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CitrographSEPTEMBER/OCTOBER 2011 • Volume 2 • Number 6Cover photo by CRB Communications SpecialistLynn SandersonSUBSCRIPTIONSU.S.Single Copies: $1.501-Year Subscription: $15.002-Year Subscription: $28.00Send Subscription Requests To:Citrus Research BoardP.O. Box 230, Visalia, CA 93279PUBLICATION OFFICEP.O. Box 230Visalia, CA 93279Phone: 559-738-0246FAX: 559-738-0607Web Site: http://www.citrusresearch.orgMargie Davidian, EditorDr. MaryLou Polek, Chief Science EditorEDITORIAL BOARDTed BatkinRichard BennettFranco BernardiDr. Akif EskalenDr. Ben FaberLouise FisherJim GordenJudy BrentProduction Manager255 38th Avenue Suite PSt. Charles, IL 60174Phone: 630-462-2919FAX: 630-462-2924jbrent@farmprogress.comCanadian & Foreign:1-Year Subscription: $30.002-Year Subscription: $56.00SCIENCE REVIEW PANELDr. Mary Lu ArpaiaJames A. BethkeDr. Abhaya DandekarDr. Akif EskalenDr. Stephen GarnseyDr. Joseph SmilanickPRODUCTION INFORMATIONDale Hahn, DesignPhone: 630-462-2308dhahn@farmprogress.comADVERTISING INFORMATIONSandy CreightonCherie AverillAd Sales ManagerAd Sales RepresentativePhone: (559) 201-9225 Phone: 402-489-9334screighton@farmprogress.com caverill@farmprogress.comADVERTISING RATESRates B/W 2/C 4/CPage ...................................... $690 ....... $860 ......$10252/3 Page Vertical................. 540 ..........700 .......... 8751/2 Page Vert/Horiz............410 ......... 580 .......... 7501/3 Page Square/Vert........ 285 ......... 455 ..........6201/4 Page................................ 200 .........370 ..........5401/6 Page Vertical..................140 ..........310 ..........4801/8 Page Horizontal............140 ..........310 ..........480*Frequency discounts: 2X–5%, 3X–7%, 4X–10%Above rates are gross; 15% discount to recognized agencies.An Official Publication of the Citrus Research BoardIN THIS ISSUE4 Editorial6 About the Cover – The NAVEKprogram for exports to Korea9 Industry Views13 Special Section: Focus onfrost protection15 Frost definitions,history andforecasting24 Weather, energy,and passive frostprotection36 Active frost protection45 Citrus Roots: Fruit Frost WarningService50 Evaluation of disinfectants for citruspackinghousesCitrograph is published bimonthly by the Citrus Research Board, 323 W. Oak Street, Visalia, CA 93291. Citrograph is sentto all California citrus producers courtesy of the Citrus Research Board. If you are currently receiving multiple copies, or wouldlike to make a change in your Citrograph subscription, please contact the publication office (above, left).Every effort is made to ensure accuracy in articles published by Citrograph; however, the publishers assume no responsibilityfor losses sustained, allegedly resulting from following recommendations in this magazine. Consult your local authorities.The Citrus Research Board has not tested any of the products advertised in this publication, nor has it verified any of thestatements made in any of the advertisements. The Board does not warrant, expressly or implicitly, the fitness of any productadvertised or the suitability of any advice or statements contained herein.September/October 2011 Citrograph 3

EDITORIALBY TED A. BATKIN, President, Citrus Research BoardMary had a little lamb…Now is the time to keepour feet to the fire indeveloping plans forarea-wide treatmentshould the ACPpopulations continue topush in to the commercialproduction areas.We all remember this poem from our childhood. So why doesit apply to all of us today? To understand this we must finishthe stanza: ”And everywhere that Mary went, the lamb wassure to go..” Now substitute Asian citrus psyllid (ACP) for“Mary” and put huanglongbing (HLB) in for “lamb”. This has been the patternworldwide for the movement of the disease and the vector relationship.We know that ACP was found in Florida in the late 1990’s, but HLB was notdiscovered until 2005. That does not necessarily mean that HLB was not therebefore, but they did follow each other in the discovery chain. The same wastrue for Brazil and other parts of the world where this devastating disease/vector combination has moved.In a recently published article from FreshFruitPortal, an online newsletter,it was reported that Mexican officials are stating that HLB will not beeradicated from Colima and that the industry will just “have to learn to livewith citrus greening (HLB) disease”. The article goes on to say that, “Onceit arrives in an area this disease cannot be practically eradicated due to thewide range of hosts, so in the future we will have to continue with agriculturedespite its presence.” Mexican officials then discussed programs to provideassistance to growers to find alternative cropping systems, basically givingup on growing limes in the Colima region…So, what does this have to do with California??? The point is that we MUSTcontinue to keep a very strong program to try and reduce the populations ofACP and learn management techniques that will keep the California industryviable when “Mary’s little lamb” shows up. The Citrus Pest and DiseasePrevention Program is starting another year of providing funding support toCDFA to locate and treat ACP populations in Southern California. Additionally,the program is maintaining a vigorous trapping and detection programin the commercial citrus groves throughout the state.Now is the time to keep our feet to the fire in developing plans forarea-wide treatment should the ACP populations continue to push in tothe commercial production areas. Already we have ACP detections in fourproduction areas of California, all in the Southern California region, butit is not a stretch to understand that the entire state is at risk of the pestmoving anywhere it wants to go. We cannot wait to start the discussionof how to manage ACP in an area-wide manner. This has been the basisof the programs that are keeping citrus production viable in Florida, andTexas has already implemented full area-wide programs to reduce theACP population in the Lower Rio Grande Valley.The California citrus industry has a very long history of providing responsiveprograms to meet the threats of invasive pests. We must keep this positivemotion going throughout the industry and prepare for the next invasion withworkable, common-sense plans that can be implemented immediately whenthe threat materializes. Oh, by the way, did I mention area-wide controlof ACP… l4 Citrograph September/October 2011

ABOUT THE COVERNavel and Valencia exports to Korea (NAVEK):plans for the 2011/2012 harvest season and beyondJ. E. Adaskaveg and J. R. CranneyIntroductionThe Republic of South Korea is theCalifornia citrus industry’s second largestexport market for oranges valuedat $112 million annually. This marketis surpassed only by exports to Canadawhich total over $145 million each year.The industry must ensure that thesemarkets are open and free of trade barriers,or significant quantities of fruitmust be diverted to domestic or otheroverseas markets. The California industryhas maintained the Korean marketthrough its compliance with quarantinemeasures designed to prevent the introductionof Septoria citri to Korea.The Navel and Valencia Export toKorea (NAVEK) lab was funded for sixof the last seven years through a USDAForeign Agricultural Service (FAS)Technical Assistance for Specialty Crops(TASC) grant. This TASC program wasdeveloped to assist commodities inovercoming trade issues – for example,Septoria spot on oranges with our tradingpartner South Korea.In one year of the seven where theindustry did not receive TASC funding,the NAVEK program was financiallysupported by orange packinghouses,which paid a fee to cover the cost ofevaluating samples.TASC grant funds have been exhaustedand are no longer available.Nevertheless, the need to screen fruitfor Septoria still exists, and the industry,at its own expense, intends to continuefollowing this successful modelfor the upcoming 2011/2012 season.The NAVEK certification program hasbeen an integral component in helpingthe California industry overcome animportant trade barrier.Overview of NAVEKThe NAVEK program was establishedto help the California orangeindustry maintain trade with Korea bypreventing the export of fruit with Sep-6 Citrograph September/October 2011Map of Korea and location of theUnshu orange growing region (Chejuisland; arrow).toria spot, a fungal disease of orangescaused by Septoria citri. The diseaseoccurs mostly in the Central Valley ofCalifornia at a low incidence but hasnot been reported on the Unshu orangegrown in the Cheju province of Korea.Thus, any citrus fruit arriving in Koreais subject to quarantine laws that aim toprevent the entry and dissemination ofdiseased fruit.The NAVEK program was initiatedthrough negotiations betweenthe USDA Animal and Plant HealthPictured on the cover, left to right:Dr. Lingling Hou, Dr. Jim Adaskaveg,Dan Felts, and Dr. Helga Förster,University of California Riverside, inthe NAVEK lab at the Kearney AgriculturalResearch and ExtensionCenter in Parlier, CA, demonstratingthe qPCR method for detectingSeptoria spot of citrus.Inspection Service (APHIS) and the KoreanNational Plant Quarantine Service(NPQS) to comply with Septoria spotquarantine restrictions and allow continuedtrade between the United Statesand Korea. The agreement was part ofthe “Work Plan” or protocol to certifyentry of “Septoria spot free” orangesgrown in California, into Korea.NAVEK has been in operation forseven years with initial research andcertification goals including: 1) Developmentof information on forecastingthe risk of infection and on managingSeptoria spot on California oranges;2) Design of rapid detection methods;3) Identification of the temporal andgeographical occurrence of the diseasein California orange production areas;4) Certification of fruit lots destined forKorea as “Septoria spot free”; and 5)Diversion of disease-positive fruit lotsfrom the Korean market.NAVEK and APHISaccomplishmentsMajor accomplishments of theNAVEK program over the years include:• The development of efficient fruitsampling strategies based on growersand packers evaluating fruit in orangegroves and packinghouses, as well assubmitting suspect fruit to the NAVEKlab following standardized protocols.• A molecular-based pathogen identificationmethod based on DNA thatallows rapid diagnosis of the diseasewith minimal labor requirements.• An “online” electronic submissionform and confidential electronic reportingsystem.• A disease-risk forecasting programto determine the need and optimal timingof protective fungicide treatments.• The identification and registrationof new pre- and postharvest fungicidetreatments for the management of thedisease.

