severe infestations can destroycrowns and kill hop plants.(Courtesy J. D. Barbour.Reproduced with permission fromCompendium of Hop Diseasesand Pests, 2009, W. Mahaffee,S. Pethybridge, and D. H. Gent,eds., American PhytopathologicalSociety, St. Paul, MN)Figure 60. Wilting, yellowing,and death of bines causedby California prionus feedingdamage. (J. D. Barbour)
38 Hop AphidAmy J. DrevesAt-A-GlanceHop Aphid◆◆Beginmonitoring in Maywhen daytimetemperaturesexceed 58 °F.◆◆Avoid excessiveapplication ofnitrogen.◆◆Intervene earlyto prevent aphidestablishment inhop cones.◆◆Rotate chemicalclasses to avoidresistance.◆◆Use selectivepesticides thatpreserve naturalenemies.Pest Descriptionand Crop DamageHop aphids (Phorodon humuli) aresmall (1/20 to 1/10 inch long), pear-shaped,soft-bodied insects that occur in wingedand wingless forms on hop. Wingless formsare pale white (nymphs) to yellowishgreen(adults) and found mostly on theunderside of hop leaves (Fig. 61). Wingedforms are darker green to brown with blackmarkings on the head and abdomen (Fig.62). Both forms have long slender antennaand two “tailpipes” (cornicles) at the endof the abdomen. Adults and nymphs havepiercing-sucking mouthparts that they useto remove water and nutrients from thevascular tissue of hop leaves and cones. Leaffeeding can cause leaves to curl and wiltand, when populations are large, defoliationcan occur. Most economic damage occurswhen aphids feed on developing cones,causing cones to turn limp and brown.Hop aphids also secrete large amounts ofsugary honeydew that supports the growthof sooty mold fungi on leaves and cones(see Sooty Mold in Disease Managementsection). Sooty mold on leaves reducesplant productivity and severe infestationsrender cones unmarketable. Hop aphidsalso can transmit plant viruses includingHop mosaic virus and American hop latentvirus that can reduce yield, both of whichare discussed under the Virus and ViroidDiseases subsection of this volume’s DiseaseManagement section.Biology and Life HistoryHop aphids overwinter as eggs onornamental and agricultural species of thegenus Prunus, including plum, cherry plum,sloe, and damson (Fig. 63). Eggs hatch inearly spring and one or two generations ofwingless aphids are produced asexually onthe overwintering host before winged aphidsare produced that migrate to developing hopplants in early May. After arriving on hop,wingless asexual females are produced. Eachfemale can give birth to 30 to 50 nymphs inits two- to four-week lifetime and more than10 overlapping generations occur duringa season. In late August, winged adultfemales are produced that migrate back tothe winter host and produce wingless sexualfemales. Winged males are produced onhop plants approximately two weeks afterwinged females are produced, and disperseto an overwintering host and mate withthe females. Eggs are laid near buds on thewinter host.Monitoring and ThresholdsYellow pan traps and suction traps(Figs. 64 and 65) are useful for monitoringthe start of spring aphid flight from winterhosts into hop yards. Monitoring shouldbegin when daytime minimum temperaturesexceed 58 to 60° F. A comprehensiveeconomic threshold does not exist for hopaphid. Most growers apply a pesticide whenan average five to 10 aphids per leaf areobserved before flowering. Generally, aphidsare not tolerated after flowering; controlwith pesticides is difficult once aphids infestcones.Figure 61. Wingless hop aphid nymphs (pale white) and adults (yellowishgreen)on the underside of an infested leaf. (D. G. James)ManagementGrowers should apply sufficient butnot excessive nitrogen, as large flushes ofnew growth favor outbreaks of hop aphids.Many aphid predators and parasitoids (e.g.,lady beetles, lacewings, predatory bugs, flylarvae, and parasitic wasps: see Beneficial
39Arthropods section) occur in hop yards.Since these natural enemies often do notestablish until after aphids arrive on hopplants and begin reproducing, however,they frequently are unable to regulate hopaphid below levels that growers will tolerate,particularly after flowering. Attractants(e.g., methyl salicylate) are available that canincrease populations of natural enemies inhop yards. Methyl salicylate has also beenshown to repel hop aphids.Unless climatic conditionsare unfavorable to reproduction anddevelopment (e.g., hot dry weather), hopaphid numbers often exceed the regulatingcapacity of their natural enemies andpesticides must be applied to limit earlyseasonpopulation growth. A number ofinsecticides are available for control of hopaphid. It is important to rotate aphicideclasses to avoid resistance. When possible,growers should use selective pesticidessuch as pymetrozine (Fulfill) that controlaphid populations while preserving naturalenemies of aphids and other hop pests. ASuperior-type oil applied to winter hostsduring the dormant or delayed-dormantperiod may reduce the number of springmigrants into hop yards. See the PacificNorthwest Insect Control Handbookat http://pnwpest.org/pnw/insects for acurrent list of registered insecticides.Figure 62. Winged form of the hop aphid. Notice the dark green to browncolor and black markings on the head and abdomen. (L. C. Wright)Figure 63. Wingless hop aphids on an overwintering Prunus sp. (L. C. Wright)Figure 64. Yellow pan trap for hopaphid. (J. D. Barbour)Figure 65. Suction trap for hop aphid.(J. D. Barbour)
40 Garden SymphylanAmy J. DrevesAt-A-GlanceGardenSymphylan◆◆Monitor fieldsfor symphylansprior to plantingor during plantestablishment.◆◆Cultivate ifnecessary tokill symphylansand disrupt theirmovement.◆◆Treat withsoil-appliedinsecticides inearly spring(preferred) or fall.Figure 66. The centipede-likegarden symphylan. Adultsare 1/8 to 1/4 inch long.(Ken Gray Image Collection,Oregon State University)Pest Descriptionand Crop DamageGarden symphylans (Scutigerellaimmaculata) are small (1/8 to ¼ inch long),white, centipede-like animals; their longantennae have a “beaded” appearance (Fig.66). Adults have 12 pairs of legs. Newlyhatched nymphs resemble adults but havesix pairs of legs with a new pair added ateach of six subsequent molts. The eggs arepearly white, spherical with ridges, andfound in clusters in the soil.Garden symphylans feed belowground on fine roots and aboveground ongrowing plant parts in contact with soil.Root feeding can reduce vigor (Fig. 67),stunt plants, cause poor plant establishmentin newly planted yards (Fig. 68), andcontribute to the decline of establishedplantings. Root damage also may increaseplant susceptibility to soil-borne pathogens.Garden symphylans are pests of hop inthe cool, moist growing regions of westernOregon, and are not known to causedamage to hop in Washington or Idaho.Biology and Life HistoryThe garden symphylan spends itsentire life in the soil or in plant materialand debris in contact with the soil surface.Nymphs and adults become active in thespring and can be found aggregating in theupper surface of soil during moist, warmweather. They move deeper in soil as itbecomes dry and cool. Eggs hatch in 12 to40 days, depending on temperature. It takesapproximately three months to completedevelopment from egg to adult. Eggs,immature nymphs, and adults can be foundtogether throughout the year. One to twogenerations occur per year.Monitoring and ThresholdsGarden symphylans often occur inpatches in hop yards and can be monitoredby one of several methods. The simplestmethod is to scout hop yards for gardensymphylan damage during warm, moistconditions. Field personnel can search thesoil surface and plant parts in contact withthe soil for garden symphylans. They canalso bait for symphylans prior to plantingby placing a cut, moistened potato halfface-down on the soil surface of a hophill. The potato should be covered with aprotective material (e.g., tarp segment), thenchecked two to three days later for presenceof symphylans. Alternatively, soil samplescan be taken during early spring or fall todetermine the presence of symphylans belowthe soil surface. Samples should be taken byshovel to a depth of 6 to 12 inches from 10to 20 different sites in the hop yard. The soilcan then be placed on a piece of dark plasticor cloth, broken apart, and symphylansobserved and counted. More samples shouldbe taken in larger fields. Although nothreshold based on economic injury levelhas been established, an average of five to 10symphylans per potato or soil sample oftenis considered a damaging level.ManagementEstablished plantings can toleratemoderate symphylan damage, however,management is critical in new plantings andduring plant establishment in early spring.No single management method has beenfound completely reliable. Cultivating fieldsimmediately prior to planting or duringearly spring in established fields directly killssymphylans, exposes them to desiccationand predators, and disrupts their movement.Symphylan mortality increases with theseverity and depth of cultivation, but caremust be taken to avoid cultivating too closeto and damaging perennial hop crowns.Natural predators, such as staphylinid andcucujid beetles, centipedes, and predaceousmites exist, but are not known to provideeconomic levels of control. No varieties areconsidered resistant.Insecticides often are needed tomanage symphylans. Insecticides should bebroadcast and incorporated as close to hopcrowns as possible to ensure penetrationinto the soil layer where symphylans live.Spring applications (April through lateMay) tend to be more effective than fallapplications (September to October),since symphylans live deeper in the soil inthe fall. Advance planning is necessary, asinsecticides registered for garden symphylanmanagement in hop have long pre-harvestintervals (65 to 90 days).
41ABOVE: Figure 67.Stunting, weak growth, andyellowing of leaves causedby garden symphylan feedinginjury. (W. F. Mahaffee)AT RIGHT: Figure 68.Severe stunting and plantdeath caused by gardensymphylan feeding injury in anewly established hop yard.Notice the aggregated patternof affected plants. (D. H. Gent)See the Pacific NorthwestInsect Management Handbookathttp://pnwpest.org/pnw/insectsfor a current list ofregistered insecticides.
42 Hop Looper and Bertha ArmywormJim D. BarbourAt-A-GlanceHop Looperand BerthaArmyworm◆◆Monitor plantsprior to floweringfor presence ofcaterpillars in hopfoliage.◆◆Treat topreventestablishment inthe upper plantcanopy afterflowering.◆◆Choosecompoundsselective forcaterpillar larvae(e.g., certain Btformulations)to preservenatural enemiesand reducethe numberof treatmentsrequired forcontrol.Pest Descriptionand Crop DamageThe larvae (caterpillars) of severalmoths and butterflies attack hop, however,only the hop looper (Hypera humuli)and the bertha armyworm (Mamestraconfigurata) commonly reach damaginglevels. The adults of both species areindistinctly mottled gray to gray-brownmoths approximately 1 inch long. Femalehop looper moths have a distinct W-shapeddark patch along the front edge of eachforewing (Fig. 69). This line is present butless distinct in males (Fig. 70). Both sexeshave an elongated “snout” that distinguishesthem from bertha armyworm moths,which have a large spot on each forewingand a white band near the rear edge of theforewing (Fig. 71).Figures 69 and 70. Left, female hop looper.Right, male hop looper. Notice the distinctW-shaped dark patch along the front edge ofeach forewing of the female. (D. G. James)Figure 71. Adult bertha armyworm. Notice thelarge spot on each forewing and the white bandnear the rear edge of the forewing. (Ken GrayImage Collection, Oregon State University)Hop looper larvae are pale greenwith two narrow white lines on each sideof the back and one on each side (Fig.72). They have four pairs of prolegs: oneeach on abdominal segments 4 to 6, andone on the last abdominal segment. Theymove with a characteristic looping motionand are active largely at night. Larvae restduring the day on the undersides of leaves,often lying along the veins or petiole (leafFigure 72. Hop looper larva. Notice the palegreen color and two narrow white lines on eachside of the back and on each side. (D. G. James)Figure 73. Larva of the bertha armyworm.Note the dark back and yellow to orangestripe on each side. (D. G. James)stem), making them difficult to see. Whendisturbed, younger instars drop to theground on a silken thread, while largerlarvae may thrash violently from side toside. Bertha armyworms are dark-backedcaterpillars with a yellow to orange stripeon each side and a tan to light brown head(Fig. 73) that lacks the “Y” marking presenton the head of other armyworm larvae.The first-instar larvae can be distinguishedfrom hop looper larvae by their black head,their occurrence in groups on leaves, and byhaving five rather than four pairs of prolegs:four on abdominal segments 3 to 6, plusone on the terminal segment.When present in large numbers, hoplooper larvae can defoliate hop plants, givingthem a characteristic lacey appearance (Fig.74). Although eggs are distributed equallyacross the surface of the plant, leaf feedingoften is more severe near the base of theplant. Later in the season, larvae feeding onhop cones can cause severe losses. Berthaarmyworm larvae also defoliate hop plants,but yield loss is caused when caterpillarschewing on the stems cause cones to fall onthe ground.
Biology and Life HistoryHop loopers overwinter as adults inprotected areas such as cracks and crevicesin tree trunks and fallen logs, sometimesat considerable distances from hop yards.The adults fly back to hop yards in spring(April) and begin laying slightly flattened,circular eggs (Fig. 75), usually on theunderside of hop leaves. Few other plantsserve as hosts for hop loopers. Eggs areapproximately 1/50 inch in diameter and,although several eggs may be laid on a leaf,all are laid singly, not in masses. Eggs hatchin approximately three days and the larvaefeed for two to three weeks, developingthrough five or six instars before pupating(Fig. 76). Adults emerge in 10 to 12 days.Three generations occur per year; however,after the first generation all life stages can bepresent in the field at the same time, makingit difficult to determine the best time forpesticide treatments.Bertha armyworms overwinter aspupae in the soil. Moths emerge in lateJune through July and lay eggs in massesof 50 to more than 100 eggs (Fig. 77) ona wide variety of host plants in addition tohop. Eggs hatch in three to five days andlarvae grow through six instars in five to sixweeks before pupating in the soil. Larvaeoften move from weed hosts to hop plantsas weeds are consumed. Two generationsper year typically occur in the PacificNorthwest.Monitoring and ThresholdsNo economic threshold has beenestablished for hop loopers or berthaarmyworms in hops. The presence of largelarvae in the upper canopy after floweringgenerally is not tolerated. The presenceof caterpillars in the hop canopy can bemonitored by placing a plastic or clothtarp along a three-foot section of hop row,grasping a bine at or just above head-height,and shaking vigorously for 10 to 15 seconds,dislodging large caterpillars to the tarpwhere they can be observed and counted.ManagementHop yards contain many predators(e.g., big-eyed bugs, damsel bugs) andparasitic wasps and flies of hop looperand bertha armyworms (see BeneficialArthropods section). Hop looper parasitismrates can reach 70%. Several pesticidesare labeled for control of hop loopers andbertha armyworms and even the largerinstars are readily controlled by theseinsecticides. Bacillus thuringiensis subsp.aizawai is effective and is highly specificto caterpillars. Use of Bt products willavoid disrupting biological control of hoploopers and bertha armyworms, as well asbiological control agents of spider mitesand hop aphid. See the Pacific NorthwestInsect Management Handbook at http://pnwpest.org/pnw/insects for a current list ofregistered insecticides.17 43AT LEFT, TOP ROW,LEFT TO RIGHT:Figure 74. Hop looper feedingresults in a characteristic laceyappearance. (D. G. James)Figure 75. A slightly flattened,circular egg of the hop looper.Notice that eggs are laid singly.(D. G. James)AT LEFT, BOTTOM ROW,LEFT TO RIGHT:Figure 76. Pupating hop looper.(D. G. James)Figure 77. Egg mass of thebertha armyworm. Eggs arelaid in groups of 50 to 100 ormore. (D. G. James)
44ABOVE: Figure 78. Adult black vine weevil with characteristicbowed antennae and mouthparts at the end of a long snout.(D. G. James)BELOW: Figure 79. Root weevil larvae are white, legless,c-shaped grubs with tan to dark-brown head capsules. Actuallength is approximately ¼ inch. (P. Greb, USDA AgriculturalResearch Service, Bugwood.org)Root WeevilsJim D. BarbourPest Description and Crop DamageRoot weevils are beetles characterized by elbowedantennae and mouthparts at the end of a long snout (Fig. 78).Several root weevil species, including the strawberry root weevil(Otiorhynchus ovatus), the rough strawberry root weevil (O.rugosotriatus), and the black vine weevil (O. sulcatus) attackhop. The black vine weevil is the largest and most common ofthese in hop. The life cycle, appearance, and damage caused bythese species are similar. Adults are oblong gray to black beetlesapproximately ½ inch long, although the strawberry root weevilis approximately ¼ inch long. The wing covers (elytra) are fusedand marked with rows of round punctures. Larvae are white,legless, c-shaped grubs with tan to dark-brown head capsules(Fig. 79).Adult weevils feed on leaves, creating rough notcheson the edges of leaves, but this feeding is not known to causeeconomic loss (Fig. 80). Economic losses can result from larvaefeeding on the roots of hop plants (Fig. 81). Root damage bylarvae reduces nutrient uptake and plant growth and increaseswater stress. The most severe damage results from late-instarlarvae feeding on roots prior to pupating in the spring.Premature leaf drop and plant death have been associated withsevere feeding damage caused by black vine weevil larvae. Heavyinfestations may require that individual plants or even wholehop yards be removed from production.
Biology and Life HistoryAdult root weevils begin feeding onleaves within 24 hours after emerging fromoverwintering sites beginning in late April.All adult weevils are females; males are notknown to occur. They cannot fly and areactive largely at night. Females must feed for25 to 30 days before they can begin layingeggs. Eggs are deposited on the soil surface,in soil crevices, and on leaves near the baseof plants. Egg-laying continues through lateSeptember and early October, with eachfemale laying an average of 300 eggs. Larvaeemerge from eggs in approximately 21 days,move through soil, and begin feeding onplant roots. Most root weevils overwinter aslate-instar larvae that pupate in the spring,but overwintering as adults can occur.Monitoring and ThresholdsPopulations of adult weevils can bemonitored with the use of grooved boardsand pitfall traps to determine when adultsare active in the spring. Scouting for leafnotching caused by adult feeding is alsouseful. Economic thresholds have not beenestablished for root weevils in hop.ManagementBiological control of root weevil inhop has been achieved using heterorhabditidand steinernematid nematodes. Nematodeapplications should be timed to coincidewith the presence of late-instar larvae, soiltemperatures above 50 °F, and adequate soilmoisture. Nematodes and foliar insecticidesare best applied in late summer or fallto reduce the abundance of large larvaefeeding on hop roots in the spring. Foliarinsecticides should be applied approximatelythree weeks after adult emergence butbefore egg-laying begins. They are moreeffective applied at night when adult weevilsare most active. See the Pacific NorthwestInsect Management Handbook at http://pnwpest.org/pnw/insects for a current list ofregistered insecticides.BELOW LEFT: Figure 80. Notched edge ofa leaf caused by adult weevil feeding. Thisfeeding injury is not known to cause economicloss. (Ken Gray Image Collection, OregonState University)BELOW: Figure 81. Root weevil larvae andassociated feeding injury on a root. (C. Baird)At-A-GlanceRoot Weevils◆◆Monitor forvine weevil adultsbeginning inApril.◆◆Treat foradults with foliarinsecticidesapproximatelythree weeksafter adults aredetected in hopyards.◆◆Treat for lateinstarlarvae inthe late summeror fall usingsoil-appliedinsecticides.◆◆Biologicalcontrol ofroot weevil inhop can beenachieved usingheterorhabditidandsteinernematidnematodes.17 45
46 Twospotted Spider MiteJim D. BarbourFigure 82. Adult female spidermite with prominent black spotson each side of the abdomen.Adults are approximately 1/50inch long. (D. G. James)Figure 83. Adult male spidermite. Males are approximately3/4 the size of females andhave a more pointed abdomen.(D. H. Gent)Pest Descriptionand Crop DamageTwospotted spider mites (Tetranychusurticae) are closely related to spiders andticks and get their name from their spiderlikeability to spin webs. Adult females aresmall, oval, yellow to yellow-green animals,approximately 1/50 inch long, with a largeblack spot on each side of the abdomen(Fig. 82). Newly hatched spider mites(larvae) have three pairs of legs, whereas allother life stages (nymphs, adults) have four.Overwintering females turn orange-red inthe fall and lose the paired black spots. Asthey begin feeding in the spring, femalesturn green and regain the spots. Adult malesare approximately 3/4 the size of femalesand have a more pointed abdomen (Fig.83). Spider mite eggs are clear to pearlywhitespheres approximately 1/200 inch indiameter (Fig. 84).Spider mites damage hop plants byfeeding on leaves and cones, sucking plantjuices from the cells. Leaf feeding causesbronzing of leaves and reduces plant vigor(Figs. 85 and 86). Severe infestation cancause defoliation and is accompanied byheavy production of webbing (Fig. 87).Most economic damage is caused by spidermites feeding on cones, which results indry, brittle, discolored (red) cones (Figs.88 and 89) that tend to shatter, reducingboth quality and quantity of yield. Spidermites in hop cones are also consideredcontaminants that lower cone quality. Wheninfestations are severe, brewer rejection ortotal crop loss can occur.Biology and Life HistoryTwospotted spider mites have a widehost range, feeding and reproducing on morethan 180 plant species, and are importantpests of many field, forage, ornamental,and horticultural crops. They overwinteras dormant or diapausing females in hopcrowns, cracks and crevices in poles, andother protected areas in fields and adjacentareas. Males do not overwinter. Femalesemerge from overwintering sites in earlyspring and immediately begin feeding onyoung shoots beneath bracts. Egg-laying canbegin as early as two days after emergence.Eggs hatch in two to five days with femalesproduced from fertilized eggs and males fromunfertilized eggs. The sex of immature stages,however, cannot be accurately determined.The larvae develop through two additionalmolts, the second instar (protonymph)and the third instar (deutonymph), beforebecoming adult mites. Developmentfrom egg to adult takes one to three weeksdepending on temperature. As many as fiveto eight overlapping mite generations perseason may occur on hop. Except whenpopulations are high, eggs and motile stagesare usually found on the undersides of leaves.Orange, diapausing females appear in lateAugust and September in response to shorterdays and cooling temperatures, at whichtime mites begin moving down plants tooverwintering sites.Figure 84. Spider mite adult,nymphs, and eggs. The eggsare clear to pearly-whitespheres approximately 1/200inches in diameter.(S. Broughton, Department ofAgriculture & Food WesternAustralia, Bugwood.org)Figures 85 (ABOVE) and 86 (AT RIGHT).Bronzing of leaves and defoliation caused byspider mite feeding. (D. G. James)
Monitoring and ThresholdsSamples should be taken weeklybeginning in mid- to late May by removingleaves and examining the undersides forthe presence of spider mites, mite eggs, andwebbing, as well as stippling and yellowingof leaves associated with spider mite feeding.Leaves can be taken at the three- to sixfootlevel early in the season, however, afterapproximately mid-June, higher leaves nearthe trellis wires should be sampled. Severalleaves from each of 10 to 30 plants shouldbe sampled depending on field size and theamount of time available. A 10X to 20Xhand lens and a pole pruner are useful mitesampling tools.A comprehensive economic thresholdbased on spider mite economic injury levelshas not been developed for hop. Mostgrowers treat when there is an average ofone to two female spider mites per leaf inJune and early July, or five to 10 mites perleaf after mid-July. However, research inthe United States and Germany indicatesthat hop plants can tolerate much highertwospotted spider mite populations withoutsuffering economic loss if cones are notinfested. Low to moderate numbers ofmites on hop foliage may be tolerated ifthe weather is mild and sufficient biologicalcontrol agents are present. However,spider mite populations can build rapidly,especially in hot, dry conditions, thereforemonitoring is important.Figure 87. Spider mite webbing is associatedwith severe infestations. (D. G. James)Figure 88 (ABOVE).Close-up of dry,brittle, and reddiscolored conesresulting from spidermite feeding. (D. H.Gent)Figure 89 (AT LEFT).Hop yard exhibitsdry, brittle, and reddiscolored conesresulting from spidermite feeding. (D. R.Smith)At-A-GlanceTwospottedSpider Mite◆◆Monitor weeklybeginning in midtolate May.◆◆Provide plantswith adequatebut not excessivenitrogen fertilityand water.◆◆Reduce dust,especially in hotdry weather.◆◆Treat toprevent coneinfestations usingfoliar-appliedmiticides.◆◆Rely onselective miticidesto reduce impacton naturalenemies andthe number ofrequired miticideapplications.◆◆Avoid the useof pyrethroid,organophosphate,carbamate, andneonicotinoidinsecticides, andlate-season sulfurapplications.◆◆Rotatechemical miticideclasses to avoidresistancedevelopment.17 47
481412108642025AVERAGE MITES PER LEAFAVERAGE MITES PER LEAF201510502520151050TSSMPredatory Mites21 June 19 July 16 August 13 SeptemberHop AphidMinute Pirate BugsMite-Eating Lady Beetles3 May 7 June 12 July 2 August 6 SeptemberFigure 90 (ABOVE): Examples of successful biological control oftwospotted spider mites (TSSM) and hop aphid when natural enemies werepresent and not disrupted by non-selective pesticides or cultural practices.200160120804002520151050Oregon2007 2008Non-treatedWaterWashington2007 2008Synthetic Early Sulfur Mid Sulfur Late SulfurEarly SulfurEarly/midSulfurMid Sulfur Late Sulfur SyntheticManagementPlant stress can be reduced byproviding adequate but not excessivefertilizer and irrigation. Spider miteproblems are often exacerbated by excessivenitrogen fertility and the presence of duston plants. Covering dirt roads with gravel,straw, or crop debris, watering or oilingroads, reducing driving speed, and plantingground covers can minimize dust. The useof ground covers also can provide habitatfavorable for spider mite natural enemies.A complex of natural enemies (e.g.,predatory mites, big-eyed bugs, minutepirate bugs, lady beetles, spiders, andlacewings; see Beneficial Arthropods section)occurs in hop yards when not disturbed bynon-selective pesticides or certain culturalpractices. Preserving endemic spider mitenatural enemies and maintaining basalfoliage on plants can enhance biologicalcontrol, potentially reducing the need forchemical controls (Fig. 90). Recruitmentof predators to hop yards using volatileattractants (e.g., methyl salicylate) also mayimprove biological control of twospottedspider mite.A number of foliar-applied miticidesare available for control of twospotted spidermites in hop. See the Pacific NorthwestInsect Management Handbook at http://pnwpest.org/pnw/insects for a currentlist of registered insecticides. Several ofthese are reported to be relatively safe topredatory insects and mites (see Table 1,page 5). Using these selective miticides canenhance biological control. Non-selectivemiticides should only be used as a last resortwhen other control tactics fail. Spider mitepopulations can be exacerbated by the useof pyrethroid, organophosphate, carbamate,and neonicotinoid insecticides used tocontrol spider mites or other arthropodpests, or by multiple applications of sulfurto control hop powdery mildew. Sulfurapplications made later in the season (i.e.,in June and July) tend to exacerbate miteoutbreaks most severely (Fig. 91).Figure 91 (AT LEFT): Effect of sulfur timingon the severity of spider mite outbreaks inOregon and Washington. Sulfur was appliedthree times at 7- to 14-day intervals in eachof the plots receiving sulfur treatments.A rotation of synthetic fungicides (Flint,Accure, and Quintec) were applied to plotsreceiving the synthetic treatment.