Septoria spot remains a quarantinedisease in Korea. At the beginning of the2010/2011 harvest season, negotiationsbetween USDA-APHIS and NPQSresulted in a new “Work Plan” outliningthe trade requirements betweenthe two countries. Because the UnitedStates does not incubate samples nor usemolecular detection of plant pathogensas an inspection criterion for any importedcommodity, APHIS successfullynegotiated the removal of fruit incubationmethods and the use of moleculardetection of Septoria spot on orangesby NPQS in Korea. Just as important,APHIS negotiated the continued use ofthe molecular detection method in theNAVEK certification program.Under the Work Plan now in effect,the inspection of oranges in Koreais based on the visual observation offruiting structures and spores of thepathogen on a standard 2% fruit samplecollected from fruit lots on arrival. Additionally,NAVEK is now a voluntaryprogram, based on following “GoodAgricultural Practices” (GAPS) thatinclude all lab and field practices developedby NAVEK.Jim AdaskavegJim CranneyThe scientific advances madethrough NAVEK have resulted in aprogram that is highly efficient andsuccessful, with minimal burden to theindustry while providing for the continuedexportation of oranges to Korea.Last year’s resultsForecasting Septoria spot: Followingthe mandatory first application ofcopper-zinc-lime between Oct. 15 andNov. 30, microclimates were monitored,and the Septoria risk assessment modelwas used as in previous years. Two criticalenvironmental parameters are accumulated:hours of temperatures lessthan 1°C (30.2°F) and precipitation.Low temperatures cause fruit rind injurynecessary for infection by the pathogen,while precipitation allows for sporulation,dissemination, germination, andfungal growth into fruit.By late December 2010, 10 to 16hours of temperatures below -1°C accumulated,and over 122 mm (4.8 inches)of precipitation occurred in Fresno andKern counties. A second application ofcopper-zinc-lime was called for based onthe Septoria spot risk assessment model.By mid-February 2011, an additional32 hours of temperatures below -1°Coccurred in Tulare County (and approximately10 additional hours in Fresno andKern counties), and total precipitationwas 191, 300, and 368 mm (7.5, 11.8, and14.4 inches) in Kern, Fresno, and Tularecounties, respectively. These were thehighest values recorded; thus, the riskfor disease was the highest in the sevenyears of the program.Septoria spot detections: As of July2011, the NAVEK lab processed 2,783navel and 276 Valencia fruit samples,for a total of 3,059 samples. For naveloranges, 11.2% or 313 of the samplesSeptember/October 2011 Citrograph 7

Citrograph asks:INDUSTRY VIEWS“In Florida, and also internationally in regionswith Asian citrus psyllid, the use of area-wide spray programs (pestcontrol districts, Citrus Health Management Areas in Florida, etc.), hasproven to be the only effective means of controlling the spread of ACP.Should we as an industry in California start addressing how to prepareourselves to utilize this mechanism for control?”The science increasingly points to the effectiveness of and necessity to implement anarea-wide approach to addressing ACP infestations. In California, at least in the commercialcitrus environment, we are still fortunate not to have to deal with major infestations(only four finds in commercial orchards to date in Ventura and Riverside counties). The“bad” neighbor concept can totally undermine the effectiveness of an ACP suppression/eradication program. One untreated block amongst treated blocks may render the wholeeffort ineffectual. Eradication of the psyllid is what should be the focus in our commercialgroves while we still deal with low levels of infestation. The focus in other industries suchas Florida obviously relates more towards suppression. The problem we currently havein California is that treatment is difficult to enforce. In Ventura, for instance, untreatedorchards exist adjacent to the find site some months after the find. Therefore, we needacceptable mechanisms to enforce where growers cannot be persuaded to treat – for thegreater good. There may be several possible vehicles, the nature of which won’t be debatedhere. One of the most obvious is using the Pest Control District powers. However, to be ableto enact some of these powers, there would need to be proactive planning and maybe eventweaking of current language to allow for enforcement, especially where compensationfor treatment is not considered. It would always be difficult to have area-wide treatmentin regions interspersed with residential development, even with the necessary powers todo so. By contrast, larger, contiguous areas, e.g. the Maricopa growing region, may evenorganize amongst themselves and put a treatment program in place which triggers as soonas a find is reported. Thus, defined sub-regions should get plans in place proactively. Youcan sense from the above that in my own mind I am not clear as to how we are going toenact a coordinated treatment program in the event of a find, for instance, in the CentralValley. We’d better get a handle on what we can do and cannot do, and sooner rather thanlater! – Etienne Rabe, Paramount CitrusCalifornia citrus growers should be optimistic. As a first positive step, California’scitrus industry has invested in the infrastructure necessary to monitor the ACP.As of yet, we have not detected the HLB disease; however, we do have the vector. Asa second step, it is now imperative that we establish an area-wide Asian citrus psyllidmanagement program in commercial citrus growing areas. The goal of an area-widespray program is to quickly suppress the psyllid population so that few psyllids willhave contact with an infected tree and therefore reduce the spread of the HLB. Acoordinated area-wide spray program will help reduce indiscriminate use of pesticides.Resistance is still a critical issue, and therefore, California’s citrus growers need toreserve their pesticide arsenals for the war against the psyllid. Maximizing the efficacyof pesticides will help provide good timing, prevent additional spray costs, andlimit environmental impact. This will help us buy time until we have better methodsfor detecting and controlling both the vector and the disease. With a coordinated,integrated approach, we will continue to learn more and continue to have success inour citrus production. – Joe Barcinas, Foothill Agricultural Research (FAR Inc.)September/October 2011 Citrograph 9

INDUSTRY VIEWSIf we’ve learned anything from Florida’s experience battling huanglongbing, it is thatthe Asian citrus psyllid must not be allowed to gain a foothold in our commercialcitrus growing areas. That means every ACP detection must be met with unequivocal andimmediate action. Unfortunately, we are already seeing reluctance from some growersto treating, based primarily on cost. That seems unbelievably shortsighted to those ofus who are in the citrus business for the long haul. Any delay in treatment, especiallyhere in Ventura County where Santa Ana winds will spread psyllids as effectively as aFlorida hurricane, could mean an out-of-control infestation before we know it. Whilethe County ag commissioners have the power to compel treatment through a nuisanceabatement action, it is cumbersome, time-consuming, and unlikely to be put in motionfor a single detection. Because ACP is so difficult for us to find with our currenttraps, a single specimen very likely means there’s a breeding population present. Asan industry, we must protect ourselves. Pest control districts can be an important toolin ensuring that timely and effective treatment of ACP occurs, but they take time toestablish. The earlier they are in place, the greater the chance we have of keeping ACPat bay. In the event that ACP populations become so great that individual treatmentsare not enough to control the spread, pest control districts could be invaluable in helpingorchestrate area-wide treatments. According to our friends in Florida, we have asingular opportunity to save our industry here in California. We mustn’t squander it.Pest control districts could be the tool that makes the difference. – J. Link Leavens,Leavens Ranches10 Citrograph September/October 2011

CPDPP asks for feedback on special survey siteThe Citrus Pest and Disease PreventionProgram is working to detectthe Asian citrus psyllid and HLB incommercial citrus groves across thestate with a crew of 23 trappers workingin 16 counties. With the help of theUniversity of California, the first phaseof constructing a special citrus invasivepest website has been completed, andindustry members are invited to providefeedback. Access will be limitedto legitimate stakeholders in the Californiacitrus industry.Go to https://crbcitrussurvey.uckac.edu/viewer. Through December, theusername is NewGrowerDemo, andthe password is RealEasy! For moreinformation, contact Richard Dunn,CPDPP/CRB data, information &management director, at rick@citrusresearch.orgor by phone at (559)738-0246.Map of Asian citrus psyllid detections in California and neighboring portions of Arizona and Mexico through 10/13/11.September/October 2011 Citrograph 11

The CRB Budget ProcessJim GordenJim Gorden is now theImmediate Past Chairmanof the Board, as he passedthe gavel to Earl Rutzwhen the new fiscal yearbegan October 1st. Gordenstarted his CRB service asan alternate in 1998-1999,and he continues as a veryactive member representingDistrict 1.As I write this, our Board is nearing the end of the annual budget process. Theadoption of a budget at the Annual Meeting on September 20th this yearcompletes a process that started in May. It is a long and deliberate processthat requires many hours of work by your representatives on the Board aswell as a number of research committee members who are not Board members. I will tryto give you a bit of insight into what is involved in getting to the point of the adoptionof the final budget.The process begins with the Research Priorities Committee which is made up of theBoard officers and chairs of the research committees as well as the chair of the CitrusClonal Protection Program Committee (CCPP). This committee reviews the establishedpriorities of the Board and writes a research funding proposal letter which is distributedto universities and research organizations across the country, inviting them to submitproposals to conduct research in these established priority areas.In July, the Committee meets again to hear, review and consider proposals whichhave been received for new research projects. Those favorably judged are heard again thefollowing month, along with proposals for continuing research; those sessions in Augustgo on for two-and-a-half days and are attended by the entire Board. Then, the variousresearch committees meet again in early September to review and discuss the merits of thevarious proposals in order to make recommendations to the full Board regarding funding.The Board hears the Research Committee recommendations at the Annual Meeting andmakes a final decision on which projects to fund.In the meantime, CRB staff members are working on developing an administrationbudget, the California Citrus Quality Council is developing the CCQC budget, and theCCPP Committee is working on their budget.At the Annual Meeting, all of these parts of the budget are pulled together to arriveat a Program funding requirement. After consideration for appropriate reserves, the totalbudget is established. The Board then approves the budget and assessment rate to providethe funds to continue the operation of the CRB Program for the next year.By the time you read this, the process will be complete and you will have received anotice of assessment for the coming year. It is a simple little document but one the makingof which required countless hours of reading, listening, discussing, and deliberationon the part of many people. lPEARSON REALTY Farm Sales Specialists for California’s Central Valley1.98 ± acs. Cold Storage Facility, Orange Cove (In Escrow) ........ $499,0004.06 ± acs. Exeter Cold Storage Facility .................................. $2,570,0006.48 ± acs. TurnKey Citrus Packing/Cold Storage .................... $2,200,0009.72 ± acs. Cutler Area Cold Storage Facility............................ $2,500,00010 ± acs. Lindsay Area Olives (SOLD)..........................................$110,00010.5 ± acs. Commercial Building Lemon Cove (In Escrow).......... $399,00015.98 ± acs. Lindsay Development Potential............................... $288,00020 ± acs. Easton Cherry Ranch...................................................$350,00020 ± acs. Lindsay Olives & House (SOLD)....................................$389,00024.47 ± acs. Woodlake Area Olives (In Escrow)........................... $242,00030.27 ± acs. Atwood Navel Ranch (In Escrow)............................ $239,00034.9 ± Porterville Navels.............................................................$472,50039.65 ± acs. Porterville Area Stonefruit (SOLD)........................... $440,00040.00 ± Cutler Table Grapes & Plums (Sale Pending)...................$500,00048.27 ± acs. Lindsay Olives (SOLD).............................................$525,00058.02 ± acs. Terra Bella Citrus, Seller Financing (In Escrow)........$575,00058.93 ± acs. Teapot Dome Navels & Open.................................. $500,00059.7 ± acs. Porterville Kiwi Ranch...............................................$805,95078.82 ± acs. Visalia Cherries, Shop Open (In Escrow)....................$895,00096.72 ± acs. Exeter Citrus Ranch.............................................$1,645,00080 ± acs. Cutler Area Open.........................................................$880,000110 ± acs. Orange Cove Pistachios, Almonds & Tangos............ $1,530,000116.94 ± acs. Cutler Area Open (SOLD)................................... $1,320,000160 ± acs. Visalia Area Navel & Open.......................................$1,950,000197.42 ± Cutler Navels, Table Grapes.......................................$3,150,000241.65 ± acs. Nice Porterville Cattle Ranch/custom home.........$1,300,000606.2 acs Stanislaus County (SOLD).........................................$4,200,000For Brochure Contact:Roy Pennebaker #0845764 (559)737-0084 orMatt McEwen #01246750 (559)280-0015 • www.citrusboys.com12 Citrograph September/October 2011

WILLOWOOD FrOstguarDFROSt pROtectANtPRIMARY BEnEFITs• Reduces frost damage• Lasts 7 to 21 days• excellent crop safety• proven frost protectant• University tested• May be applied byground or air• consult the label forspecific crop instructionssECOndARY BEnEFITs• On certain crops,Willowood Frostguardhas been shown toprovide secondarybenefits such as:• a reduction in icemarkings• a more rapidregrowth rate aftertreatmentWILLOWOOd FROsTguARdis a spray concentrate of antifrostmaterials which can beapplied prior to anticipated frostconditions.every year, hundreds ofmillions of dollars in crop lossesare sustained worldwide dueto killing freezes... WillowoodFrostguard can give growersextra insurance that theprotection of a few degrees canafford.For more information on Willowood USA products or tofind a distributor near you contact us at:CORPORATE OFFICE:1600 NW Garden Valley Blvd #120 • Roseburg, OR 97471541-679-9963 • Fax: 541-679-4650www.WillowoodUSA.comALWAYS ReAD AND FOLLOW LABeL DIRectIONS • © 2011 Willowood USA

no inversion, then most active frost protection methods areless effective. In fact, under strong wind conditions, activeprotection can sometimes cause more damage than good.If the temperatures near the surface fall below acritical damage temperature and there is little or noinversion, then most active frost protection methodsare less effective. In fact, under strong wind conditions,active protection can sometimes cause more damagethan good.Radiation frostRadiation frost — or simply “frost” — is a common occurrencein California. Frost events are characterized by clearskies, calm winds, and temperature inversions. Frost eventsoccur because of heat losses in the form of radiant energy.Under clear, nighttime skies, more heat is radiated awayfrom an orchard than it receives, so the temperature drops.The temperature falls faster near the ground or orchard floorcausing a temperature inversion to form – i.e., temperatureincreases with height above the ground.If you measure high enough, the temperature will reach thepoint where it begins to decrease with height, known as a lapsecondition. The level where the temperature profile changesfrom an inversion to a lapse condition is called the ceiling. Aweak inversion (high ceiling) occurs when the temperaturesaloft are only slightly higher than near the surface. Whenthere is a strong inversion (low ceiling), temperature increasesrapidly with height. Most frost protection methods are moreeffective during low ceiling, strong inversion conditions.In an inversion, temperature increases with heightup to a point. The ceiling of an inversion is the heightwhere temperature begins to decrease with increasingheight. This decrease with additional height is knownas a lapse condition.Frost sensitivityCitrus plants are indigenous to the humid tropical regionsof China, the Southeastern Asian countries including theFig. 1. Typical air temperature profiles with height for advection freeze andradiation frost nights.16 Citrograph September/October 2011western border areas of India and Pakistan, and the islandsof the Philippines and Indonesia. Cultivation of citrus wasintroduced into the West Indies by Columbus, and it laterspread to Florida and eventually to California.In the tropical citrus producing regions, citrus grows continuouslywhere warm weather prevails throughout the year,and frost injury is not a problem. In California, however, citrusis grown under a variety of weather conditions ranging froma moderate coastal climate to warm inland valleys to hot drydesert conditions, and intermittent frost events can sometimescause severe damage. Frost protection methods are used toavoid or ameliorate freezing temperatures that can lead toinjury and reduced yield.Plants are not damaged by freezing temperature but byice formation inside plant tissues. Ice crystals, which are largerelative to plant cells, form in the space between cells. Icecrystals grow by drawing water out of the cells leading to dehydrationof the cell. Subsequently, when the ice melts, the cellwall is damaged. Thus, anything that reduces the chance of iceformation inside the plant tissue helps to avoid freeze injury.Also, any factors that resist cell dehydration help theplants to tolerate ice formation and avoid injury. It is wellTable 1. Percentage offruit frozen for differenttime periods at 25°F incontrolled chambers.Time (hours) Damage (%)0.5 – 1.0 52.0 – 3.0 35 – 404.0 – 5.0 55 – 606.0 – 7.0 65 – 708.5 80Source: Nov. 1980 “CitrusNotes”, UCCE Tulare County.Based on studies conducted inFlorida.known that drought-tolerantplants also tend to be tolerantof freeze injury because thoseplants tend to have more solublesolids (sugars) in their cells,and more solids work againstdehydration. Thus, plant tissuethat accumulates soluble solidsin the hardening process (i.e.when plants are exposed to coldtemperature) has more time toaccumulate solutes and tends tobe freeze-injury tolerant.The longer the temperatureis below the critical temperature,the more likely that ice crystals will form and cause damage.Florida studies have reported that the number of frozenfruit increases with the time the temperature remains belowthe critical temperature (Table 1).Fruit damage can occur in the peel or thepulp. Peel damage occurs as moisture on thesurface freezes. Following the frost event, thedamaged area collapses and is invaded by decayorganisms followed by premature fruit drop.Note that waterspots can freezeon the peel whentemperatures areabove 32°F (0°C)when the dew pointtemperature is low.If the fruit is wetgoing into a lightfrost event night,operating windFig. 2. Ice marking onlemon peel.machines to dry the fruit during the day couldreduce peel damage (Figure 2). During a hardfrost or freeze, juice moves from the juice vesiclesand into the peel. Later, this water evaporates

to the atmosphere, the vesicles dry and collapse, and crystals(hesperidin) may form in 5-10 days, giving the fruit an offflavor(Figure 3).On frost nights,if the temperatureis low enough, extracellularwater freezes,drawing waterout of the plant cells.Fig. 3. Drying of vesicles followingfrost damage.If cell desiccation islimited, the waterwill move back intothe cells as temperatures rise the next morning without causingdamage. During this process the leaves will take on a blackwater-soaked appearance, but they regain a normal appearanceas warming takes place. If desiccation is severe, cell walldamage causes cell death.Either ice marking of the peel or internal damage of pulpcan result in fruit loss as a packable unit. When frost eventsoccur, packing houses and regulatory organizations such asthe agricultural commissioner’s office initiate an intensive fruitinspection program to examine each lot of fruit harvested forthe presence of damage.Scion varieties vary in their sensitivity to frost damage.Based on records of severe frost events in Texas and California,oranges are the most frost tolerant and limes are the mostsensitive. Tangelos, grapefruit, and lemons are moderatelytolerant. Mandarins exhibit variable degrees of injury; Earlymaturing varieties should be planted in frost prone areas. FruitSpot damage, 1990 Freeze. Photo by Neil O’Connellshould be harvested prior to the frost season (i.e. Novemberto early December).Rootstock has an influence on frost tolerance. As earlyas 1911, it was known that trifoliate orange rootstock improvedfrost tolerance. Navel oranges are more frost hardySeptember/October 2011 Citrograph 17

when grown on trifoliate rootstock than on sweet orangerootstock. Rough lemon was the most susceptible rootstock,sweet orange was less tender, sour orange was fairly hardy,and trifoliate was very frost resistant. From the 1963 freeze inFlorida, Cleopatra mandarin and sour orange were the mostresistant rootstocks. During the severe 1990 California freezeevent, trees on sour orange and trifoliate suffered the leastamount of damage whereas trees on rough lemon rootstsocksuffered the most.Hardening occurs when temperatures decline in thefall and the physiological activity level of the tree begins todrop. In this lowered state of activity, the tree is “hardened”and less susceptible to potentially damaging temperatures.Generally, daytime temperatures below 60°F (15.6°C) andnighttime temperatures below 40°F (4.4°C) will harden thetrees. This tolerance is lost following a few days of warmerweather.Late-season pruning tends to maintain a higher level ofphysiological activity as the trees enter winter; therefore, pruningactivity should be completed well in advance of the frostseason. For example, mature Valencia trees topped in Octoberexperienced severe splits in eight-inch scaffold branches duringthe 1990 freeze in the San Joaquin Valley. The applicationof pesticide oils to the trees can exacerbate frost injury, soavoid applications shortly before the frost season.Critical damage temperatures for citrus are related to:• Scion variety• Rootstock variety• Physiological activity• Maturity of foliage, fruit• Time of pruning• Age of treeCritical damage temperatures for citrus are related notonly to scion and rootstock variety but also to the maturity offoliage and fruit. Mature citrus leaves can tolerate 23° to 29°F(-6.1° to -1.7°C), and dormant wood will stand 20°F (-6.7°C)for up to four hours. Immature feather growth can be damagedon a night with a low temperature as high as 30°F (-1.1°C).Young orchards cool more quickly and the trees experiencelower temperatures for a longer duration than large trees ina mature orchard. Therefore, active frost protection methodsshould be started earlier in young orchards. Fruit that is moremature and higher in soluble solids (sugars) will withstandlower temperature as will larger fruit and fruit with a thickerpeel. Critical damage temperatures for citrus fruits are listedin Table 2.Table 2. Fruit temperatures at which freezing begins*Temperature (°F)Green oranges 28.5 to 29.5Half-ripe oranges, grapefruit and mandarins 28.0 to 29.0Ripe oranges, grapefruit and mandarins 27.0 to 28.0Button lemons (up to ½-inch diameter) 29.5 to 30.5Tree-ripe lemons 29.5 to 30.5Green lemons (larger than ½-inch diameter) 28.5 to 29.5Buds and blossoms 27.0*Data are from the former US Weather Bureau - Fruit Frost Service.History of California frost eventsFrequencyRadiation frost conditions are common in California,occurring on several nights in a typical winter - particularlyin inland and desertvalleys. Freeze (or advectionfrost) eventsare less common butfar more damaging. Onaverage, major freezesoccur every 10 to 20years with the most recentin December 1990.The most recent severeradiation frost eventswere in 1998 and 2007.Table 3. Details on major citrusfrost events in California.Year Estimated fruit damage TulareCounty only*1990 $281,000,000 **1998 $291,584,1332007 $418,547,000* Source: Tulare County AgriculturalCommissioner.** Estimates of carry-over damage into1991 not available.Losses due to these events are illustrated in Table 3 withinformation pertaining to Tulare County.Post-freeze remedial actionFollowing a frost event, proper management is required torecover from the damage. The management includes: fruit salvage,care for damaged trees, and modified cultural practices.Fruit salvage: Generally, removing frozen fruit is notcost-effective unless there is some economic value. Damagedlemons and navels will substantially drop fruit on their own.Valencias and 2,4-D treated navels may drop fruit but over alonger period time. Decisions on such removal may involveconsideration of the aggravation of red scale control from thepresence of scale-infested fruit left on the tree or the interferenceof the frozen fruit with the new crop harvest. Removalof dropped fruit from the orchard floor is recommended tominimize the spread of fungal pathogens such as Septoriaand Phytophthora.Care of frost/freeze damaged trees: It is impossible to determinethe full extent of severe frost/freeze injury for severalmonths; therefore, avoid pruning until 6-12 months after thefrost/freeze event. In severe cases, dieback may continue forthe entire season and in subsequent years. Early pruning oftenleaves limbs that continue to die back and/or the removal ofsome limbs that could recover. Early pruned trees do not recoveras quickly as trees pruned later. Management dependson the severity of damage as described below.1. Severe damage to canopy including frameworkbranches. Defoliation of the entire tree is likely. The recommendationis to delay pruning to allow the tree to define thelimit of damage. Topping and hedging allows for rapid andrelatively inexpensive removal of damaged wood. If necessary,hand pruning allows for selective removal of damagedbranches for the retraining of framework branches.2. Severe damage where the top and crown limbs are killedbut the trunk shows little injury. No action is needed untilthe full extent of injury is known (usually after midsummer).Remove the entire top of the tree – cutting below all largeareas of injured bark. Numerous sprouts on the trunk commonlyappear by late summer. New heads of trees will developfrom these sprouts. Select the uppermost good sprout and cutoff the trunk just above this sprout. Slope the cut downwardaway from the sprout. Then, choose 2-3 other sprouts properlyspaced to form a new head and favor their growth by pinchingback any sprouts that crowd them. Leave all formed sprouts18 Citrograph September/October 2011