Minor Arthropod and Slug Pests17 49Hop Flea BeetleAmy J. DrevesPest Descriptionand Crop DamageHop flea beetle (Psylliodes punctulatus)adults are small (1/12 inch long), bronze toblack metallic beetles (Fig. 92) with stronglydeveloped hind legs that allow the beetleto jump like a flea when disturbed. Theeggs are whitish-yellow, oval, less than 1/60inch in diameter, and deposited singly or ingroups of three or four near the roots of hopplants. Mature larvae are approximately 1/5inch long and off-white with a brown head.Adult beetle feeding in spring causesshothole damage on leaves on young bines(Fig. 93). Adults emerging in the fall mayfeed on young cones. Larval feeding onhop roots causes surface tracking and smalltunnels. Infestations resulting in economicdamage are uncommon and occur primarilyin Oregon.Figure 92. Adult hop flea beetles feedingon a hop leaf. Adults are approximately1/12 inch long and bronze to metallicblack in color. (F. Weihrauch)Biology and Life HistoryHop flea beetles overwinter as adultsin plant debris, in cracks in poles, underbark, and around the margins of hop yards.Adults become active March to May andbegin feeding on growing hop bines andweeds. The beetles mate and lay eggs duringMay and June with most eggs deposited inthe upper 1/4 inch to 1 inch of soil aroundhop plants. Larvae hatch in June and feedon hop roots for approximately four to fiveweeks before pupating in the soil. Adultsemerge in three to five weeks and feed onlow-growing foliage around hills beforemigrating to overwintering sites. Onegeneration occurs each year.Monitoring and ThresholdsGrowers should scout fields in earlyspring, looking for shothole damage onleaves and for the presence of jumpingbeetles. Beetles are easier to observe if theleaves are not disturbed during scouting.White or yellow sticky traps can be placed atthe bases of bines to detect spring-emergingblack beetles. No thresholds are establishedfor flea beetles on hop. Healthy, rapidlygrowing hop plants usually quickly outgrowfeeding damage. Larger plants can withstandmore feeding injury.ManagementTrap crops (crops more attractive tothe pest than hop) such as Chinese mustardor radish can be used to intercept beetles beforethey enter hop yards. Beetles should betreated in the trap crop to prevent migrationinto hops. Plowing or tilling weeds and hopresidue in the fall to destroy overwinteringsites may be beneficial. Biological controlusing commercial formulations of entomopathogenicnematodes may help to reducepopulations of overwintering beetles andconsequently reduce flea beetle damage toplant roots. Nematodes should be applied tomoist soil during the summer before mostlarvae pupate. No insecticides are labeled forcontrol of hop flea beetle in hop, but foliarorsoil-applied systemic pesticides used forcontrol of hop aphid usually provide control.See the Pacific Northwest Insect ManagementHandbook at http://pnwpest.org/pnw/insects for a current list of registeredinsecticides. Whenflea beetles migratefrom hosts outsidea hop yard, most ofthe infestation willbe localized on theborders and spottreatment of bordersmay be effective.Treat early in theseason when plantsare young and lessthan three feet tall.At-A-GlanceHop FleaBeetle◆◆Monitor hopsfor flea beetleadults and leafdamage inMay and June,especially ifalternative fleabeetle hosts arepresent nearby.◆◆Need fortreatment isunlikely.◆◆Certaininsecticidesapplied for aphidcontrol usuallycontrol fleabeetles.Figure 93. Severe feeding damage caused by hop fleabeetle resulting in a “shothole” appearance. (F. Weihrauch)
50 SlugsAmy J. DrevesAt-A-GlanceSlugs◆◆Monitor for slugpresence on hillsin early spring.◆◆Cultivatebetween rows todirectly kill slugsor expose themto weather andpredators.◆◆Damagecaused by otherpests such asflea beetles orcucumber beetlescan be mistakenfor slug damage.◆◆Slime trailsindicate thepresence of slugs.◆◆Iron phosphatebait is availablefor slugmanagement(Oregon only).◆◆Bait at plantingtime in an Oregonyard with ahistory of sluginfestation.Pest Descriptionand Crop DamageSlugs are a problem in Pacific Northwesthops primarily in Oregon. While severalspecies can be found in hop yards, the mostcommon is the gray field slug, Deroceras reticulatum(Fig. 94). These soft-bodied mollusksrange in length from ¼ inch to 2 inches andare light gray to dark brown with a networkof mottled colors. The underside of the foot iswhitish with a darker zone. The mantle (i.e.,area on top just behind the head) is roundedat both ends and generally lighter in colorthan the rest of the body. As in all slugs, thereis a respiratory pore behind the mid-pointand on the right side of the mantle. The bodyof the slug behind the keel (i.e., the foot) hasa boat-like shape running down the top to thetail. When disturbed, the watery slime trail ofthis slug turns from clear to milky white.Slugs are most active at night or earlymorning, especially when humidity is highand temperatures are cool. They retreat intocracks, soil crevices, and sheltered areas byday to protect themselves from predators anddehydration. Very little activity takes place inextremely cold or hot weather. Slugs feed onnewly developing shoot tips and leaves of hopplants, resulting in ragged leaves with irregularlysized holes. Damage tends to be heaviestalong the edges of hop yards where weedyor grassy borders serve as a habitat for slugs.When populations are high, slugs can destroythe growing tips of hop shoots.Biology and Life HistoryThe gray field slug completes one totwo generations per year. Young adults or eggsoverwinter under leaf residue, in soil cracks,and in sheltered areas under the soil surface.In the spring, mating and egg-laying usuallyfollow within one to three weeks after slugactivity is noticed. Eggs are laid in clutchesof 10 to 40, totaling 200 to 400 eggs in alifetime. The spherical eggs are laid in a gelatinousmass and are transparent when laid butbecome cloudy just before hatching. The immatureslugs resemble adults but are smaller.The average life span of a slug is nine to 13months. All slugs have both male and femalereproductive organs, so that self-fertilizationand egg-laying can occur in any individual.Monitoring and ThresholdsIn areas where slugs may be present,growers can monitor for slugs by carefullyobserving hop shoots during the pest’s criticalstage of emergence in the early spring. Openbait traps (in Oregon, where bait is registered,see below) or slug blankets/boards can beplaced on the ground near hop hills to monitorfor slugs. After several nights, the trapsshould be turned over and checked for thepresence of slugs. Treatment should be consideredif the field has a history of slug damageor if excessive damage to foliage or growingtips is observed and slugs are determinedto be present.ManagementThe most effective control of slugs canbe achieved in early spring when temperaturesbegin to warm and hop plants start togrow. The hop plant is at its greatest risk ofslug damage when plants are young. Wherebaits are registered, it is best to bait at plantingtime or just before shoots emerge inspring if a yard has a history of slug damage.Managing hop yards so that plants emergequickly in the spring can help to escape theworst period of slug damage.Increased use of irrigation and moistwarm springs favor slugs in hop yards. Soilcultivation in early spring between hopplants can kill slugs and also expose themto predators and desiccation. Birds, frogs,snakes, Sciomyzid flies, harvestmen (daddylong-leg spiders), and carabid ground beetlesprey on slugs. Parasitic nematodes and naturallyoccurring ciliates (protozoans that moveby means of small hairs or cilia) can infectthe bodies of slugs.No chemical treatments/baits forcontrol of slugs are labeled for use on hopsin Washington or Idaho; Oregon has a 24c“Special Local Needs” registration for ironphosphate (Sluggo). Iron phosphate baitsmust be ingested by slugs, and slug deathtakes three to six days. Feeding activity,however, is stopped almost immediately.Iron phosphate baits works at most temperaturesand slugs will not recover afteringesting the bait.See photo opposite page.
Western Spotted Cucumber BeetleJim D. Barbour17 51Pest Descriptionand Crop DamageAdult western spotted cucumberbeetles (Diabrotica undecimpunctataundecimpunctata) are small (1/4 to 1/3inch long), yellowish-green beetles with11 distinct black spots on the wing covers(Fig. 95). Eggs are yellow, oblong, andapproximately 1/50 inch long. Larvae are1/20 to ¾ inch long and have one veryshort pair of legs on each of the three bodysegments immediately behind the head.Large larvae are white except for the headand the last abdominal segment, which arebrown. Adults feed on pollen, flowers, andfoliage of many plants. Adult feeding is notgenerally of economic importance in hopexcept when beetles attack the growing tipsof newly planted hops or developing hopflowers. Larvae feed on the roots of manyplants but have not been reported as aneconomic pest of hop.Biology and Life HistoryWestern spotted cucumber beetlesoverwinter as fertilized females on vegetationwithin field borders and on plant debris.They may be active on warm winterdays. Eggs are deposited in the soil near thebase of host plants in early spring and hatchin seven to 10 days. A single female can laybetween 200 and 1200 eggs. Larvae completedevelopment and pupate in the soil bylate spring, and adults emerge in early Julyin western Oregon. The complete life cyclerequires 30 to 60 days. Two generations peryear occur in the Pacific Northwest.Monitoring and ThresholdsHop is not a favored host of westerncucumber beetle and is seldom attacked innumbers warranting management. Groundbeetles (Carabidae) prey on eggs and aparasitic fly attacks adult cucumber beetles.Avoiding unnecessary use of broad-spectrumpesticides may help to preserve naturalenemies. No insecticides are registered forcontrol of western spotted cucumber beetleon hop.ManagementPreventing establishment of weedhosts in fields and field borders may reducerisk of attack. Hop yards near favored larvalhosts such as cucurbits and corn may havea higher risk of attack by adult beetles.Certain insecticides applied for control ofhop aphid likely provide some control ofwestern spotted cumber beetles.At-A-GlanceWesternSpottedCucumberBeetle◆◆Monitor foradults prior toflowering of hopplants.◆◆Need fortreatment isunlikely.◆◆Certain foliarinsecticidesapplied for hopaphid are likelyto control thisinsect.Figure 94. Gray field slug. Slugsrange in size from ¼ to 2 inches inlength. (J. Berger, Bugwood.org)Figure 95. Adult western spotted cucumberbeetle. (J. N. Dell, Bugwood.org)
52 Beneficial ArthropodsDavid G. James and Amy J. DrevesAt-A-GlancePredatoryMites◆◆Predatorymites areimportantbiocontrol agentsof spider mites.◆◆Somepredatory mitesfeed on aphidsand on hoplooper eggs.◆◆Alwaysmonitor forpredatory mitesas well as spidermites.◆◆Predatorymites movefaster than pestmites.◆◆Adults caneat three to 10spider mites and/or eggs a day.◆◆Considerpopulationdensity ofpredatory mites(1 predatorto 20 pests)before applyingmiticides.◆◆Always usemiticides andinsecticides thatare nontoxic orpartially toxic topredatory mites.Conservation biological control seeks to preserve and enhance populations ofresident beneficial organisms in cropping systems. When a crop environment is “friendly”to beneficial arthropods, biological control provided by endemic populations of predatorsand parasitoids can contribute substantially to pest management. In hops, beneficialarthropods can provide partial or complete control of spider mites and aphids, dependingon the population densities of pest and prey, environmental conditions, and grower culturalpractices. The foundations of reliable conservation biological control include: 1) properidentification of beneficial organisms; 2) preservation of beneficial arthropods through useof selective pesticides that have low toxicity to beneficial insects and mites (see Table 1,page 5; see also http:www.koppert.nl./Side_effects.html and http://ipmnet.org/phosure/database/selctv/selctv.htm); and 3) modification of cultural practices to provide refuge andextra-floral nectar and pollen resources for beneficial organisms (e.g., border plantings,hedgerows, ground covers). A generalized summary of the seasonal development and activityof several key beneficial (predatory) arthropods is illustrated in Figure 124, page 61. For moreinformation on IPM and conservation biological control, see the Pacific Northwest InsectManagement Handbook at http://pnwpest.org/pnw/insects.Predatory MitesA number of predatory mites occur onhop in the Pacific Northwest including thephytoseiids Galendromus occidentalis (westernpredatory mite) and Neoseiulus fallacis, andthe anystid, Anystis spp. (whirligig mite).All feed on spider mites and Anystis spp.also feed on aphids and on hop looper eggs.G. occidentalis and N. fallacis are generallypale tan colored, pear-shaped, shiny andmore active than spider mites (Figs. 96-98).Predatory mites move faster than pest mites.They range in size from 1/50 to 1/25 inchin length and have needle-like mouthparts,which they use to puncture spider mitesand suck out body contents. Predatorymites feeding on spider mites change color,temporarily reflecting their meal. Eggs ofphytoseiid mites are oblong and slightlylarger than the spherical eggs of spider mites(Fig. 98). Nymphs are smaller and lighter incolor but otherwise are miniature versions ofthe adult. Anystid mites are velvety red andup to 1/10 inch long (Fig. 99).Biology and Life HistoryPredatory mites (Phytoseiids) passthrough four stages before becoming adults:egg, larva, protonymph, and deutonymph.Eggs generally require high humidity forsurvival and hatching, a condition providedby the hop leaf surface. Larvae and nymphsare active predators, consuming spider miteeggs and motiles. Phytoseiids develop fasterthan spider mites, with G. occidentalis andN. fallacis completing development within aweek during the summer. Mating is requiredfor reproduction and females (66 to 75% ofthe population) lay 1 to 5 eggs per day forup to six weeks. Adults can eat three to 10spider mites and/or eggs a day, dependingon temperature. Up to 12 generationsof predatory mites may occur on hopduring the growing season and very largepopulations can develop by mid-summer.Figures 96 and 97. Adult predatory mite, Neoseiulus fallacis.Notice shiny appearance and distinctive pear shape. (D. G. James)