until a balance between root and top is established. Graduallyremove unnecessary sprouts.3. Medium damage where a considerable part of the top iskilled but the trunk and main crown limbs show little damage.Do no pruning for several months until the full extentof damage is visible. Save as much framework as possible.Cut below all serious bark injuries. When injured limbs areremoved, cut back to the best available strong new shoots. Insome cases, control the distribution of the framework branchesusing a light pruning during the first season. However, nothingis lost by delaying pruning a full year. After injured brancheshave been cut to new leaders, further pruning consists ofgradual thinning of excessive sprouts over a period of years.Otherwise, sprouts will crowd and interfere with the growthand branching of the leaders forming the new framework.4. Light damage where only foliage and small twigs aredamaged. No special treatment is required when light damageoccurs.Cultural operations: With canopy reduction from defoliationand dieback of branches, consider the following adjustmentsin cultural operations:1. Irrigation. Water requirements of the tree are oftenreduced if many branches are lost. Revise your evapotranspiration(ET) estimates based upon the canopy size.2. Fertilization. Adjust applications downward becausethe canopy is rebuilding. Consider the application of zinc andPACIFIC DISTRIBUTING, INCDistributor forOrchard-Rite®wind machines forfrost protection &Tropic Breeze®original partsSalesServiceNewUsedPortableStationary24 HourEmergencyService559-564-3114Woodlake, CAwww.orchard-rite.comRandy Quenzer, Sales559-805-8254randyquenzer@pdi-wind.comJeff Thorning, Sales559-972-9937jeffthorning@pdi-wind.comSeptember/October 2011 Citrograph 19

foliar applications of Lo-biuret urea to the new flush.3. Pest Management. Because of canopy regrowth, it isnecessary to monitor thrips activity for damage to new flushgrowth.4. Weed Management. Minimize weed growth, but carefullycheck herbicide labels for use on stressed trees.5. Survey of Damage following 1990 freeze. A survey ofgrowers by UC Cooperative Extension following the freezedeveloped the following information:a. Damage was least to trees on sour orange rootstockfollowed by trees on trifoliate. This confirmed what had beenobserved historically. Regrowth was also more rapid withthese rootstocks.b. If turned off, low volume irrigation systems froze andcould not be restarted.c. Tree damage was less where low volume systems wereturned on at the beginning and never shut down during thefreeze.d. Tree damage was less with low volume systems particularlywith mini-sprinklers rather than with furrow irrigation.e. With little to no inversion, some wind machines werenot used after the first night of protection. Estimates of thesalvageable crop remaining after the first night did not justifythe operation of wind machines without an inversion.Predicting and measuring temperaturePredicting when the temperature will fall to a critical valueis important for starting active frost protection methods. Inaddition, the duration of temperature below the critical valueis important for assessing potential damage. Starting at theproper temperature is important because it avoids losses resultingfrom starting too late, and it saves energy by reducingthe operation time when using various methods.The first place to start is to access your regional forecastfrom the National Weather Service (NWS) office. Dependingon the local NWS office, considerable information is availableon the forecast minimum temperature and hourly trends duringthe night. Internet links to regional NWS Office websitesare provided on the UCD website: http://biomet.ucdavis.eduunder the heading “Valuable Links”.After selecting your regional NWS Office link, click on“Weather Tables” and you can set up the ability to monitorseveral important weather forecast variables on an hourlyinterval. For example, you can see an hourly forecast oftemperature, dew point temperature, wind speed, etc. duringthe night. Private weather forecast services also disseminateregional forecasts on the Internet. While public and privateforecasts are useful, it is also a good idea to develop your ownsite-specific frost night temperature forecast.During a radiation frost event night, the temperaturetrend tends to follow a square root function. Assuming thatthe predicted minimum temperature (Tp) is correct and it occursat the end of the sunrise hour, the nighttime temperaturetrend from the temperature T 2 , which occurs two hours afterthe end of the sunset hour until the end of the sunrise houris estimated using equations 1 and 2.20 Citrograph September/October 2011T i T2 b i 2 Tp T2b n 2 (1)(2)

T ibIn equation 1, i =0 for the time at the end of the hourwhen sunset occurs, i=2 for two hours after i=0, and i=n forthe end of the hour when sunrise occurs the next morning.T i is the temperature at the end of the i th hour following thesunset hour.Figure 4 illustrates how the temperature trend modelworks using data from the University of California LindcoveResearch and Extension Center (LREC) during 22-23 December1990. The figure shows that sunset occurred at 5:08p.m. on 22 December and sunrise occurred at 7:26 a.m. on 23December. The end of the hour when sunset occurs is 6:00p.m. and the model begins two hours later at 8:00 p.m. TheobservedT2 b itemperature 2 (1) at 8:00 p.m. is T 2 =28.0°F. The predictionmodel Tp Tstops 2 at the end of the hour when sunrise occurs. Sincesunrise occurs at 7:26 a.m., predicted minimum temperaturewill n 2be reached(2)at 8:00 a.m. The National Weather Service(NWS) or a private forecast service forecasted the minimumtemperature (Tp=20.7) to occur at 8:00 a.m.. The value for bis calculated using equation 1:Tp T220.67 28.04 7.3equation 1: b 2.129n 2 14 2 3.464Then, the predicted temperature for each hour is calculatedusing equation 2. Table 4 shows the hourly temperaturetrend calculations for I = 2 to 14 using the data from Figure 4.Table 4. Calculations ofdata from Figure 4 and equation 2.i 2 , b i2,, b i 2 ,and T i using thei i 2 b i 2 Tii i 2 b i 2 TiF2 0.000 hr 0.000 28.04o F3 1.000 2 0.000 -2.129 0.000 25.91 28.044 1.414 3 1.000 -3.011 -2.129 25.03 25.915 1.732 4 1.414 -3.687 -3.011 24.35 25.036 2.000 5 1.732 -4.258 -3.687 23.78 24.357 2.236 6 2.000 -4.760 -4.258 23.28 23.788 2.449 -5.215 22.837 2.236 -4.760 23.289 2.646 -5.632 22.418 2.449 -5.215 22.839 2.646 -5.632 22.4110 2.828 -6.021 22.0211 3.000 -6.387 21.6612 3.162 -6.732 21.3113 3.317 -7.061 20.9814 3.464 -7.375 20.67hr10 2.828 -6.021 22.0211 3.000 -6.387 21.6612 3.162 -6.732 21.3113 3.317 -7.061 20.9814 3.464 -7.375 20.67value. Air temperature should be monitored in the orchardduring the night to accurately determine and update the timeto start active protection methods.Many types of thermometers and alarms are available foruse in frost protection. Each orchard should have a minimumtemperature-registeringthermometer mounted in a fruit-frostshelter. The temperature is read periodically during the nightfor decisions on active protection start-up and for updates onthe current air temperature while sprinklers, heaters, or windmachines are in operation. Minimum-registering thermometerswill also record the lowest temperature during the night.During the following day, the minimum temperature isrecorded and the thermometer is reset to make it ready forFig. 4. Observed and predicted hourly mean temperatures(°F) during a radiation frost on December 22-23, 1990 at theUniversity of California Lindcove Field Station.It is important to remember that this prediction methodwill work only during frosts with calm, clear nights duringwhich the temperature drops because of long waveband radiationlosses from the surface. It will not work in a locationsubject to micro-scale cold air drainage from nearby mountainvalleys or during freezes.Note that this prediction method is fairly accurate, but it isnot perfect. In Figure 4, the observed temperatures are alwaysslightly above the predicted temperatures, but the predictedtemperatures are above the observed temperatures on somefrost nights. Damage could result if you rely only on the temperatureprediction; the model should be used as a guide totell approximately when the temperature will reach a criticalIce marking on leaves, 1990 Freeze. Photo by Joe ConnellSeptember/October 2011 Citrograph 21

the following evening. Electronic recording devices, includingwireless sensors, are available for producing a record oftemperatures over time.Mobile thermometers are available for updating temperatureinformation and predictions during the night. Theseinclude hand-held units for orchard temperature, units formeasuring the internal temperature of the fruit and unitsthat can be mounted in vehicles for recording surroundingambient air temperature.Additional informationCooperative extension viticulture farm advisors developedsome video training units on frost protection for grapevines,but much of the material is also useful for citrus growers.There are four training units available in both English andSpanish. The training units include:• Active Frost Protection: Water• Active Frost Protection: Wind Machines• Passive Frost Protection• Methods of Measuring TemperatureNote that over-plant sprinklers are commonly used forgrapevine frost protection, but over-plant irrigation of citrusfor frost protection is not recommended due to limb breakage.The training units are available from the UC Davis Departmentof Land, Air and Water Resources website http://lawr.ucdavis.edu/ce_frost_protection.htm.All three authors are with the University of California.Neil O’Connell has been a UC Cooperative Extension farmadvisor in Tulare County since 1981, dealing exclusively withcitrus and avocados. In addition to his role as citrus farm advisor,O’Connell has collaborated on a number of field studieson frost protection including several projects with co-authorSnyder. Joseph H. Connell has worked with citrus for 33 yearsas a UCCE farm advisor in Fresno and Butte counties. Dr.Richard L. Snyder is a Biometeorology Specialist, UC CooperativeExtension, headquartered in the Department of Land,Air and Water Resources, UC Davis.ReferencesAllen, C.C. 1957. A simplified equation for minimum temperatureprediction. Mon. Wea. Rev. 85, pp. 119-120.Bagdonas, J.C., Georg, J.C., and Gerber, J.F. 1978. Techniquesof frost prediction and methods of frost and cold protection.Technical Note No. 157. World Meteorol. Org. WMONo. 487. Geneva, Switzerland, pp. 1-160.Hagood, L.B. 1967. An empirical method for forecastingradiation temperatures in the Lower Rio Grande Valley ofTexas, Southern Regional Technical Memo. No. 33, NationalWeather Service.Snyder, R.L. and J.P. de Melo-Abreu. 2005. Frost Protection:Fundamentals, Practice and Economics. Vol. 1. ISBN:92-5-105328-6. Food and Agriculture Organization of theUnited Nations, Rome, 223p.Young, F.D. 1920. Forecasting minimum temperatures inOregon and California. Mon. Wea. Rev., 16, Supplement, pp.53-60. lAg Crime:The Valley’s newest cash crop• Equipment• Copper wire• Pumps• Produce• Vehicles• ChemicalsPipkin Detective Agency is helping ag operations Valley-wideprotect their assets – and capture the thieves.We are the finest locally owned protection and investigationsagency in the Valley because we hire the best.Ag is your business.Catching thieves is ours.Call today to learn how we can help protect your assets.HQ: 4318 W. Mineral King, Visaliapipkindetectiveagency.com 559-622-8889theautomaticdefrosterfromWIND MACHINE LLC20513 Avenue 256Exeter, California 93221-9656telephone: 559.592.4256fax: 559.592.4194www.amarillowind.com• DependableYour best insurance against frost damage• Unbeatable ServiceAvailable 24 hours a day, every day!• CommittedExcellence built into every componentFrost protection is investment protection22 Citrograph September/October 2011

Cold air drainage dammed by Hwy. 65 in Central California, 1990 Freeze. Photo by Joe Connell.transfer mechanisms is extremely important for good frostprotection management.RadiationRadiation is electromagnetic energy that can transferthrough air or even empty space. All objects with a temperatureabove absolute zero radiate energy, and the amountof energy radiated depends on the 4 th power of the objecttemperature in Kelvin units. Note that Kelvin (K) units arethe same magnitude as degrees Celsius (°C), but 273 K= 0°C=32°F. Sunlight is a good example of radiation. Because thesun is very hot (about 6000 K), considerable energy is radiatedfrom the sun, and the Earth receives some of that energy.Radiation from the sun is called “solar” or “short waveband”radiation. The radiation received from the sun is the mainsource of energy for life on Earth.The Earth is much cooler than the sun (about 315 K), butobjects on Earth also radiate energy to their surroundingsdepending on the 4th power of their absolute temperatureand emissivity properties of the object.The ground, which is nearly a perfect emitter, at 32°Fwill emit about 315 W m -2 of energy to the sky. This is aboutequivalent to having three 100W light bulbs per square yardon the ground. Three 100W light bulbs per square yard is thesame as 14,520 100W light bulbs per acre, so there is clearlya lot of energy loss even at 32°F.Fortunately, the sky also has a temperature, and a clear skyemits energy downward at about 210 to 230 W m -2 when theground is about 32°F depending on humidity. When the skyis overcast, it is warmer than a clear sky, and the downwardradiation is even higher. Therefore, less radiation energy is loston a cloudy than a clear night. Radiation at Earth temperaturesis called “terrestrial” or “long waveband” radiation. Ifan object emits more radiation energy than it receives fromother sources, it will cool. Therefore, the ground cools at nightbecause the warmer surface emits more energy upwards thanthe clear sky emits downwards.ConductionConduction is heat transfer from one molecule to anadjacent molecule through a solid. A good example is thetransfer of heat through a metal rod. If one end of the metalSeptember/October 2011 Citrograph 25

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Ice marking on leaves, 1990 Freeze. Photo by Joe Connellsible” heat flux with the symbol “H”. The reason that theair temperature drops during the night is because the air istransferring heat to the surface to partially replace the surfaceenergy losses to net radiation. The idea behind most activemethods of frost protection is to try to replace the sensibleheat loss from the air by using water, heaters, wind machines,etc. On a clear, calm night, the downward sensible heat fluxis about 50 W m -2 , so replacing that amount of sensible heatloss would normally stop the air temperature from dropping.If the net radiation is less negative, the transfer of sensibleheat to the surface is reduced and the air will cool moreslowly. Similarly, if there is more ground heat flux to the surface,i.e., a more negative G, then there will be less transferof sensible heat from the air and the temperature will coolmore slowly.Thus, any weather or management factor thatreduces the magnitude of the net radiation loss or increasesthe magnitude of the negative ground heat flux is beneficialfor frost protection.Assuming no phase changes, e.g., dew or frost formation,the net radiation must equal the sum of the ground andsensible heat fluxes. The more negative the net radiation, themore downward sensible heat transfer is needed to replace thelost energy and the faster the air temperature drops. When itis clear, the sky is colder, the downward radiation is less, andthe net radiation is more negative; implying a bigger radiationloss of surface energy. Consequently, the surface temperaturedrops faster on a clear than a cloudy night.Table 2 shows a typical nighttime energy balance for arange of cloud cover conditions. During a typical radiationfrost night, the upward ground heat flux is generally about50% of the net radiation and the downward sensible heat fluxis also about 50% of the net radiation. However, improvingthe transfer and storage of soil heat can increase the upwardground heat flux and reduce the downward sensible heat flux.Under cloudy or foggy conditions, the downward radiationis increased because the temperature of the clouds or fog ishigher than for clear skies. This leads to more downward terrestrialradiation, less negative net radiation, and lower valuesfor the ground and sensible heat flux to the surface (Table 2).Thus, radiation frosts are less likely when skies are overcast.Table 2. Typical energy fluxes during nighttime for a range ofcloud cover conditions with a surface temperature of about32°F (0°C). The unit for energy transfer is watt per squaremeter (W m -2 ) or Joule per second per square meter (J s -1 m -2 ).Energy Transfer Method 0% Clouds 33% Clouds 67% Clouds 100% CloudsWatt per square meter (W m- 2 )Conduction (from the soil) -53 -28 -10 -1.5Convection (from the air) -53 -28 -10 -1.5Downward Radiation +209 +259 +295 +312Upward Radiation -315 -315 -315 - 315Net Radiation -106 -56 -20 -3Wind is a factor in frost protection because it mixes theair and increases the amount of energy transferred from theair to the crop. When wind speeds increase to more than 5mph (2.2 m s -1 ), the increased convective heat transfer is oftengreat enough to balance the radiation heat losses, and radiationfrosts are unlikely. Normally, when freezing temperaturesoccur under windy conditions, the event is an advection ratherthan a radiation frost event. Latent heat is only a factor whenwater is present, so it is generally ignored except when irrigationis used for frost protection.HumidityHumidity is an important factor in frost protection becauseof phase changes, which convert sensible to latent (evaporation)or latent to sensible (condensation) heat and becausemoist air absorbs more radiant energy. When the surfacetemperature drops to near the dew point temperature, con-28 Citrograph September/October 2011

September/October 2011 Citrograph 29

densation can occur releasing latent heat and reducing the rateof temperature drop. Also, air with high water vapor contentabsorbs more upward terrestrial radiation, and thus air withhigher humidity cools slower than drier air.Wet-bulb temperature: If water evaporates into theair and the only source of energy for the evaporationis sensible heat in the air, the sensible heat decreases, sothe temperature drops, and the water vapor content increasesdue to evaporation. When the air reaches 100%relative humidity, the temperature is at the wet-bulb.Dew point temperature: If the air is cooled withoutchanging the water vapor content of the air, the relativehumidity will increase. When it reaches 100% relativehumidity, the temperature of the cooled air will be atthe dew point.Cold, dry wind increases evaporation rates from wet surfacesand can cool wet plant parts to damaging temperatures.If an object is wetted and not rewetted during a frost night, itwill cool to approximately the wet-bulb temperature, which isthe temperature measured with a wet-bulb thermometer in apsychrometer (Figure 1). The wet-bulb temperature is alwaysbetween the air and dew point temperatures, and it falls loweras the dew point decreases. Even when the air temperatureis above 32°F (0°C), wet plants can have ice formation if thedew point is sufficiently low that the wet bulb temperature isbelow 32°F (0°C).Spot damage or “ice marking” on citrus and other fruit issometime attributed to wet spots on fruit that were cooled todamaging temperatures because evaporation caused coolingof the wet spots to the wet-bulb temperature.The dew point temperature is defined as the air temperatureat which the air becomes saturated with water vapor(reaches 100% relative humidity) when the air is cooled byremoving sensible heat without changing the water vaporcontent of the air. When the air temperature is at the dewpoint, the number of water molecules evaporating from apure, flat water surface is equal to the number condensingonto the surface.The dew point is important in meteorology because it isdirectly related to the amount of water vapor in the air andit can be used to determine other humidity variables (e.g.,vapor pressure, relative humidity, wet bulb temperature, andvapor pressure deficit) that are often used in agriculture. Inaddition, the dew point temperature is often used to predictthe next morning’s minimum temperature. Consequently, itis extremely important for frost protection.A simple method to measure the dew point temperatureinvolves cooling a surface until water vapor begins to condenseon the surface. This is the principle used in a chilled-mirrorhygrometer, which is used to measure the dew point. Unfortunately,a chilled mirror hygrometer uses complicated electronicsto measure the dew point temperature and thereforeit is expensive. A simple, inexpensive method involves usinga shiny can, a thermometer, and ice water as shown below.Dew point TemperatureSlowly add ice cubes to the water to lower the cantemperature. Stir the water with a thermometer whileadding the ice cubes to insure the same can andwater temperature. When condensation occurs,note the dew point temperature.Fig. 2. A simple method fordetermining dew point temperature.When the dew point is below 32°F,adding salt to the ice water canlower the temperature to identify thedew point temperature.During low dew point, freezing conditions, since an icewatermixture will only chill the can to 32°F, it is sometimesdifficult to get the water cold enough for condensation tooccur on the outside can surface. Adding salt to the ice-watermixture will help to melt the ice and cool the water to a lowertemperature with a potential minimum of approximately18°F. When the dew point temperature is well below freezing,sometimes white frost rather than dew will form on the outsideof the can. When this occurs, you have measured the “frostpoint” rather than the dew point temperature. For the samevapor pressure, the frost point temperature will be slightlyhigher than the dew point temperature. However, for mostagricultural operations, there is little difference and they canbe used interchangeably.Fig. 1. The upper instrument is an aspirated psychrometer andthe lower is a sling psychrometer. The cotton wick on a wetbulbthermometer is wetted with distilled water. The aspiratedpsychrometer blows air across the wet thermometer bulbwith a battery-powered fan inside the instrument. Swingingthe instrument ventilates the sling psychrometer. Whenventilated, the temperature of a wet-bulb thermometer dropsbecause evaporation removes heat from the thermometer.The wet-bulb temperature is noted when the temperature ofthe wet-bulb thermometer stops dropping.30 Citrograph September/October 2011Humidity conversionsThe equations for converting humidity expressions aretypically given using the metric system and degrees Celsius.To convert from degrees Fahrenheit (°F) to degrees Celsius(°C), use the following equation:o 5 oC F 32(1)9o 5Example: C 5 23 329

To convert from the dew point temperature (T d ) in °C toother expressions for humidity, first calculate the vapor pressure(e) expressed in kilopascals (kPa), where 1.0 kPa = 0.145psi (pounds per square inch). 17.27Tde 0.6108exp kPa (2) Td 237.3 where exp() is the exponential function in a computerprogram or e x on a calculator.Example: For T d =23 o F=-5 o 17.27( 5)C: e 0.4210.6108exp 5 237.3 Note that 1 Atmosphere of barometric pressure is thesame as 101.325 kPa or 14.7 psi. To calculate the saturationvapor pressure (e s ) in kPa at air temperature (T) in °C, usethe following equation: 17.27Te s 0.6108exp kPa (3)T 237.3 Example: For T=41 o F=5 o 17.27(5) C: e 0.872 0.6108exp 5237.3 At the saturation vapor pressure, the number of watermolecules vaporizing equals the number condensing onto aflat surface of pure water. The saturation vapor pressure isonly a function of temperature, and equation 3 is a simpleformula for calculating the saturation vapor pressure from theair temperature (T). Relative humidity (RH) is calculated as: e RH 100 % (4) e s 0.421Example: For e=0.421 kPa and e s =0.872 kPa RH 48.3 100 0.872 If RH and T are available, the vapor pressure (e) is calculatedusing T and equation 3 to determine e s and RH e e skPa (5) 100 48.3 Example: For RH=48.3% and e s =0.872 kPa e 0.421 0.872 100 The dew point temperature T d (°C) is calculated from thevapor pressure (e) in kPa by first calculating:ln e 0.6108b (6)17.27where ln() is the natural log function, which is found in computerprograms and on most calculators and then calculatingthe dew point as: b T d 237 .3o C (7)1b ln0.4210.6108Example: For e=0.421 kPa b 0.0215517.27 0.02155 and Td 5.0 237.31( 0.02155)Use Eq. 8 to convert from T d ( o C) to T d ( o F).o9F 5o C32(8)9F 41 5Example: For T=5 o oC: 5 32Protect your bottom line withcitrus crop insurance fromBuckman-Mitchell, Inc.Insurance & Financial ServicesBMI has insured farming operations like yoursfor over 95 years, and we have over 40 yearsof crop insurance experience. Let us maximizeyour hard-earned dollars with a cropinsurance program that meets your needs –that’s the Buckman-Mitchell difference.Contact Steve Chrismansteve@bminc.com559-635-5342500 N. Santa FeVisalia800-828-3795License # 0A96361# 0011334September/October 2011 Citrograph 31