Figure 98. Adult predatory mite, Galendromusoccidentalis, lower right, with its opaque,oblong egg. Above left is a twospotted spidermite adult. Predatory mites range in size from1/50 to 1/25 inch in length. (D. G. James)Most hop yards in Washington haveboth G. occidentalis and N. fallacis presentin proportions that vary with location andyear. Galendromus occidentalis is betteradapted to hot, dry conditions, whileN. fallacis flourishes under cool, moistconditions, thus dominating the phytoseiidfauna in Oregon hop yards. Neoseiulusfallacis is shinier and faster than G.occidentalis and is able to feed on pollen aswell as spider mites, enabling persistence inhop yards even when spider mite numbersare low. Mature females of both speciesoverwinter in hop yard leaf litter, debris,soil, or pole fissures. Activity resumes inMarch to April when spider mites colonizenew hop growth.Little is known about the biology ofAnystis mites (Fig. 99), which are becomingmore frequent in hop yards as pesticideinputs lessen. They are active predatorsof mites, aphids, and small insects likethrips. They are very rapid movers and arelong-lived as adults. Development fromegg to adult takes more than a month, butadults eat large numbers of mites, up to 40per day. Two generations occur per year.Anystis mites’ biology complements therapid developmental biology of phytoseiidsand it is expected that they will becomean important component of IPM as use ofbroad-spectrum pesticides decreases.Predatory MitesMonitoring, Importance in IPM and Compatibilitywith PesticidesPredatory mites are readily monitored by sampling andexamining leaves with a hand lens or microscope. Their rapidmovement easily distinguishes them from slower-moving spidermites. A definitive guide to determining the number of predatorymites needed to give good biological control of spider mites onhop has not been developed. Generally, early-season populations ofpredatory mites in hop yards are too small (fewer than one per leaf)to control a rapidly expanding mite population. However, by Julypredatory mite numbers are often large enough (1 to 5 per leaf) toprovide control of spider mites. A predator:prey ratio of 1:20 or loweroften will result in acceptable biological controlAlthough predatory mites, particularly G. occidentalis and N.fallacis, are very important in the biological control of spider mitesduring July and August, acceptable biological control only occurswhen insect predators of spider mites, such as mite-feeding ladybeetles also are present.Predatory mites are extremely sensitive to broad-spectrumpesticides. However, many new generation insecticides, miticides,and fungicides are non-toxic to predatory mites and should beused in preference to those that are not. Predatory mites also canbe conserved by providing in-yard and adjacent refugia that harboroverwintering populations.A generalized summary of the seasonaldevelopment and activity of key predatoryarthropods is illustrated in Figure 124, page 61.Figure 99. An anystid mite, Anystis spp. Notice the velvety red color. These mitesare relatively large (1/10 inch) compared to other predatory mites. (A. J. Dreves)17 53
54 Predatory Lady BeetlesWashington, Oregon, and Idaho hop yards are readily colonized by several species oflady beetles (Coccinellidae), which play a major role in suppressing spider mite and aphidAt-A-GlanceAphid-FeedingLady Beetles◆◆Lady beetleadults and larvaehelp controlspider mites,thrips, aphidsand other smallinsects.◆◆Monitor foraphid-feedinglady beetles;one adult everysecond or thirdplant can helpsuppress aphids.◆◆Always uselady beetlecompatibleinsecticides tocontrol aphids.populations. Four species of primarily aphid-feeding lady beetles and two species of mitefeedinglady beetles are most frequently seen and are discussed separately below.Aphid FeedersTransverse Lady BeetleCoccinella transversoguttataFigure 100. Adult stage of the transverse lady beetleis approximately ¼ inch long and rounded with distinctnarrow black markings on the wing covers. (D. G. James)DescriptionThe adult is approximately ¼ inchlong and rounded. The wing covers (elytra)are orange with distinct, narrow transverseblack markings (Fig. 100). The body andpronotum (area between the head andwing cases) are black with small whiteor yellow patches. The yellowish-orange,spindle-shaped eggs are laid in batches. Thealligator-shaped larva is purple-blue withorange markings.Biology and Life HistoryTransverse lady beetles are native toNorth America but declining in abundancethroughout much of Canada and theeastern United States. However, they are stillrelatively common in eastern Washingtonand are frequently found in hop yards.Overwintered beetles fly into hop yardsduring April and May and feed on newlyestablished colonies of hop aphids. In someyears, C. transversoguttata is very common,but in others it can be scarce; the cause ofthese population fluctuations is unknown.Transverse lady beetles are also found inother aphid-affected crops such as tree fruit.Adults may consume up to 100 aphids aday depending ontemperature. Larvaeare also voraciousfeeders. When preyis scarce adults cansurvive (but notreproduce) on nectar,honeydew, andpollen. Larvae moltthrough four instarsbefore pupating.The life cycle fromegg to adult takesapproximatelythree weeks duringsummer.Convergent Lady BeetleHippodamia convergensDescriptionThe adult is approximately 1/4 inchin length and more oval-shaped than round(Fig. 101). The wing covers are orange tored, typically with 12 to 13 black spots.However, the number of spots is variableand some individuals have none. Thefirst section between the head and thorax(pronotum) is black with two convergingwhite stripes and white edges. The smallhead is almost covered by the front of thethorax. Legs and antennae are short. The eggis approximately 1/20 inch, bright yellow,elongated, and pointed at one end. Eggs arelaid in clusters. The alligator-shaped larva isdark gray to blackish blue with two small,indistinct orange spots on the pronotumand four larger ones on the back (Fig. 102).The pupa is orange and black and oftenattached to the upper surface of a leaf.Biology and Life HistoryConvergent lady beetles are nativeand common in hop yards. They also areavailable commercially. Females lay 200 to500 eggs, which hatch in five to seven days.Development through larval and pupalstages takes three to six weeks dependingon temperature and food availability,with one to two generations a season. Thelargest populations in hop yards occurduring spring; convergent lady beetles tendto disappear when weather becomes hot.Field evidence suggests that populationsmigrate to cooler, high-elevation areasin summer and aestivate (enter summerdormancy). Congregations of millions ofinactive convergent lady beetles may befound during July to August in the BlueMountains of northeastern Oregon andsoutheastern Washington states (Fig. 103).Most of these beetles overwinter in themountains before migrating back to valleyareas in spring.
Multicolored Asian Lady BeetleHarmonia axyridis55DescriptionAdults are strongly oval and convex,approximately ¼ inch long (Fig. 104). Theyare highly variable in color and pattern,but most commonly are orange to red withmany to no black spots. Some individualsare black with several large orange spots.The first section between the head andthorax is straw-yellow with up to five blackspots or with lateral spots usually joined toform two curved lines, an M-shaped mark,or a solid trapezoid. Eggs are bright yellowand laid in clusters of approximately 20 onthe undersides of leaves. Larvae are elongate,somewhat flattened, and adorned withstrong round nodules (tubercles) and spines(Fig. 105). The mature larva (fourth instar)is strikingly colored: the overall color isblack to dark bluish-gray, with a prominentbright yellow-orange patch on the sides ofabdominal segments 1 to 5.Biology and Life HistoryThis exotic species is considered to beprimarily forest-dwelling, but it appears tobe well adapted to living in hop yards and isoften the most common lady beetle speciespresent.Unmated females overwinter in largecongregations, often in buildings or caves(Fig. 106). Mating occurs in spring and eggshatch in five to seven days. In summer, thelarval stage is completed in 12 to 14 daysand the pupal stage requires an additionalfive to six days. In cool conditionsdevelopment may take up to 36 days.Adults may live for two to three years. H.axyridis is a voracious predator, feeding onscale insects, insect eggs, small caterpillars,and spider mites, as well as aphids. Adultsconsume 100 to 300 aphids a day and up to1200 aphids may be consumed during larvaldevelopment.Multicolored Asian Lady Beetle.Convergent Lady Beetle. ABOVE LEFT: Figure 101. Adult is approximately 1/4 inch long, more ovalthan round, typically with 12 to 13 black spots on the wing cases. (R. Ottens, Bugwood.org)ABOVE RIGHT: Figure 102. Alligator-shaped larvae are gray to blackish-blue with six orange spots.(D. G. James) BELOW: Figure 103. Congregating adults during aestivation. (D. G. James)TOP: Figure 104.Adult is oval, convex, andapproximately ¼ inch long.They are highly variable incolor and pattern, but mostcommonly orange to red withmany to no black spots.MIDDLE: Figure 105. Larvaeare elongate and somewhat flatwith round nodules and spines.Mature larvae are black to darkbluish-gray, with prominentbright yellow-orange patcheson the sides.BOTTOM: Figure 106.Overwintering H. axyridiscongregated under a rock.(3 photos, D. G. James)
56A generalizedsummary ofthe seasonaldevelopmentand activity ofkey predatoryarthropodsincluding ladybeetles isillustrated inFigure 124,page 61.Seven-Spot Lady Beetle.AT RIGHT: Figure 107.Adult is relatively large(approximately 3/8 inch)and has a distinctive “1-4-2”pattern of black spots on thewing cases. (D. G. James)FAR RIGHT, TOP: Figure 108.Larvae are dark gray withorange spots. (R. Otten,Bugwood.org)FAR RIGHT, BOTTOM:Figure 109. Pupal stage lasts3 to 12 days. (D. G. James)Seven-Spot Lady BeetleCoccinella septempunctataDescriptionThis species is comparatively large(approximately 3/8 inch), with a white orpale spot on either side of the first sectionbetween the head and thorax (Fig. 107). Thebody is oval and domed. The spot patternis usually 1-4-2, black on the orange or redwing cases. Eggs are spindle-shaped andsmall, approximately 1/25 inch long. Larvaeare alligator-like, dark gray with orangespots on segments 1 and 4 (Fig. 108), andgrow to the same length as adults beforethey pupate (Fig 109).Biology and Life HistoryThis exotic species is a newcomer tohop yards, unknown before approximately2000. Currently, it is well established andoften as common and important as H.axyridis in controlling hop aphids. Adultsoverwinter in protected sites near fieldswhere they fed and reproduced the previousseason. In spring, emerging beetles feed onaphids before laying eggs. Females may lay200 to 1,000+ eggs during a period of oneto three months commencing in spring orearly summer. The spindle-shaped eggs areusually deposited near prey, in small clustersof 10 to 50 in protected sites on leaves andstems. Larvae grow from 1/25 to 3/8 inchin 10 to 30 days depending on the supply ofaphids. Older larvae may travel up to 36 feetin search of prey. The pupal stage lasts fromthree to 12 days depending on temperature.Adults are most abundant in mid- to latesummer and live for weeks or months,depending on availability of prey and timeof year. One to two generations occur beforeadults enter winter hibernation.Aphid-Feeding Lady BeetlesMonitoring, Importance in IPM and Compatibilitywith PesticidesAphid-eating lady beetles are extremely important to naturalsuppression of hop aphids. Growers should encourage the speciesdescribed here to colonize and reside in hop yards. Attraction andconservation of lady beetles is more effective and sustainable than thepurchase and introduction of H. convergens, which tend to rapidlyleave hop yards after released. Despite feeding primarily on aphids,these lady beetles also can feed on spider mites, thrips, and othersmall insects, and thus contribute generally to biological control.Lady beetles can be monitored by simply walking through yardsand conducting timed counts. Alternatively, they can be sampledby shaking foliage over a tray. A mean of one adult lady beetle everysecond or third plant represents a significant population capable ofresponding to aphid population increases. Lady beetles are compatiblewith many new, selective insecticides and miticides but are negativelyaffected by older, broad-spectrum pesticides.