1990 Freeze. Photo by Neil O’ConnellPhase changesFor phase changes from water vapor to liquid water andfrom liquid water to ice, latent heat is converted to sensibleheat and the temperature rises. For phase changes from iceto liquid water and from liquid to water vapor, sensible heatis removed from the air to break the hydrogen bonds. Thesensible heat is converted to chemical energy or “latent”heat as the water vapor content of the air increases and thetemperature decreases. The latent is converted back to sensibleheat when the water molecules condense out of the air.The energy needed to convert between the different phasesof water in both directions is given in Table 1. Phase changesare important when using water for frost protection.From Table 1, it is clear that cooling 1 gram of water from68°F to 32°F (20°C to 0°C) will release about 20 calories ofenergy and freezing it at 32°F (0°C) will convert about 80calories of energy from latent to sensible heat. Thus, coolingand freezing 1 gram of water will release 100 calories to warmthe environment. On the other hand, evaporating 1 gram ofwater at 32°F (0°C) will convert about 600 calories of energyfrom sensible to latent heat, which cools the environment.Consequently, to break even, one must cool and freeze aboutsix times more water than is evaporated:600 calories6 .20 80 caloriesFortunately, evaporation rates are low during frost nights,and sufficient water can usually be frozen to supply moreenergy than is lost to evaporation. A higher application rateis needed to compensate for greater evaporation on nightswith high wind speeds and low dew points.Ice nucleationWater melts but it does not necessarily freeze at 32°F(0ºC). For freezing to occur, either “homogeneous” or“heterogeneous” nucleation must occur. When the watertemperature is below 32°F (0°C), the energy is unstable andhomogeneous freezing can occur because agitation causesice crystals to form. As the super-cooled water temperaturedecreases, the energy state becomes increasingly unstable andfreezing is more likely to occur. Water can also freeze if icenucleation-active(INA) particles are introduced triggeringice crystal formation (heterogeneous nucleation). The mainsource of ice-nucleating materials on crop plants is bacteria,and they are most effective in the 23° to 32°F (-5º to 0ºC)temperature range. The potential for frost damage decreasesas the concentration of ice nucleating bacteria is reduced. Thismay be accomplished using copper bactericides (althoughsome INA bacteria are copper resistant) or by applyingcompetitive non-INA bacteria. While this method is knownto work, it is not widely used for frost protection of citrus,which harbors relatively low concentrations of INA bacteria.Air temperature and dew formationThere are actually more molecules of air in a cubic meterthan the number of stars that we know in the Universe. Inaddition, the air molecules commonly move at about thespeed of sound. Because they move fast, they often have32 Citrograph September/October 2011

collisions, so they generally don’t travel very far. When airmolecules strike your skin, they transfer energy to your skin,and you sense those collisions as heat, so this type of energyis called “sensible heat”. You don’t feel the air moleculesstriking your skin because they are extremely small. Theydo, however, make you feel warmer because there are a lotof them and they constantly impart small amounts of energyto your body.The sensible heat content of the air is quantified usinga thermometer to measure temperature. When the airtemperature rises, the air molecules move faster, more willstrike a thermometer, more energy is transferred, and thethermometer temperature reads higher. When the temperaturecools, the air molecule are moving slower, fewer willstrike the thermometer, and the thermometer reading drops.When the surface temperature cools until the air becomessaturated, dew will form. Water vapor molecules,like those in other gases, have a velocity near the speed ofsound, and they continually strike nearby surfaces. Whenthe surface air temperature is at the dew point, the samenumber of water molecules will condense onto a surface asevaporate from the surface. Dew forms when the number ofwater molecules striking the surface and forming hydrogenbonds with other water molecules is slightly greater thanthe number of molecules breaking hydrogen bonds andseparating off as a gas.Technically, the dew point is defined as the temperaturereached when air is cooled, without changing the water vaporcontent of the air, until the air becomes saturated with watervapor. Once the surface temperature reaches the dew pointand dew starts to form on the surface, a slower air temperaturedrop results as the latent heat changes to sensible heatand replaces some of the energy lost to radiation.Passive frost protection methodsSite selectionSite selection is the single most important frost protectiondecision. Since cold air is denser than warm air, duringradiation frost events it flows downhill and accumulates in lowspots. These low cold areas should be avoided when seekinga subtropical orchard site. The tops of hills are prone to frostdamage during advection frosts. In general, it is best to planton slopes where cold air can drain away from the orchard.Subtropical trees are best planted on south-facing slopeswhere the soil and the orchard can receive and store moredirect energy from sunlight. It is wise to plant rows in a downhilldirection to allow cold air to drain through the orchard.Cold air drains downhill much like water, so any vegetation,buildings, etc. that block the down slope flow of cold air andforce it to back up into the orchard will increase frost damagepotential. There are examples where berm walls, fences, etc.have been used to funnel cold air around orchards reducingthe potential frost damage.The most severe freezes often occur during micro-scaleadvection when cold air drains into an orchard. Cold air canaccumulate in canyons upslope from orchards when the cold airis prevented from draining into the orchard by prevailing winds.If these winds stop, the cold air can drain into the orchard andcause damage. These micro-advection freezes occur frequentlyin California and they cause considerable damage. Real-time800-992-2304TheSOURCEfor all yourcitrus treeneedsSuperCitrus TreesB&B Trees • SeedlingsStarter Trees/Citrus LinersRootstock SeedBudwood of all TypesW W W . C I T R U S T R E E S O U R C E . C O MSeptember/October 2011 Citrograph 33

1990 Freeze. Photo by Joe Connell.measurements in the upslope canyons can identify potentialproblems. In some cases, helicopters or some other method offrost protection can reduce or eliminate cold accumulation inthe upslope canyons and prevent damage in the orchards below.Investigation of a possible site for an orchard shouldinclude a review of any available temperature records duringfrost episodes and any records on the extent of damage.Temperature recording stations can be situated at the sitebeing considered during the winter to document minimumtemperatures and the durations. This information not onlyprovides spatial minimum temperature data but it can alsobe compared to a nearby reference weather recording stationto improve frost forecasting.Soil tillage and water contentHeat absorption and storage is enhanced by a firm, undisturbedsoil surface, so fall tillage should be avoided. If tillageis necessary, it should be done early enough to allow the soilto settle and firm up before the arrival of the frost season.Thermal conductivity and the heat content of soils are affectedgreatly by the soil water content. On a daily basis, heatis transferred into and out of approximately the top foot (0.3m) of soil. When the soil is near field capacity, heat transfer andstorage in the upper soil layer is better, so more heat is storedduring daylight for release during the night. Field capacity isthe water content after gravitational water has drained fromthe soil (usually 1-2 days after rainfall or irrigation).Considerable differences between thermal conductivityand heat capacity are observed between dry and moist soils. Ifthe soil water content is near field capacity, additional wettingof the soil is unwarranted. Wetting the soil to a depth belowone foot is unnecessary because temperature variation isinsignificant below that depth. On an annual basis, however,heat transfer below one foot is important and could affectfrost protection if a soil profile is dry for a long period of time.Therefore, wetting is prudent when the soil is dry for severalweeks prior to frost season.Ground cover and mulchesWhen grass or weeds are present in an orchard, sunlightis reflected from the surface and less energy is stored in thesoil. Therefore, the orchard is more prone to frost damage.Vegetative mulches usually reduce the transfer of heat intothe soil and hence make orchards more prone to damage.A typical cultural practice is for tree prunings to be stackedbetween the tree rows and shredded in place. Research in aKern County orchard during a frost episode where a thicklayer of shredded orchard prunings was present on the orchardfloor demonstrated that the mulch caused lower nighttimetemperatures than where the prunings had been removed.Large variations in ice nucleating bacteria concentrationshave been observed on different crops. The concentrations ofINA bacteria on citrus are low. However, the concentrationof ice nucleating bacteria on grass and weed ground coversis typically high. Therefore the presence of ground coverwithin orchards or cereal crops around orchards increasesthe concentration of INA bacteria and the freezing potential.CoversIn home orchards, covers are sometimes used to decreasethe net radiation and convection energy losses froma tree reducing the potential for frost damage.34 Citrograph September/October 2011

MulchesClear plastic mulches that increase heat transfer into thesoil typically improve heat storage and hence provide passivefrost protection. Black plastic mulch is less effective for frostprotection than clear plastic. Wetting the soil before coveringwith clear plastic provides the best heat storage and protection.If there is doubt about whether to keep or remove plasticmulch from an orchard, a simple test measuring the minimumsurface temperature on covered and non-covered ground willdetermine which is coldest.Tree wrapsOn young trees, tree wraps block light from reachingthe trunks, inhibit unwanted suckers from developing onthe trunk, and prevent sunburn during the summer. Theyalso protect the bark from sprays used in weed control, andprovide some protection from frost. Wraps are most effectiveduring short duration frost episodes. Florida researchers havereported temperature increases of 4° to 8°F (2.2° to 4.4°C)from using rigid polystyrene (thick-walled) foam, and 3° to6°F (1.7° to 3.3°C) for fiberglass and polyurethane wraps.Protection was less for rigid polystyrene (thin-walled) andclosed-cell polyethylene foam.SummaryThis article discusses the basic concepts of weather andenergy balance, which are important for frost protection,and provides some guidelines on passive methods of frostprotection. Passive methods, including site selection, soilwater management, ground cover and mulch management,tree wraps, etc. are discussed.In general, choosing orchard sites that are less prone tofreezing temperature is perhaps the best protection method.Removing objects that block cold air drainage is extremelyimportant for existing orchards. The use of fences and bermwalls to control cold air drainage can be beneficial in somelocations depending on topography. If the soil is dry, then wettingthe upper one foot of soil can improve soil heat storageand can provide protection. The soil should be wetted a fewdays prior to an expected frost event to improve daytime heatstorage. In most cases, ground covers increase the potentialfor frost damage and should be removed at least during frostseason. Tree wraps can provide protection for young trees ifthe right materials and procedures are used.Dr. Richard L. Snyder is a Biometeorology Specialist withUC Cooperative Extension, based in the Land, Air and WaterResources Department at UC Davis. Since joining the Universityin 1980 with a Ph.D. in agricultural climatology, hehas focused much of his research on freeze and frost protectionand is regarded internationally as a leading authority on thesubject. Neil O’Connell is the UCCE citrus and avocado farmadvisor in Tulare County, and Joseph Connell is the UCCEcitrus farm advisor in Butte County.ReferencesSnyder, R.L. and J.P. de Melo-Abreu. 2005. Frost Protection:Fundamentals, Practice and Economics. Vol. 1. ISBN:92-5-105328-6. Food and Agriculture Organization of theUnited Nations, Rome, 223p. lSeptember/October 2011 Citrograph 35