Mite FeedersMite-Eating Lady BeetlesStethorus picipes, S. punctillumDescriptionMite-eating lady beetles are black,tiny (1/25 to 1/16 inch), oval, convex, andshiny, covered with sparse, fine, yellowishto-whitehairs (Fig. 110). Emerging adultsare reddish-orange for a few hours beforeturning black. The white, oval eggs are lessthan 1/50 inch long, and turn dark justbefore the larvae emerge (Fig. 111). Eggs arelaid singly, usually on the underside of leavesnear the primary vein, and adhere tightlyto the leaf. The newly hatched larva is grayto blackish and has many long-branchedhairs and black patches (Fig. 112). Thelarvae grow from 1/25 to 1/16 inch long,becoming reddish as they mature, at first onthe edges of the body. Just prior to pupationthe entire larva turns reddish. The pupaeare black and flattened, somewhat pointedon the posterior end, with the entire bodycovered with yellow hairs (Fig. 113).Mite-Feeding Lady BeetlesMonitoring, Importancein IPM and Compatibilitywith PesticidesMite-eating lady beetlesare critical to good biologicalcontrol of spider mites. One ortwo Stethorus beetles are usuallysufficient to control an early-seasonmite “hot spot,” preventing it fromspreading into a larger outbreak.In combination with predatorymites, Stethorus may maintain nondamaginglevels of spider mitesduring July and August. Monitoringcan be conducted by examiningleaves in the field or a laboratoryby looking for tiny alligator-likelarvae or mobile pinhead-sized blackdots. The beetles also can be shakenfrom bines and collected onto atray. Stethorus spp. are susceptibleto broad-spectrum insecticidesand miticides such as abamectin.However, many narrow-spectrumpesticides are compatible with thesurvival of these important predators.TOP: Figure 110. Adult mite-eating lady beetlesare 1/25 to 1/16 inch long. MIDDLE: Figure111. White, oval eggs are less than 1/50 inchlong. BOTTOM: Figure 112. Newly hatchedS. picipes larva is dark and hairy, with blackpatches. (3 photos, D. G. James)Biology and Life HistoryStethorus picipes (a native species) ismost commonly found in hop yards butS. punctillum (exotic) also occurs. Bothspecies are found in hop yards not exposedto broad-spectrum pesticides and arevoracious spider mite feeders, consuming 50to 75 mites per day. Overwintering occursas non-reproductive adults in protectedhabitats (e.g., in ground debris, under bark)away from hop yards. Adults emerge fromhibernation sites in late March and April, andseek out spider mite colonies in hop yards,which they are able to do extraordinarilywell. Once prey is found, female Stethorusfeed and lay eggs (approximately 15 eggs perday), rapidly exterminating small colonies ofmites. Larvae develop through four instars,pupating after 12 days. Development fromegg to adult takes approximately three weeksand three to four generations are producedduring spring-summer. Adults live for fourto eight weeks during summer and thrive attemperatures between 68 and 95 °F.At-A-GlanceMite-FeedingLady Beetles◆◆Monitor formite-eating ladybeetles.◆◆Learn torecognize “blackdot” adults andalligator-typeblack larvae.◆◆Thesevoracious spidermite feedersconsume 50 to75 mites per day.◆◆Spider mite“hot spots” canbe suppressedby 1 or 2 miteeatingladybeetles.◆◆Use onlyinsecticides andmiticides safe tomite-eating ladybeetles.17 57Figure 113. Pupae of the miteeatinglady beetle S. picipes.Notice the pointed posteriorend and yellow hairs coveringthe body (D. G. James)
58 Predatory BugsThe predatory bugs described here are true bugs, belonging to the insect orderAt-A-GlancePredatoryBugsHemiptera. Predatory bugs have shield-like, thickened forewings and suck out the bodycontents of their prey through tubular, stylet-like mouthparts. All of the predatory bugsfound on hop feed on more than one type of prey, consuming the eggs, immatures, andadults of a wide variety of prey including mites, aphids, caterpillars, and thrips.◆◆Recognizeand identifypredatory bugs.◆◆Predatorybugs areimportant inearly seasonsuppressionof mites andaphids.◆◆Predatorybugs alsofeed on eggs,immature andadult thrips,loopers andother soft-bodiedarthropods.◆◆Monitorpredatorybugs by shakesampling ordirect counts onfoliage.◆◆Always useinsecticides andmiticides safe topredatory bugs.Minute Pirate BugOrius tristicolorDescriptionAdults are 1/12 to 1/5 inch long,oval, and black or purplish with whitemarkings on the forewings (Fig. 114). Thewings extend beyond the tip of the body.The tiny (1/100 inch) eggs are embedded inplant tissue with the “lid” exposed, throughwhich the nymph emerges (Fig. 115).Newly hatched nymphs are transparent witha slight yellow tinge, turning yellow-orangeto brown with maturity (Fig. 116). They arefast-moving, wingless, and teardrop-shaped.Figure 114. Adult minute pirate bug(Orius tristicolor). Adults are 1/12 to 1/5inch in length. (D. G. James)Biology and Life HistoryMinute pirate bugs overwinter asadults in leaf litter or under bark andusually emerge from hibernation in lateMarch or early April. They feed on mites,aphids, thrips, hop loopers, and other softbodiedinsects. Eggs take three to five daysto hatch and development from egg toadult through five nymphal stages takes aminimum of 20 days. Females lay an averageof approximately 130 eggs over a 35-dayperiod and several generations are producedduring spring and summer. When prey isnot available, minute pirate bugs are able tosurvive feeding on pollen and plant juices.Adults and immatures can consume 30 to40 spider mites or aphids per day. Minutepirate bugs are efficient at locating preyand are voracious feeders. They aggregatein areas of high prey density and increasetheir numbers more rapidly when there is anabundance of prey. Minute pirate bugs arecommon predators in low-input hop yardsand contribute significantly to control ofspider mites, aphids, and hop loopers.Figure 115. First-instar nymph and egg ofthe minute pirate bug (Orius tristicolor).Eggs are extremely small (1/100 inch) andembedded within leaves. (D. G. James)Figure 116. Minute pirate bug (Orius tristicolor)nymph. Notice that nymphs are wingless andteardrop-shaped, and older ones are yelloworangeto brown in color. (D. G. James)
Big-Eyed BugGeocoris pallensDescriptionBig-eyed bugs are oval, somewhat flattened,and 1/10 to 1/5 inch in length. Theyare usually gray-brown to blackish and have awide head with prominent, bulging eyes (Fig.117). Antennae are short and enlarged at thetip. Big-eyed bugs walk with a distinctive“waggle” and emit an unpleasant odor whenhandled. Eggs are cylindrical, ribbed, andpink or yellowish-white with a distinctive redspot. Eggs hatch into nymphs that resembleadults except they are smaller and lack wings.Figure 117. Adult big-eyed bug (Geocorispallens) is 1/10 to 1/5 inch long, gray-brownto blackish in color, and has a wide head withprominent bulging eyes. (D. G. James)Biology and Life HistoryEggs are deposited singly or inclusters on leaves near potential preyand hatch in approximately a week.Development from egg to adult throughfive nymphal stages takes approximately30 days under summer conditions. Bothadults and nymphs are predatory, but cansurvive on nectar and honeydew when preyis scarce. Nymphs may consume up to 1600spider mites during development and adultsfeed on 80 to 100 mites a day. Big-eyedbugs prey on a wide variety of insects andmites smaller than themselves. They feedon eggs and small larvae of hop loopers andother caterpillar pests, as well as all stagesof thrips, aphids, and mites. Two to threegenerations a year occur between Apriland September. Adults overwinter in leaflitter or debris, or under bark. The relativeabundance of Geocoris pallens in Oregon islow compared to other natural enemies.Predatory MiridDeraeocoris brevisDescriptionAdult predatory mirids (Deraeocorisbrevis) are oval, shiny black with palermarkings, 1/10 to 1/5 inch long andapproximately 1/12 inch wide (Fig. 118).Eggs are elongate, approximately 1/25 inchlong, and inserted into plant tissue, often atthe mid rib of a leaf, with only the “lid” anda respiratory horn visible (Fig. 119). Nymphsare mottled pale gray with long gray hairson the thorax and abdomen (Fig. 120). Acottony secretion covers most of the body.Dark areas on the thorax and abdomen giveit a spotted appearance. The eyes are dull red.Biology and Life HistoryDeraeocoris overwinters as an adultin protected places such as under bark or inleaf litter. Overwintered adults emerge fromhibernation during March to April and feedon nectar of willow catkins and other earlyspring flowers. They seek out prey and beginlaying eggs in late April or May. Nymphsof the first generation occur two to threeweeks later. Nymphs develop through fivestages in approximately 25 days at 70 °F.Females lay up to 250 eggs during theirlifetime and adults consume 10 to 20 aphidsor mites a day. Nymphs can eat 400 miteeggs a day. Deraeocoris adults and nymphsare important predators that prey on a widevariety of small insects and mites includingaphids, thrips, leafhoppers, scale insects,small caterpillars, and spider mites. Twoor three generations are produced betweenMay and September. Deraeocoris is abundantin many agricultural and non-agriculturalhabitats in the Pacific Northwest.TOP: Figure 118.Oval, shiny black adultpredatory mirids.59ABOVE: Figure 119.Elongated predatory mirid eggsare inserted into plant tissue.AT LEFT: Figure120. Predatory mirid(Deraeocoris brevis)nymph. Nymphs aremottled pale gray withlong gray hairs on thethorax and abdomen.(3 photos, D. G. James)
60Assassin BugsReduviidaeDescriptionAdults are blackish, brown, or reddishwith a long, narrow head; round, beadyeyes; and an extended, three-segmented,needle-like beak (Fig. 121). They are larger(2/5 to 4/5 inch) than other predatory bugs.Eggs are reddish-brown, skittle-shaped, laidin a raft of 10 to 25 or more, and coatedwith a sticky substance for protection (Fig.122). Nymphs are small versions of adults,although early instars are often ant-like.Figure 121. An adult assassin bug feeding on a beetle larva. Adult assassinbugs are relatively large (2/5 to 4/5 inch), blackish, brown, or reddish incolor, and have a long, narrow head and beak. (D. G. James)Biology and Life HistoryAssassin bugs are long-lived and consumelarge numbers of insects and mites duringtheir lifetime. Adults may live for morethan one season and nymphs are slow todevelop. Population densities of assassinbugs are usually low but they provide useful,consistent, and long-term feeding on aphidsand caterpillars in hop yards. They are mostfrequently found in yards with a groundcover. Populations of assassin bugs in hopyards in Oregon tend to be relatively low.Damsel BugsNabis spp.Figure 122. A raft of eggs laid by an assassin bug. Notice the reddish-browncolor, distinctive skittle shape, and clustering of eggs. (D. G. James)Predatory BugsMonitoring, Importance in IPM and Compatibilitywith PesticidesPredatory bugs are an important component of IPM, providingcontrol and suppression of spider mites, aphids, loopers, and thrips.They are particularly important early in the season, when predatorymites have not fully established, helping to suppress spider mitepopulations. They also exert significant control on aphid populations.The abundance of predatory bugs in hop yards is likely to increaseas broad-spectrum pesticide use decreases and greater use is made ofground covers. Monitoring of predatory bugs is best done by visualscanning of foliage or by taking canopy shake samples.DescriptionDamsel bugs are mostly yellowish,gray, or dull brown, slender insects up to½ inch long with an elongated head andlong antennae (Fig. 123). The front legsare enlarged for grasping prey. Cylindricalwhite eggs are deposited on leaf surfacesnear potential prey. Nymphs look like smalladults but are wingless.Biology and Life HistoryAdult damsel bugs overwinter inground cover, debris, and protected sites.They emerge from hibernation in Apriland soon begin laying eggs. Numerousoverlapping generations occur during theseason. Both adults and nymphs feed onsoft-bodied insects and mites includingaphids, loopers, spider mites, leafhoppers,small caterpillars, and thrips. A number ofdamsel bug species are seen in hop yards,particularly those with a ground cover.
61Figure 123. Adult damsel bug. Note that damsel bugs are mostly yellowish, gray, or dull brown,slender insects up to ½ inch long with an elongated head and long antennae. (D. G. James)Predatory Mitespredatory mitesbecome activeat shootemergence andprey on mitespopulations increasemostly in lowercanopy, providingsuppression ofspider mitesgreatest abundanceof predatory miteswhen spider mitesincreasepredatory mites continuefeeding on spider mites,overwinter in soil near hopcrown and protected areasin and near hop yardsGeneralized informationpresented only for keygroups of predatoryarthropods. Imagesdepict adult stages.Many other naturalenemies occur inhop yards and cancontribute to control ofspider mites, aphids,and caterpillar pests.See text for detailedinformation on thebiology, life cycle, andimportance of theseand other beneficialorganisms.Lady BeetlesStethoruslady beetlesfly into yardsand feed onaphids andmitesStethorusactively seekout and eatspider mitesPredatory Bugspopulations declineas aphids areconsumed andtemperature increases;some species dormantpopulationsincrease, helpingto suppress miteoutbreakspredatory bugs appearand feed on mites,aphids, caterpillarlarvae, and thripsabundance increaseswith aphid resurgencegreatest abundance ofStethorus lady beetlespopulations increase,feeding on mites andother pestsoverwinter as adultsin protected areasnear hop yardsoverwinter as adultsin protected areasnear hop yardsoverwinter as adultsin leaf debris or otherprotected areas in ornear hop yardsDormancy Emergence Training Flowering Harvest Post-harvestFigure 124. Seasonal development and activity of four key groups of predatory arthropods that occur on hop: predatory mites, aphideatinglady beetles, mite-eating (Stethorus) lady beetles, and predatory bugs. Information is generalized; multiple factors influencethe presence and abundance of beneficial arthropods in hop yards. Detailed sections for each of these predator groups appear on thepreceding pages, beginning p. 52; other beneficial arthropods are detailed in the pages following. (Illustrations by Joel Floyd)
62 Parasitic Wasps(Parasitoids)At-A-GlanceParasiticWasps◆◆Wasps areimportantparasitoids ofeggs, larvae, orpupae of hoploopers andother caterpillarpests.◆◆Predatorywasps such asyellow jacketsand hornetscan removecaterpillars andaphids.◆◆Encourageflowering groundcovers thatprovide nectarfor wasps.◆◆Useinsecticides andmiticides safe towasps.DescriptionParasitic insects that attack andkill other insects are termed parasitoids.Many species of wasp parasitoids attackeggs, larvae, or pupae of hop pests such asloopers, cutworms, leafrollers, and aphids.There are several families of parasiticwasps; some have a noticeable stinger/ovipositor specialized for piercing theirhosts. Each family is distinguished primarilyby differences in wing venation. Adultsare usually small, varying from less than1/12 inch to 1 inch long, with two pairs ofmembranous wings folded over their backs.They are black-brown to metallic blue incolor and have medium to long segmentedantennae. Some are slender with longbodies (Ichneumonidae) (Fig. 125); otherssmaller (
64 17Predatory and Parasitic FliesA number of fly species from at least five families are known as predators or parasitoidsof hop pests in the Pacific Northwest.At-A-GlancePredatory& ParasiticFlies◆◆Identify andmonitor adultand larvalpredatory flies.◆◆Predatory fliesfeed on aphids,spider mites,thrips, and theeggs and adultsof small insects.◆◆Useinsecticides andmiticides safe topredatory flies.◆◆Encourageflowering groundcovers thatprovide nectarfor predatoryflies.Hover FliesThe yellow-and-black-banded adulthover fly resembles a stinging bee or wasp,but only has one pair of wings (Fig. 128).Hover flies lay single white, oblong eggs nearaphid infestations. The adult is not predaceousbut feeds on flower nectar. The larvae areapproximately ¼ to ½ inch long, green to lightbrown, with a wrinkled-looking body that isblunt at the rear and pointed at the mouthend (Fig. 129). The pupae are pear-shaped andgreenish to dark brown (Fig. 130). A numberof species occur in hop yards and may be blackand yellow or black-and-white banded.Hover flies overwinter as pupae inthe soil or above ground in leaves and plantmaterial. The adult flies become active duringspring (April and May), laying eggs on leavesand stems of hop plants harboring aphids.Hover flies are good fliers, disperse widely, andseek out aphid infestations very effectively.Larvae feed on aphids for approximately 7to 10 days and then pupate. The larvae arevoracious feeders: as many as 300 to 400aphids may be consumed by one larva duringdevelopment.Adult hover flies may be monitoredusing yellow sticky traps; the maggot-likelarvae can be found amongst aphid colonies.Hover flies are an important component ofbiologically based hop aphid management. Incombination with lady beetles and predatorybugs, they can provide rapid control of aphidinfestations. Hover flies are generally sensitiveto broad-spectrum pesticides.Figure 128. Adult hover fly. The adult hoverfly resembles a stinging bee or wasp, but onlyhas one pair of wings. (D. G. James)Figure 129. Hover fly larva attacking hop aphid.Larvae are ¼ to ½ inch long. (D. G. JamesFigure 130. Hover fly pupa.(D. G. James)Dance FliesThe adults are small to mediumsized(< ¼ inch), dark-colored flies witha humpbacked thorax, long taperingabdomen, and slender legs. Dance flies arepredators as adults and larvae, consumingsmaller insects like aphids. Adults fly anduse their front legs to grasp small insects onthe wing and pierce them with their sharpsnout. The larvae are pale and cylindricaland live in the soil or decaying vegetation,preying on small insects and mites. Adultsalso visit flowers and swarm for mating.The larvae are generally found on moistterrestrial soil or rotten wood and arepredacious on various arthropods.Adult dance flies may be monitoredusing yellow sticky traps. Their value inhop yards is undetermined but they maycontribute to suppression of hop aphids.