Active frostprotectionJoseph Connell, Neil O’Connell and Richard L. SnyderIntroductionIn this issue of Citrograph, types of frost events and thesensitivity of citrus to frost damage are presented in the article“Frost definitions, history, and forecasting”, and passiveprotection methods are discussed in the article “Weather,energy, and passive frost protection”.Freeze or advection frost events are associated with freezingtemperatures and wind speeds greater than 5 mph. Theseare rare in California, and most active protection methodsare relatively ineffective during a freeze event, so this articlewill emphasize methods to protect against “radiation frost”or simply “frost” events.Note that the damage to plants from freezing temperaturesis caused by ice formation and dehydration of plant cells thatleads to cell wall damage regardless of whether the event isa frost or a freeze. Since freeze events are rare in California,plant injury from ice formation will be called “frost” damagein this article, and protection methods will be called “frost”protection.Frost events, which are common in California, occur onnights with freezing temperature, calm winds, clear skies, andinversions where temperature increases with height above theground. Passive protection involves cultural management thatis done in advance of a frost night that reduces the need foractive protection and reduces the possibility of injury due tofreezing temperature. It includes ground cover, soil, water, andice nucleation active (INA) bacteria management as well asgood site selection, insulation and covers.In this article, we discuss the use of energy-intensivemethods, e.g., wind machines, irrigation, etc., to replace sensibleheat losses resulting from a net loss of radiation energyduring the night.Active frost protection methodsWind machinesWind machines provide protection by increasing themixing of warmer air aloft with cold air near the surface, resultingin an increase in the orchard temperature (Figure 1).The amount of protection afforded depends on the inversionstrength that would occur without using the wind machines.In general, the temperature increase in an orchard, afterstarting the machines fans, is about equal to the sum of 1/3 ofthe difference between the 5-foot and 40-foot (1.5 and 12.2m) measured temperatures. For example, if the temperatureat 40 feet is 9°F higher than the 5-foot temperature, the 5-footPhoto by Randall Priester,used by permission ofPacific Distributing.36 Citrograph September/October 2011

temperature should rise by about 3°F=9°F/3 after startingthe fans. When using wind machines for frost protection, thefans should be started when the temperature measured at the5-foot height reaches the critical damage temperature.In general, one wind machine is recommended for each 10acres, but the number can be increased in strongly frost-proneorchards. The effectiveness of the wind machines increaseswhen there is a strong inversion with a low ceiling such thatthe temperature increases rapidly with height.In an inversion, temperatures increase with height upto a point. The ceiling of an inversion is the height wheretemperatures begin to decrease as you go even higher.It is important to test your orchard for inversion strengthbefore investing in wind machines. This can be done by mountingelectronic thermometers on a pole in the orchard at 5 and40 feet in height. Keep temperature records at the two heightson clear, calm nights. The inversion strength is the differencein the upper and lower temperature; the larger the difference,the stronger the inversion. Helium-filled balloons canbe used to measure the upper temperature. Tie an electronicthermometer or a minimum recording thermometer to theballoon and raise it to about 40 feet in height. Compare thetemperature difference between 5 and 40 feet to determinethe inversion strength.With Wind MachineNo fanHeight (m)1612840483624120Height (ft)-5 0 5 10Temperature (°C)Fig. 1. This figure shows the effect of wind machine operationon the temperature profile during a radiation frost event.HelicoptersIn an inversion, helicopters push warm air aloft down tothe surface. If there is little or no inversion, helicopters areless effective. The area covered by a single helicopter dependson the helicopter size, weight, and weather conditions. Pilotsload helicopter spray tanks with water to increase the weightand provide more thrust. Under severe freezes with a highinversion, one helicopter can fly above another to enhancethe downward heat transfer.A helicopter should pass over the entire orchard every 30minutes during mild frosts and more often during severe frosts.September/October 2011 Citrograph 37

Thermostat-controlled lights at the top of the canopy are usedto help pilots see where passes are needed. Downward heattransfer continues to move down-slope after reaching thesurface, so concentrate flying on the upper end of orchardson slopes. The pilot should monitor temperature on the helicopterand change altitude until the highest temperature isobserved to determine the best flight altitude. A ground crewshould monitor orchard temperature and communicate withthe pilot where flights are needed. Lights around the orchardperimeter are beneficial to help the pilot. Flights are stoppedwhen the air temperature upwind from the orchard has risenabove the critical damage temperature.SprinklersWhen sprinklers are first started, the air temperaturecan initially drop to as low as the wet-bulb temperature, andwet plant surface temperatures can drop to the wet-bulbtemperature until sufficient water freezes to raise the wetsurfacetemperature to around 32°F. When using the systemfor frost protection, the sprinklers should be started whenthe wet-bulb temperature (T w ) is above the critical damagetemperature (T c ) on the first night of a series of frostnights. If freezing conditions persist for several nights, thesprinklers can be stopped during the day if T w >32°F (0°C),but they must be started when T w >32 on nights following theinitial frost night. If the sprinklers are wet and the wet-bulbtemperature is below 32°F (0°C), the heads can freeze upand become nonfunctional. Do not turn off the sprinklersduring the day unless the sun is shining on the crop and thewet-bulb temperature exceeds 32°F (0°C).The wet-bulb temperature can be measured directly witha psychrometer (see Figure 1 on p. 30). For direct wet-bulbtemperature measurements, a cotton wick on the wet-bulbthermometer is wetted with distilled or de-ionized waterand air is moved across the wick until the temperature of thewet-bulb thermometer stabilizes. Ventilation is accomplishedby swinging a sling psychrometer; if using an aspirated psychrometer,air is blown across the wetted bulb by an electricfan. If the temperature is below 32ºF (0ºC), the water on thecotton wick should be frozen and aspirated until the temperaturestabilizes. Touching the wick with cold metal or icewill cause freezing. When the water on the wick is frozen, thetemperature is called the “frost-bulb” rather than wet-bulbtemperature.Both frost-bulb and wet-bulb temperatures exist for temperaturesbelow the 32ºF melting point. The difference is thatthe saturation vapor pressure over ice is lower than over liquidwater. This means that water vapor that strikes the surfacefrom the air is more likely to attach to ice than to a watersurface. For a given water vapor content of the air, the frostbulbwill be slightly higher than the wet-bulb temperature.If a psychrometer is unavailable, the air temperature forstarting sprinklers on the first night of a frost event is determinedusing the dew point temperature (T d ) and the criticaldamage temperature. To determine the starting temperaturefrom Table 1, locate the critical temperature column and themeasured dew point temperature row. (Note that criticaltemperatures in the heading row correspond to wet-bulb“We Cover More Ground!”DISCOVER THE CHINOOK FAN…** Increased Radius Coverage by 80 to 150Feet with Same HP Draw** Air Flow Starts 14” from Hub** Donier Swept Tip - Reduces Tip Drag** Exclusive “Trailing Edge Wedge”Technology Widens Sector Angle &Increases Air Velocity** Advanced Flow Design** Increased Horsepower** Less Fuel Consumption** Quality Built, Affordable, Fast PaybackReturns“...there is no dead zonebelow the prop like withother machines.”Bill HenriSolar Powered Weather StationsFrost AlarmsVia voice or text messageReal-Time Data & AlertsVia email or smart phoneControl pumps & valvesDegree days, chill hours,irrigation schedulingWestern Weather Group— providingsystem design, sales, installation &support. Over 30 years experiencewith Campbell Scientific weatherstations and equipment.Yakima, WA 98903509-248-0318 Phonehfhauff@gmail.comVisit us at www.hauff.com to Find Out More!John’s Crane ServiceTulare, CA559‐352‐9834Authorized DealerWestern Weather Group530-342-1700info@westernwx.com38 Citrograph September/October 2011

temperatures). Taking the critical damage temperature andthe dew point into account, select the air temperature (T) tostart the sprinklers from Table 1. Measure this air temperaturein or upwind from the orchard. For example, if T d =23°F(-5.0°C) and T c =30°F (-1.1°C), then start the system when theTable 1. Minimum turn-on and turn-off air temperatures(°F) for sprinkler frost protection for a range of criticaldamage and dew point temperatures (°F)*Dew point Critical Damage Temperature ( o F)o F 22.0 23.0 24.0 25.0 26.0 27.0 28.0 29.0 30.0 31.0 32.032 32.031 31.0 32.730 30.0 31.7 33.329 29.0 30.6 32.3 34.028 28.0 29.6 31.2 32.9 34.627 27.0 28.6 30.2 31.8 33.5 35.226 26.0 27.6 29.2 30.8 32.4 34.0 35.725 25.0 26.5 28.1 29.7 31.3 32.9 34.6 36.324 24.0 25.5 27.1 28.6 30.2 31.8 33.5 35.1 36.823 23.0 24.5 26.0 27.6 29.1 30.7 32.3 34.0 35.6 37.322 22.0 23.5 25.0 26.5 28.1 29.6 31.2 32.8 34.5 36.1 37.821 22.5 24.0 25.5 27.0 28.5 30.1 31.7 33.3 34.9 36.6 38.220 22.9 24.4 25.9 27.4 29.0 30.6 32.1 33.7 35.4 37.0 38.719 23.4 24.9 26.4 27.9 29.4 31.0 32.6 34.2 35.8 37.5 39.118 23.8 25.3 26.8 28.3 29.8 31.4 33.0 34.6 36.2 37.9 39.5*Select the appropriate critical temperature column and the row withthe measured dew point temperature. Then, read the correspondingair temperature from the table. This table assumes a barometricpressure of 29.92 inches of Mercury (101.3 kPa), so it is appropriateto use the table up to about 1000 feet elevation.Table 2. Dew point temperature (°F) for a range of airtemperature and relative humidity*RelativehumidityAir Temperature (ºF)% 32 36 40 44 48 52100 32 36 40 44 48 5290 29 33 37 41 45 4980 27 30 34 38 42 4670 23 27 31 35 39 4360 20 23 27 31 35 3950 16 19 23 27 30 3440 10 14 18 21 25 28*Select a relative humidity in the left column and an air temperaturefrom the top row. Then find the corresponding dew point in the table.air temperature is at or above 34.0°F (+1.1°C).If the system is turned off when the wet-bulb temperature(T w ) is greater than 32°F (0°C) during daytime following anight of frost protection, be sure to start it again before the T wfalls below 32°F (0°C) on following frost nights. If the system isstarted after T w