Long-legged FliesThese small to medium-sized (¼ to 3/8 inch), slender flieshave metallic green, blue, to bronze coloration, long legs, and large,prominent eyes. The wings are clear with some darker markings,depending on species. The larva is maggot-like. Both larvae andadults prey on small insects such as aphids, thrips, and spider mites.Adult long-legged flies commonly sit on hop leaves and maybe monitored using timed counts or yellow sticky traps. Their valuein hop yards is undetermined but they likely contribute to somedegree to suppression of aphids and spider mites.65Tachinid FliesThese parasitic flies are gray-black, robust, and have stoutbristles on their body similar to house flies (Fig. 131). Tachinidsparasitize the caterpillars of moth pests of hop including armyworms,cutworms, leafrollers, and hop loopers (Fig. 132). Tachinids typicallydeposit a single egg directly on or inside the body of a caterpillar,and the developing maggot feeds inside the host, eating away nonessentialorgans first, then emerging from the moribund caterpillar orpupa. The adult fly emerges after two weeks. There are two to threegenerations a year in Washington. Five species of tachinid fly attacklarvae of the hop looper in Washington, with levels of parasitismlater in the season up to 30%. Tachinid flies tend to be less commonin hop yards in Oregon as compared to those in Washington.Tachinid flies can be monitored using yellow stickytraps. The value of tachinid flies in hop yards has not been fullyinvestigated but recent research shows that they do have an impacton hop looper populations, particularly in Washington. They aresusceptible to pesticides, therefore should become more frequent inhop yards as broad-spectrum chemical inputs decrease.Predatory MidgesPredatory midges are fragile-looking and gnat-like (less than1/8 inch long) with antennae that curl back over their heads. Thetiny larvae are yellowish to red-orange (Fig. 133) and are easily seenusing a 10X hand lens. Predatory midges are most often foundfeeding amongst aphids, spider mites, thrips, and the eggs of otherinsects and mites. Predatory midges are most frequently seen duringpest outbreaks. In some parts of the Pacific Northwest, a predatorymidge species (Feltiella sp.) specialized for feeding on spider miteshas been observed, however the occurrence of this species in Oregonis rare. Other species may occur, including Aphidoletes spp., whichspecialize on aphids. Adult predatory midges feed on nectar andhoneydew and lay 70 to 200 eggs near aphid or mite colonies. Alarva during development consumes 40 to 100 mites or aphids.Pupation occurs on the ground and pupae overwinter. The life cycleoccupies three to six weeks with three to six generations per year.Predatory midge adults can be monitored using yellowsticky traps. The value of predatory midges to biological control ofspider mite and aphid is significant, particularly when there is anoutbreak of these pests. Mid-summer colonies of spider mites inlow-input hop yards can be suppressed by predatory midge larvae incombination with other predatory insects and mites. Most broadspectruminsecticides and miticides used in hop yards are toxic topredatory midges.Figure 131. Adult tachinid fly. (D. G. James)Figure 132. Top, hop looper larva killed by a tachinidfly larva, which has now pupated. Bottom, a tachinid flylarva exiting a hop looper larva. (D. G. James)Figure 133. Larvae of a predatory midge. Larvaeare less than 1/8 inch long. (D. G. James)
Snakeflies67DescriptionRelated to lacewings (Order:Neuroptera), snakeflies are voraciousfeeders of a wide variety of small insects.Adult snakeflies are weak flyers with long,transparent wings. The common name,snakefly, derives from the superficiallysnake-like appearance that is suggested bythe unusually long “neck” (frontal thorax)and long, tapering head (Fig. 137 A-B).Biology and Life HistorySnakeflies have four stages in theirlife cycle: egg, larva, pupa, and adult. Bothlarvae and adults are predatory, feedingon aphids, thrips, hop looper eggs, smallcaterpillars, spider mites, and other smallprey. The larvae usually live under treebark or on the ground in decaying organicmaterial. Snakeflies are arboreal; hop yardsprovide a good temporary habitat duringspring and summer. They can be monitoredusing yellow sticky traps or by shaking hopbines over a tray. Snakeflies are susceptibleto many broad-spectrum pesticides.Figure 137 A and B. Adult snakefly. Notice the unusually long “neck”that is a characteristic of these insects. (D. G. James)Lacewings and SnakefliesMonitoring, Importance in IPM andCompatibility with PesticidesLacewings and snakeflies can be monitored by shakingbines over a tray or by using yellow sticky traps. In conjunctionwith key predators, their importance in biocontrol is considerable,contributing to suppression of aphids, mites, and hop loopers.Broad-spectrum pesticides are harmful to lacewings and snakeflies,but some newer selective materials appear safer to these closelyrelated arthropods.Insect PathogensNaturally occurring diseasessometimes contribute to managementof hop pests. In particular, outbreaks ofBacillus thuringiensis, a bacterial infection,and viruses occasionally result in populationcrashes of hop looper. Once pathogens takehold, they can almost eliminate hop looperpopulations. Diseased caterpillars are easy tospot; they are dark brown to black and hangfrom one pair of claspers or are draped overleaves (Fig. 138). They emit a foul-smellingodor and basically become liquefied,releasing endospores of Bacillus thuringiensisto infect other caterpillars. Mites andaphids may also succumb to pathogensbut the incidence of this is generally low inthe Pacific Northwest, unless the season isunusually cool and wet.Figure 138. A hop looper larva infected witha bacterium. Diseased caterpillars are darkbrown to black and hang from or are drapedover leaves. (D. G. James)At-A-GlanceInsectPathogens◆◆Watchfor diseasedcaterpillars.◆◆Diseasedcaterpillars aredark in color,smell bad, andhang loosely.◆◆Diseaseusually leadsto epidemicand looperpopulationcrash.
68At-A-GlanceSpiders◆◆Spiderpresence in hopsis a good signof low pesticideinput.◆◆Spiders oftenserve as buffersthat limit initialexponentialgrowth of preypopulations.◆◆Spiders mayhelp regulateaphids andcaterpillars.◆◆Use insecticidesand miticides safeto spiders.SpidersDescriptionSpiders are common residents inmost low-chemical-input hop yards andcan reach high densities on the groundfloor and in the hop canopy. Some of thecommon spiders found in hop yards includejumping spiders (Figs. 139 and 140), crabspiders (Fig. 141), sheet web weavers, andsac spiders. Spiders are one of the mostabundant predators in hop yards.Biology and Life HistorySpiders often serve as buffers thatlimit the initial exponential growth ofprey populations. However, the specificrole of spiders as effective predators hasreceived little attention and is difficultto demonstrate. There is evidence inmany ecosystems that spiders reduce preypopulations. They are generalists that acceptmost arthropods as prey in their webs orin their paths. They eat the eggs and larvaeof all the insects and mites that infesthops. Spiders disperse easily to new areasin hop yards and colonize rapidly by aerialballooning and walking between bines.They are also blown around with the windand debris. The abundance and diversityof spiders in hop yards is linked to thelarge-scale landscape complexity (hop yardmargins, overwintering habitat, weediness)and local management practices (pesticideuse, tillage practices).Figure 140. A jumping spider. (D. G. James)Figure 141. A crab spider feeding on a wasp.(D. G. James)Figure 139. A jumping spider (Phidippia sp.)feeding on a beetle larva. (D. G. James)SpidersMonitoring, Importancein IPM and Compatibilitywith PesticidesSpiders can be monitored byshaking bines over a tray. The valueof spiders to biocontrol is thoughtto be considerable, but has yet tobe evaluated. Most pesticides harmspiders, but populations tend torecover rapidly.
Weed ManagementRobert ParkerWeeds have many definitions. Inhop yards they are plants that interfere insome way with production, whether directlyimpacting the growth and yield of theplants themselves or indirectly inhibitingproduction by interfering with fieldoperations.Weeds compete with hop plants fornutrients, water, and—to some extent—light. Hop by nature grows tall, thereforecompetition for light is usually not as greata problem as it can be with most row crops.Some weeds also provide an environmentfor certain pathogens to survive when hopplants are not actively growing. Generallyspeaking, as weed density increases in thehop yard, yields decrease. Therefore weedmanagement must be considered in anoverall integrated pest management programin hops.Hop is a perennial crop and weedscan be a problem year around. Summerannual weeds, those germinating in thespring or summer, are found in the growingcrop. They can interfere with sprayingoperations, distort sprinkler patterns insprinkler-irrigated yards, and interfere withharvest. However, winter annual weeds,those germinating in the late summer orfall, usually do not have much direct impacton hop growth. Winter annual weeds can,however, cause indirect problems in hopyards by depleting stored soil moisture,interfering with hop yard maintenanceduring the off season, and slowing springfield operations. Perennial weeds, thoseplants that live more than two years, cancreate problems similar to those posed byannual weeds. Perennials are much moredifficult to control and are frequently spreadwith tillage operations.A few representative annual andperennial weeds are pictured in Figures 142to 148. The pages following contain basicinformation on planning and executing anintegrated weed management program inhops as well as photos of many of the weedsthat can be problematic in hop yards.69RepresentativePerennial WeedsRepresentative Annual WeedsTOP ROW, LEFT TO RIGHT: Figure 142. Prickly lettuce. Figure 143. Common lambsquarter.BOTTOM ROW, LEFT TO RIGHT: Figure 144. Kochia. Figure 145: Puncturevine. (R. Parker)FROM TOP:Figure 146: Canada thistle.Figure 147: Field bindweed.Figure 148: Blackberry.(R. Parker)
70 17Planning a WeedManagement ProgramSeveral factors should be consideredwhen planning a weed managementprogram in the hop yard. Factors such asweed species, tillage, row spacing, irrigation,and herbicides all need to be integrated todevelop an effective weed control strategy.(See “Identification” sidebar, opposite.)The photos presented in this section areintended to aid in the identification ofweeds at various stages. Weed seedlings areshown first, with other stages on the pagesfollowing.Figure 149. Canada thistleseedling. (R. Parker)Figure 150. Common lambsquarterseedling. (R. Parker)Figure 150. Common lambsquarterseedling. (R. Parker)Figure 152. Kochia seedling.(R. Parker)Figure 153. Shepherd’s purseseedling. (R. Parker)Figure 154. Common groundselseedlings. (R. Parker)PreventionThe first line of defense in hop yardweed control is to prevent weeds frombecoming established. It is very difficult toprevent weed seed from infesting a hop yard,as weed seed and reproductive propagulesare easily transported from outside areas intoa yard via animals, birds, wind, equipment,irrigation water, and many other means.However, cleaning equipment before movingit from one field to another and controllingweeds around the field borders will lessenthe establishment of weeds within theyard. Cultivating or mowing weed growtharound the field border not only reduces thepotential for weed seed movement into thefield, but also improves air circulation andhelps eliminates refuge areas for insect pests.As weeds arise, further spread can bediscouraged through diligence and immediatecontrol of new weeds before they are allowedto produce seed.Weed seed germination is triggered byoptimum temperature, adequate moisture,and field operations that expose seed tolight. Not all weed seeds located in the soilwill emerge each year because most weedseeds have an inherent dormancy factor.For example, approximately 26% of kochiaand 3% of common lambsquarter seed willgerminate each year. With certain summerannual weeds, secondary dormancy willoccur and seed germination stops whentemperature increases to a critical point.Winter annual weeds generally will notgerminate until soil temperatures and/orday length begins to decrease. Perennialherbaceous weeds begin to grow when soiltemperatures reach a certain point and willcontinue to grow until they either set seedor temperatures drop to a critical point.
71Figure 155. Blue mustardseedlings. (R. Parker)Figure 156. Pigweedseedling. (R. Parker)Figure 158. Sunflower seedling. (P. Westra,Colorado State University, Bugwood.org)Figure 159. Flixweed seedling.(R. Parker)WeedSeedlingIdentification◆◆Accurateweedidentificationshould be the firststep in any weedmanagementprogram.◆◆Many weeds(e.g., hairynightshade,commonlambsquarter,and pigweed)look similar in theseedling stage,however theirsusceptibility tocontrol measurescan be quitedifferent.◆◆To aid inproper seedlingidentification, aseries of commonweed seedlingsaffecting hopsare presented inFigures 149 to160.◆◆Proper weedidentification isimportant forselecting themost effectiveand economicaltreatment in thehop yard.Figure 157. Puncturevineseedlings. (R. Parker)Figure 160. Purslane seedlings.(Utah State University Archive, Bugwood.org)
17 72Figure 161. Redroot pigweedplant. (R. Parker)Cultural (Non-chemical)TacticsTillage has a major impact on weedspectrum and population. Weed seedresponse to burial and exposure to lightvaries with the species. Disking in thespring stimulates certain seeds to break seeddormancy and allow germination.The use of a fall-planted cover cropcan reduce weed emergence the followingspring. Fall tillage may stimulate germinationof certain summer annual weed seeds, whichare then killed by freezing fall temperatures.Summer annual weed populations will belower in fall-tilled areas planted to a fallplantedcover crop. Fall-planted cover cropsand weeds can then be killed with glyphosatebefore hop shoots emerge.HerbicidesThe number of herbicides availablein hop production is limited; however,herbicides are becoming more widely usedfor controlling weeds. Herbicide selectionshould be based on the weed spectrum ineach yard. It is extremely helpful for hopproducers to keep records of previous weedinfestations. Perennial weeds such as Canadathistle, field bindweed (wild morning glory),and Bermudagrass usually occur in patchesinitially. Scattered patches and individualweeds can be spot-treated with an herbicide,rogued, or cultivated. Soil-active herbicidesapplied during the dormant period maynot provide adequate weed control becauseof inadequate moisture or mechanicalincorporation after application. Tools suchas disking and post-emergence herbicideapplication can be utilized to control weedescapes. One disadvantage to disking isthat soil disturbance can stimulate weedseed germination in the growing seasonand also disking can deposit dust on hopfoliage which could enhance the buildupof spider mites. Field scouting immediatelyafter weeds emerge is important to identifyweeds and provide the information neededto choose a post-emergence herbicide thatmatches the weed spectrum.Figure 162. Aptly named redrootpigweed root. (R. Parker)Figure 167. Puncturevine fruit. (R. Parker)Figure 163. Powell amaranthinflorescence. (R. Parker)Figures 164, 165, and 166. Prickly lettuce.From Top: Plants at various stages of growth;close-up of leaves; mature plants. See alsoFigure 142. (3 photos, R. Parker)Figure 168. Puncturevine plant. See alsoFigure 145. (R. Parker)
Nine herbicides are registered foruse in hop production: trifluralin (Treflanand several other trade names), norflurazon(trade name Solicam), clopyralid (tradename Stinger), 2,4-D amine (various tradenames), glyphosate (various trade names),clethodim (trade names Select and others),carfentrazone (trade name Aim), paraquat(trade names Gramoxone, Firestorm,Parazone, and Paraquat), and pelargonic acid(trade name Scythe).Trifluralin and norflurazon areprimarily soil-applied and are applied priorto annual weed emergence. Trifluralin mustbe mechanically incorporated into the soil,whereas norflurazon may be mechanicallyincorporated or incorporated into the soilby sufficient overhead moisture. Clopyralid,glyphosate, and 2,4-D are post-emergenceherbicides applied to actively growingweeds. Clopyralid is selective on somebroadleaf weeds, particularly those in thesunflower, nightshade, pea, and smartweedfamilies. Clopyralid will control manyperennial weeds in those plant families. 2,4-D controls a broader spectrum of annualbroadleaf weeds and suppresses or controlsmany perennial broadleaf weeds found inhop yards. Glyphosate is non-selective andwill control both annual and perennialbroadleaf and grass weeds. However,glyphosate will kill or seriously injure hopplants if the allowed to contact hop foliage.Clethodim is selective in controlling mostannual and perennial grass weeds found inhop yards. Pelargonic acid, while registered,is not widely used.Paraquat effectively controls emergedweeds before hop emergence and issometimes tank-mixed with norflurazon.The two herbicides used as desiccantsare carfentrazone and paraquat; these areutilized to “burn back” basal leaves andsuckers, aiding in air circulation and theremoval of inoculum of the powdery anddowny mildew pathogens. Carfentrazoneis the most active product in burningback or desiccating hop foliage and willalso control some annual broadleaf weeds.Paraquat, although not as active as adesiccant, will control both annual grassand broadleaf weeds and provide top kill ofsome perennial weeds. Paraquat can be usedto control broadleaf weeds prior to binetraining.Specific herbicide use guidelines canbe found in the annually updated PacificNorthwest Weed Management Handbookavailable from the Idaho, Oregon, andWashington Extension Services and onlineat http://pnwpest/pnw/weeds. Table 3presents a summary of the effectiveness ofherbicides and cultural control practices forseveral common weeds in hop yards.73Figure 173. Horseweed plant.(R. Parker)Figure 174. Mature horseweedplants. (R. Parker)Figure 175. Horseweedinflorescence. (R. Parker)Figure 169. Henbit plant. (R. Parker)Figure 171. Henbit flower. (R. Parker)Figure 170. Kochia plant. See also matureplant, Figure 144. (R. Parker)Figure 172. Field bindweed flowers. See alsoplant, Figure 147. (R. Parker)Figure 176. Horseweed buds.(R. Parker)
17 74Figure 177. Common mallow. (R. Parker)Figure 181. Blue mustard plant. (R. Parker)Figure 178. Common purslane plants. (R. Parker)Figure 182. Blue mustard seed pods. (R. Parker)Figure 179. Common purslane flowers. (R. Parker)Figure 183. Severe blue mustard infestation. (R. Parker)Figure 180. Individual purslane plant.(S. Dewey, Utah State University, Bugwood.org)Figure 184. Common sunflower plants. (J. D. Byrd,Mississippi State University, Bugwood.org)
Table 3. Efficacy Ratings for Weed Management Tools in Hops75RATING SCALE: E = Excellent (90-100% control); G = Good (80-90% control); F = Fair (70-80% control); P = Poor (
76Calculating Treated Acres versus Sprayed AcresHerbicide rates given on an herbicide label are usually given in pounds, pints,or quarts per acre. An acre is equal to 43,560 square feet. Herbicides in hop yards,particularly foliage desiccant control products, frequently are applied in bands overthe row. Confusion commonly occurs in interpreting how much herbicide should beapplied when the herbicide is used to treat only a portion of each field. To illustratethis, if a 4-foot band is applied only over the row, 10,890 feet or 3,630 yards of rowwould have to be treated to equal one treated or broadcast sprayed acre. Assuminghops are planted in rows spaced 14 feet apart and the herbicide label indicates theherbicide is to be applied at 2 pints per acre, it would mean that 2 pints of herbicideis enough to treat 3.5 field acres of hops. Since 2 pints equal 32 fluid ounces, eachplanted acre of hops will receive only 9.14 fluid ounces of herbicide.Figure 185. Flixweedinflorescence. (R. Parker)Figure 188. Common groundsel.(R. Parker)Figure 191. Bermudagrass plants.(R. Parker)Figure 186. Flixweed plant inflower. (R. Parker)Figure 189. Mature inflorescence of Canadathistle. See also Figure 146. (R. Parker)Figure 192. Bermudagrass inflorescence.(R. Parker)Figure 187. Quackgrass.(S. Dewey, Utah StateUniversity, Bugwood.org)Figure 190. Quackgrass plant and rhizome.(S. Dewey, USU, Bugwood.org)Figure 193. Bermudagrass stolon.(R. Parker)
Table 4. Common Symptoms of Herbicide Injury on HopHerbicide use carries an inherent risk of crop damage. When using herbicides, read and carefullyfollow label instructions to minimize crop injury and maximize weed control. Table 4 presentsherbicide injury symptoms commonly observed on hop. Figures 194 to 204 display typical symptomsassociated with herbicides commonly used in hop yards.77Herbicide2,4-DcarfentrazoneclethodimclopyralidglyphosatenorflurazonparaquatSymptomsLeaf cupping usually will be exhibited on sprayed foliage and developingleaves may be malformed. Some stem twisting may be observed.Symptoms seldom occur above the zone of spray contact (Figs. 194, 195).Sprayed foliage will exhibit chlorotic (yellow) and necrotic (brown) stemtissue, with stem cracking reported on some hop varieties. Sprayedgrowing points are killed. Chlorotic and/or necrotic spotting will beobserved on leaves (Fig. 196) and stems (Fig 197) if the herbicide drifts.No symptoms have been observed on hops even at extremely high rates.The young growth of treated grasses will eventually turn yellow or brownand the leaves in the leaf whorl can be easily separated from the rest ofthe plant.Upward leaf cupping (Fig. 198) and some stem twisting sometimes will beexhibited, particularly on sprayed foliage. Leaf cupping is seldom observedabove the zone of spray contact (Fig. 199).Leaves may be chlorotic, necrotic, and malformed (Figs. 200, 201). Leafveins will often remain green while the areas between the leaf veins arechlorotic. Developing stems have shortened stem internodes (Fig. 201).Cones may be malformed. Plants are often severely injured or killed.Symptoms may persist into the next growing season.Leaf veins may be chlorotic to complete white (Fig. 202). The symptomsare usually temporary.Sprayed foliage will exhibit chlorotic and necrotic leaf tissue (Fig. 203).Stem cracking may be observed on some varieties. Sprayed growingpoints are killed. Chlorotic and/or necrotic spotting will be observed onleaves and stems if herbicide drifts (Fig. 204).Figure 195. Injury caused bydirect exposure of leaves to2,4-D. Leaves above the zoneof herbicide contact appearhealthy. (R. Parker)trifluralinRoot tips may be club-shaped and stems may emerge slowly if herbicidetreatedsoil is thrown over the root crowns when incorporating theherbicide. Occasionally stems are thickened where they emerge from thesoil.Figure 196. Yellowing andspotting of leaves caused bycarfentrazone. (D. H. Gent)Figure 194. Leaf cupping and stem twisting due to 2,4-D. Notice that upperleaves above the zone of herbicide contact appear healthy. (R. Parker)Figure 197. Necrotic spottingon stems due to carfentrazone.(D. H. Gent)
78Figure 198. Severe cupping of leaves due to high rate ofclopyralid applied to control Canada thistle. (D. H. Gent)Figure 199. Slight cupping of leaves due to clopyralid.Notice that leaf cupping is not apparent on leavesabove the zone of herbicide contact. (R. Parker)Figure 200. Severe yellowing, bleaching, and malformationof leaves on newly emerged shoots caused by a fallapplication of glyphosate on Willamette. (D. H. Gent)Figure 201. Yellowing and stunting of leaves and shoots causedby a fall application of glyphosate on Columbus. (M. E. Nelson)Figure 202. Yellowing of leaves causedby norflurazon. Affected plants generallyrecover. (R. Parker)Figure 203. Yellowing and death of leavescaused by paraquat applied for springpruning during cold weather. (D. H. Gent)Figure 204. Yellow spots on leavescaused by paraquat drift. (R. Parker)
Nutrient Management and ImbalancesDavid H. Gent79Several nutrients can occur at deficientor toxic levels in Pacific Northwest soils, andthe situation can be difficult to diagnose.Symptoms may be similar among variousconditions or may vary with the samecondition, depending on variety and theenvironment. General symptoms associatedwith nutrient imbalances are describedin this section, as well as known nutrientinteractions with diseases and arthropodpests. Fertilization recommendations varywidely in published literature, differingamong production regions, varieties,irrigation methods, soil pH, and seasons,therefore fertility recommendations are notprovided. Local experts should be consultedfor specific recommendations appropriate foryour hop yard.BoronBoron deficiency can result in delayedemergence of shoots, stunting, distortionand crinkling of young leaves (Fig. 205), andyellowing and death of shoot tips (Fig. 206).Leaves of affected plants may be small, brittle,and develop a fluffy-tipped appearance dueto impaired development of lobes (Fig. 207).Deficiencies are most common in acid soils.Boron deficiency has been suggested as acontributing factor for red crown rot.CalciumSymptoms of calcium deficiencydevelop first in young tissues and at growingpoints. Symptoms can be similar to borondeficiency, and may include yellowing ofgrowing points, reduced development ofleaves, and yellowing and death of leafmargins. Excessive calcium can interferewith uptake of other nutrients and inducedeficiencies in other positively charged ions(e.g., ammonium, magnesium, potassium).IronIron deficiency is first observed onyoung leaves as yellowing between veins,while veins remain green (Figs. 208 and209). Iron deficiency is most common inalkaline soils, although it can be inducedin highly acid soils (approximately pH 5.7or less) because of enhanced solubility anduptake of manganese.Figure 205. Stunting, distortion, and crinkling of young leavesassociated with boron deficiency. (J. Portner)Figures 206 and 207. Misshapen shoot tip and misshapen, “fluffy-tipped” leaf,both due to boron deficiency. (J. Portner, P. McGee)ABOVE: Figure 208. Close-up of yellowed leafdue to iron deficiency. (D. H. Gent)AT RIGHT: Figure 209. Yellowing of theyoungest leaves resulting from iron deficiency.Notice that symptoms are less pronounced onolder leaves. (J. Portner)
80Figure 210. Yellowing anddeath of tissue between leafveins caused by magnesiumdeficiency. (C. B. Skotland)Figure 211. Weak growthand yellowing of lower leavesassociated with nitrogendeficiency. (J. Portner)Figure 212. Weak growth andreduced side arm developmentassociated with zinc deficiency.(C. B. Skotland)MagnesiumSymptoms appear first on older leavesas yellowing between leaf veins, followedby death of these areas and defoliation (Fig.210). Magnesium deficiencies are mostcommon in acid soils or where excessivepotassium was applied.ManganeseManganese becomes limited inhigh-pH (alkaline) soils and can be presentat toxic levels under low-pH (acidic)conditions. Symptoms of manganesedeficiency are yellowing of young leaves andwhite speckling. Manganese accumulationin plant tissues increases at pH below 5.7,which interferes with iron uptake and caninduce an iron deficiency.MolybdenumMolybdenum deficiencies appearfirst in older leaves as yellowing and whitespeckling. Deficiencies have been reportedon hops grown in acidic soils (pH 5.7or less). In some plants, molybdenumdeficiency can be misdiagnosed as a nitrogendeficiency since affected plants can have ageneral yellowing.NitrogenSymptoms of nitrogen deficiencyinclude poor growth, stunting, and a generalyellowing of plants that is most pronouncedon older leaves (Fig. 211). Cones ofnitrogen-deficient plants are smallerthan cones on plants receiving adequatenitrogen. Excessive nitrogen fertilizationcan increase incidence of several diseasesand arthropod pests, including powderymildew, Verticillium wilt, spider mites,and hop aphid. Excessive nitrogen can alsoreduce alpha acid levels of cones. Effortsshould be taken to balance crop demandswith nitrogen inputs and to avoid overapplicationof nitrogen.The form of nitrogen can also affectcertain diseases. Fusarium canker appears tobe favored by ammonium-based nitrogenfertilizers, whereas nitrate-based fertilizersfavor Verticillium wilt. These interactionsprobably involve complex relationshipsbetween the fertilizer components, the soilpH, and the availability of other nutrients(i.e., manganese and zinc).PhosphorusSymptoms of deficiency first appearon lower leaves as down-curved, dark-greenleaves with a dull appearance. Bines are thinand weak. Affected cones may have a browndiscoloration. Studies in England indicatethat although symptoms may not be apparent,yield can decrease significantly when hopplants are deficient in phosphorous.Excessive phosphorous fertilizationmay induce zinc deficiencies, particularly inalkaline soils or soils otherwise marginally deficientin zinc. Phosphorous acid compoundsoften are applied as foliar fertilizers and cansuppress downy mildew, black root rot, and,to a lesser extent, powdery mildew.PotassiumPotassium deficiency results in weakbine growth and reduced burr formation.Symptoms develop first on older leaves,appearing as a bronzing between veins.These bronze areas become an ashy gray, andleaves may be shed prematurely. Excessivepotassium fertilization also may inducemagnesium deficiencies.SulfurDeficient plants have stunted growth,spindly stems, and yellowing of younger leaves.Sulfur is commonly deficient in the acidic,coarse-textured soils of western Oregon.ZincPlants deficient in zinc have weakgrowth, short lateral branches, and poorcone production (Fig. 212). Leaves are small,misshapen, yellow, curl upward, and canbecome brittle (Fig. 213). In severe casesaffected plants may die. Zinc deficienciesoccur frequently when soil pH is greater than7.5, which is common in central Washington.Zinc applications also can cause remission ofsymptoms associated with Apple mosaic virus.Figure 213. Cupped, brittle leaves caused byzinc deficiency. (J. Portner)
IndexAabamectin 5, 6, 7, 57action threshold 2Aeolothrips fasciatus 63Agrobacterium tumefaciens 27Alfalfa mosaic virus 33Alternaria 17Alternaria alternata 8Alternaria cone disorder 8American hop latent virus 28, 38Anystis mite 52Aphelenid spp. 62Aphelinus spp. 62aphid-feeding lady beetles 54-56Aphidius spp. 62Aphidoletes spp. 65Apple fruit crinkle viroid 32Apple mosaic virus 29Arabis mosaic virus 32Armillaria root rot 27assassin bugs 60Aster yellows phytoplasma 33BBacillus pumilus 3, 4, 5Bacillus thuringiensis 3, 4, 5, 43, 67bacterial diseases 27banded thrips 63bare-bine disease 33basal spikes, downy mildew 11bermudagrass 76bertha armyworm 42-43beta-cyfluthrin 4, 5bifenazate 5bifenthrin 5big-eyed bugs 59bindweed. See field bindweedbiocontrol. See biological controlbiological control 52augmentative vs. conservation 3conservation biological control,principles 52blackberry 69black hunter thrips 63black mold 27black root rot 9black vine weevil 44blue mustard 71, 74boron 79boscalid 5Botrytis cinerea 17Bracon spp. 62brown lacewings 66Ccalcium 79calculating treated acres vs. sprayed acres76California prionus beetle 36-37Canada thistle 69, 70, 76Candidatus Phytoplasma asteris 33carfentrazone 4, 5, 73, 75, 77carlavirus complex 28caterpillars 42Caution (signal word) 4chemical characteristics (aroma v. bittering)of hops 12chlordane 35Chrysopa spp. 66Chrysoperla spp. 66Cladosporium 27clethodim 4, 5, 73, 75, 77clopyralid 5, 73, 75, 77Coccinella septempunctata 56Coccinella transversoguttata 54common groundsel 70, 76common lambsquarter 69, 70cone 18, 20cone tip blight 16Coniothyrium minitans 23conservation biological control 52convergent lady beetles 54copper 5crab spiders 68crown gall 27cucumber beetle 51Cucumber mosaic virus 33cyfluthrin 5cymoxanil 4, 5Ddamsel bugs 60dance flies 64Danger (signal word) 4deficiency. See nutrient managementDeraeocoris brevis 59Deroceras reticulatum 50Diabrotica undecimpunctataundecimpunctata 51dicofol 5dimethomorph 5disease susceptibility of hop varieties 12downy mildew 8, 10-14susceptibility by variety 12E81economic injury level 2economic threshold 2eelworms 34efficacy ratings for weed management tools75entomopathogenic fungi 3ethoprop 4, 5, 37Ffamoxadone 5Feltiella sp. 65fenpyroximate 5field bindweed 69, 73flag shoots, powdery mildew 18flea beetle 49flixweed 71, 76folpet 4, 5fosetyl-al 5, 6, 14fungal diseases 8Fusarium avenaceum 16Fusarium canker 8, 15, 23Fusarium cone tip blight 16Fusarium crookwellense 16Fusarium sambucinum 15, 16GGalendromus occidentalis 3, 52garden symphylan 40-41Geocoris pallens 59glyphosate 5, 31, 72, 73, 75, 77, 78gray field slug 50gray mold 17green lacewings 66green peach aphid 28groundsel 70, 76
82HHarmonia axyridis 55Hemerobius spp. 66henbit 73heptachlor wilt 35herbicide injury symptoms 77herbicides 72, 73, 75calculating treated acres vs. sprayed acres76injury symptoms 77table of efficacy ratings 75Heterodera humuli 34heterorhabditid nematodes 45hexythiazox 5Hippodamia convergens 54honeydew 24, 38hop aphid 24, 28, 38-39hop cyst nematode 34hop flea beetle 49Hop latent viroid 32Hop latent virus 28hop looper 42-43Hop mosaic virus 28, 38Hop stunt viroid 30horseweed 73hover flies 64Humulus japonicus latent virus 33hunter thrips 63Hypera humuli 42IIPM. See integrated pest managementimidacloprid 5Insect Management Handbook 1insect pathogens 67integrated pest managementdefinition 1principles of 1-3International Organization for BiologicalControl 4IOBC rating system 4, 5iron 79Jjumping spiders 68Kkaolin 5kochia 69, 70, 73Llacewings 66lady beetles 54-57aphid-feeding 54-56convergent 54mite-feeding 57multicolored Asian 55seven-spot 56transverse 54lambsquarter 69, 70Leptothrips mali 63long-legged flies 65Lysiphlebus testaceipes 62MMacrosiphum euphorbiae 28magnesium 79, 80malathion 5mallow 74Mamestra configurata 42manganese 80mechanical controldefined 3mefenoxam 5metalaxyl 5, 6mineral/petroleum oil 5, 7minute pirate bugs 58mite-eating lady beetles 57mites 46predatory 3, 52-53twospotted spider mites 46-48mollusks 50molybdenum 80monitoring 3See also individual disease, arthropodand weed entries for monitoringinformation pertaining to specificpestsmorning glory. See field bindweedmoths 42multicolored Asian lady beetle 55myclobutanil 5Myzus persicae 28NNabis spp. 60naled 5natural enemy identification 2nematodes 3, 32, 34, 45hop cyst nematode 34Neoseiulus fallacis 3, 52nettlehead disease 33nitrogen 80norflurazon 5, 73, 75, 77, 78nutrient management 79boron 79calcium 79iron 79magnesium 80manganese 80molybdenum 80nitrogen 80phosphorous 80potassium 80sulfur 5, 47, 48, 80zinc 80OOlipidium brassicae 33Orius tristicolor 58Otiorhynchus ovatus 44Otiorhynchus rugosotriatus 44Otiorhynchus sulcatus 44
83PPacific Northwest Pest ManagementHandbooks 1paraquat 5, 73, 75, 77, 78parasitic flies 64tachinid flies 65parasitic wasps 62parasitoids 62pelargonic acid 5, 73pesticide resistance management 6pesticide “signal word” 4pesticide toxicity ratings 4, 5pest identification 2, 3Petunia asteroid mosaic virus 33Phacidiopycnis 22Phomopsis tuberivora 22Phorodon humuli 28, 38-39phosphorous 80phosphorous acid 5, 9, 80Phytophthora citricola 9phytoseiids 52pigweed 71, 72Pimpla sanguinipes 62Plant Disease Handbook 1Podosphaera macularis 18life cycle 19potassium 79, 80potato aphid 28powdery mildew 18-21susceptibility by variety 12Powell amaranth 72Praon spp. 62predatory arthropods activity chart 61predatory bugs 58-61assassin bugs 60big-eyed bugs 59damsel bugs 60minute pirate bugs 58predatory mirids 59predatory flies 64-65dance flies 64hover flies 64long-legged flies 65midges 65predatory midges 65predatory mirids 59predatory mites 52-53predatory thrips 63banded thrips 63black hunter thrips 63six-spotted thrips 63prickly lettuce 69, 72principles of integrated pest management1-3prionus beetle 36-37Prionus californicus 36-37pruningillustration of thorough vs. incomplete13qualityimpacts on downy mildew 14impacts on powdery mildew 20timingimpact on downy mildew 14Prunus necrotic ringspot virus 29Pseudoperonospora humuli 10life cycle 13Psylliodes punctulatus 49puncturevine 69, 71, 72purslane 71, 74pymetrozine 5, 39pyraclostrobin 5pyrethrin 3, 5Qquackgrass 76qualitative resistance 6quantitative resistance 6quinoxyfen 5Rred crown rot 22, 79redroot pigweed. See pigweedReduviidae 60resistance management 6Rhizoctonia solani 27root weevil 44-45rough strawberry root weevil 44Ssampling 3Sclerotinia sclerotiorum 23Sclerotinia wilt 23susceptible varieties 23Scolothrips sexmaculatus 63Scutigerella immaculata 40seven-spot lady beetle 56shepherd’s purse 70shoestring root rot. See Armillaria root rotshothole damage 49signal wordCaution, Danger, Warning 4six-spotted thrips 63slugs 50snakeflies 67sodium borate 5sooty mold 24, 38spider mites 46spiders 68spinosad 5spirodiclofen 5spirotetramat 5spiroxamine 5spotted cucumber beetle 51steinernematid nematodes 45Stethorus picipes 57Stethorus punctillum 57Strawberry latent ringspot virus 33strawberry root weevil 44sulfur 5, 47, 48, 80impacts of timing on spider mites 48sunflower 71, 74
84TTablesTable 1 5Table 2 12Table 3 75Table 4 77tachinid flies 65tackweed. See puncturevinetebuconazole 5Tetranychus urticae 46thiamethoxam 5thrips, predatory. See predatory thripsTobacco necrosis virus 33toxicity ratings for pesticides 4transverse lady beetles 54trap crops 49treated acres vs. sprayed acres, calculating76trifloxystrobin 5trifluralin 5, 73, 75, 77Trichogramma wasps 62TSSM 46-482,4-D 5, 73, 75, 77twospotted spider mite 46-48VVerticillium albo-atrum 25, 26, 34Verticillium dahliae 25, 26Verticillium wilt 25-26, 34, 35susceptibility by variety 12Vulgichneumon brevicinctor 62virus and viroid diseases 28-33WWarning (signal word) 4wasps, parasitic 62weed managementcalculating treated acres vs. sprayed acres76cover crops 72cultural tactics 72disking 72efficacy ratings for tools 75herbicides 72injury symptoms 77non-chemical tactics 72planning a program 70prevention 70tillage 72weed seed 70Weed Management Handbook 1weed seedlings, identifying 70, 71weevil 44western predatory mite 52western spotted cucumber beetle 51whirligig mite 52white mold 23. See Sclerotinia wiltsusceptible varieties 23XXiphinema diversicaudatum 32Zzinc 80