est to start the micro-sprinklers first and then use the windmachines if frost conditions worsen during the night.The use of micro-sprinklers in conjunction withwind machines is better than the use of either methodalone. It is best to start the micro-sprinklers first andthen use the wind machines if frost conditions worsenduring the night.Surface irrigationSurface (flood and furrow) irrigation is commonly used forfrost protection in California. The benefit derives from theconversion of latent to sensible heat from the cooling water.Both convection of air warmed by the water and upwardradiation are enhanced. In surface irrigation, freezing of thewater is undesirable because the formation of ice above theliquid water prevents heat transfer from the warmer waterunder the ice crust.Surface irrigation should be started early enough so thatwater reaches the end of the field before the air falls to thecritical damage temperature. As it moves down the field, thewater cools, so the runoff water should not be re-circulated.Warmer water provides more protection. The furrows shouldbe as wide as possible because the radiation and sensible heattransfer depends partially on the water’s surface area. Runningwater under the trees provides some protection, but damageis mostly on the top and sides of trees, so running water underthe edges of the tree canopy will provide better protection.If an inexpensive source of energy is available, heating thewater will provide more protection. Cost-effectiveness fromheating water depends on the capital costs for the infrastructureto heat the water, the cost of energy, and the severity ofthe frost event.HeatersIn small orchards and in field-planted citrus nurseries,heaters are sometimes employed for frost protection. Emissionsfrom heaters are regulated in California under air qualityTemperature ( o F)323130292827Height (ft)605040302010Heaters3530 (-1.1) 30 (-1.1)26 00.0 -3.3Fig. 2. Schematic diagram showing the energy effect of a smudge pot heateron temperature and circulation within an inversion.40 Citrograph September/October 201118.315.212.29.16.13.0Height (m)0.0-0.6-1.1-1.7-2.2-2.8Temperature ( o C)standards and are enforced by air quality control districts.A list of approved heaters that meet air quality standards isavailable from local air quality district offices.Oil or liquid propane fuels are commonly used in returnstack heaters. Heat output from a return stack unit burningfuel at a rate of +3.28 lb/hour is 105,000 Btu/hour. About27% of the energy output is radiation, with 10% going to theground, 9% to the trees, and 9% to the sky.For frost protection, radiant energy is more efficient thanheated air, and the radiation emanates best from a hot, solidsurface (e.g., a steel smokestack of a heater). The radiationsource should be kept as close to the plant as possible withoutcausing burn damage. A portion of the combustion isconverted to sensible heat as heated air and gasses from theflame. As this heated air rises and mixes within the inversion,it can warm the leaves, fruit, and branches. If the fires are nottoo hot, a circulation effect can result that enhances frostprotection. If the fires are too hot, the heated plume can riseabove the inversion ceiling, and the energy is lost.The number of heaters per acre depends on the heatoutput, typical inversion strength, and presence of cold spotsthat require more heat. In general, radiant energy transferis more efficient than sensible heat transfer, so heating anorchard is more efficient with more heaters having a smallerheat output. The amount of protection for each site can bedetermined only by the grower and their years of experiencewith the frost problem on that site.Heaters provide freeze protection by direct radiation tothe plants around them and by causing convective mixing ofair within the inversion layer. When heaters are operated, theheated air rises. As the heated air rises, it cools until it reachesthe height where the ambient air has the same temperature.Then the air spreads out and, eventually, the air descendsagain. A circulation pattern much like that of a gravity furnaceis created (Figure 2). If the inversion is weak, the heatedair cools, but it rises too high and a circulation pattern is notproduced. As a result, heaters are less efficient when thereis no inversion. Making fires too hot will also make heatersless effective because the heated air risesabove the inversion ceiling, and the circulationpattern is not created.Heaters should be evenly distributedthroughout the orchard being protected;however, they should be concentratedsomewhat more on the edge of theupslope or upwind side of the orchard.Considerable time is needed to light heaters,so sufficient time is needed to finishlighting all heaters before the temperaturefalls to critical levels.One method of heater distributionis the border heating technique. On theupwind edge, where prevailing air driftenters the orchard, use two heaters pertree on the outside and one heater pertree in the first two rows in from the edge.On the downwind side of the orchard, useone heater per tree on the outside. On theremaining two sides of the orchard, useone heater per tree on the outside andfirst row in from the outside. To reduce

Citrus RootsPreserving Citrus Heritage FoundationIf you have found Our workinteresting and engaging...Please Support YourFoundation during thisspecial time of giving:Buy our books, cratelabels, make a cashcontribution ...Orgive to Citrus RootsFoundation your cratelabels, books, citrusmemorabilia ...youwill save FED and CAtaxes to the full extentallowed.Give this year ...forsome in Congressare considering the withdrawal ofcharitable deductions.Our website is a reference centerwww.citrusroots.comOur “Mission” is to elevate the awarenessof California citrus heritage throughpublications, education, and artistic work.We are proud of our accomplishments as avolunteer organization, which means eachdonated dollar works for you at 100% [forwe have no salaries, wages, rent, etc.]. Alldonations are tax deductible for income taxpurposes to the full extent allowed by law.Citrus Roots – Preserving CitrusHeritage FoundationP.O. Box 4038, Balboa, CA 92661 USA501(c)(3) EIN 43-2102497The views of the writer may not be the same as this foundation.Fruit FrostWarning ServiceRichard H. BarkerExcerpts from Barker, Richard H. and Pulley, ThomasM., Citrus Powered the Economy of Orange Countyfor Over a Half Century Induced by a Romance.From 1908 through 1916 a group, later to be known as thePomona Valley Orchard Protection Association, was activelytrying to understand and learn about frost protection. Thisgroup, as well as Sunkist, was mostly responsible for establishingthe Fruit Frost Warning Service in California.The growers’ inability to know when temperatures wouldbecome critical for their trees and crop, and their inability toadequately protect them, led to the strong demands placedon their political representatives for the establishment of afederally funded frost service. Washington responded! FloydD. Young, a hydrologist, in 1917 was asked to research thefrost problem of the citrus growers and establish a service.The first district was started in Pomona during that year. From1917 to 1921, most of the work was done in the research areadetermining freezing points for lemons and oranges, testingthermometers, inversion studies and testing types of protectiondevices.In 1921, the Weather Bureau was planning to eliminatethe service. Two things were responsible for changing theBureau’s mind. First, there was a major freeze in January 1922which proved that growers with proper protection and usingthe Fruit Frost Warning Service could save their trees andcrops. The use of orchard heating greatly increased after the1922 freeze and continued through the 1930s. Secondly, thecitrus growers were organized and spoke through one voice.Charles C. Teague, president of Sunkist, aggressively establisheda strong lobby bringing together the Mutual OrangeDistributors (MOD) and independents to keep the service andto share in the per diem costs of providing transportation tosample temperatures. From this loud roar, the growers won.It is humorous to read how they first got the forecast to thegrowers. In Pomona, a motorcyclist would first ride throughthe groves obtaining temperatures every night; then he rodeback to the grower’s home to warn him when the temperaturedropped to a critical level. In some towns, the early frostforecast was given through the fire whistle. At eight o’clock,two whistles followed by five more meant the temperaturewould fall to 25 degrees (a level where protection was needed).Forecasts also were given to telephone exchanges and theoperator would read the release. Mr. Young told the story ofa local telephone company which decided it was not going toprovide the forecast service. The growers called a meeting andSeptember/October 2011 Citrograph 45

Photos tell the story...Orchard heaters shown at dawn in Covina still burning. (1949).The studio of 50,000-watt, clear-channel station KFI, ownedby Earle C. Anthony, Inc. Note the Packard in front of thestudio at 141 N. Vermont Ave. Anthony provided both FloydYoung and the host of the noontime farm report withprestigious Packards to drive around town. (A study made inthe 1920s supports Anthony’s interest in the citrus industry;the findings indicated that as a professional group, citrusgrowers owned more motor vehicles than all others.)Floyd Young’s home.Young would often do his nightly broadcast from home.Note his bed in the background (top, far left). (1952).KFI ownerEarle C.Anthony.An employee reading theinstrumentation. He would thenphone the result to Mr. Young.The distribution location andshowroom of Earle C. Anthony,California Packard Distributors.Atop the building (at 1000 S.Hope St., Los Angeles) were thebroadcast towers for KFI radio.46 Citrograph September/October 2011

decided to order their phones be taken out unless the frostservice dissemination was continued. Service was restored.Movies were also interrupted to give the forecast. This practicecontinued for a number of years.Forecasts were later broadcast by radio starting with KHJ,moving to KNX and then to KFI. At eight o’clock nightlyduring the winter months, starting on November 15th toFebruary 15, the voice of Floyd D. Young of the Fruit FrostWarning Service was broadcast from his office in the PomonaSouthern Pacific’s weather bureau and its valueto the publicIbid, p. 7Excerpts from Sunset Magazine, June 1898, page 35: “Thework of the United States Weather Bureau and the Signal Servicehas been constantly growing in value and the benefits havebeen more and more apparent to agricultural interests. But theclimate peculiarities of the Pacific Coast and the sensitive characterof the fruit products, as well as their great value, has ledto the establishment by the Southern Pacific Company of whatis in effect an auxiliary weather bureau. The Southern PacificCompany has supplied suitable instruments and observationsare taken at each of the 181 stations at 7:00 A.M. and 2:00 P.M.The Southern Pacific Company extends this service to manyother points without charge by forwarding over its own wires allimportant predictions…”This can also be assessed by their response in 1937 tothe worst freeze on record. Following are excerpts froma speech prepared by Ronald S. Hamilton, Meteorologistin Charge of the National Weather Service Officefor Agriculture and Fire Weather in Riverside. He stated:Ibid, p. 92“Railroad companies had a very vital interest in preventingdamage to trees and crops, since much of their annual revenuesdepended on the movement of fruit. On the strength of the firstwarning of impending cold, before the arrival of the freeze, onerailroad company began to bring tank cars from all parts of itsline into Southern California. More than a hundred cars used totransport coconut oil were rushed southward from San Francisco.Cars that were used to carry gasoline, molasses, alcohol, roadoil, and even fish oil could be transported. There was not timeto clean the cars, and many gallons of molasses were burned inthe heaters during the freeze. The delivery of orchard heater oilto so many locations required a large number of trains. At timesas many as a dozen oil trains operated simultaneously along a50-mile railway line. To minimize confusion, all cars were pooledby the railroads, regardless of ownership.“All railroad crews were pressed into service, and additionalcrews had to be brought in from all over the state, and somecame from as far away as Texas. Movement of every other typeof freight in the state, except the most perishable, had less prioritythan oil. Toward the end of the freeze the railroads notifiedthe citrus growers that their own supplies of oil for locomotivefuel were practically exhausted and the cars would have to bediverted to their own use, if rail operations were to continue.Orchard heater oil transported by rail during the freeze totaled4,435 cars, or a little over 53 million gallons. At one time duringthe freeze, unfilled orders totaled 18 million gallons. It was estimatedafter the freeze that the total amount of orchard-heateroil that was delivered during the freeze from all possible meanswas between 80 and 100 million gallons, costing between $4 and$5 million dollars. During the freeze, a strike was in progress thattied up many ships on the Pacific Ocean. If it had not been for thisstrike, reducing the need for diesel fuel by these ships which werenot in service, orchard heater fuel would have been completelyexhausted before the end of the freeze.”Post Office. If firing was expected, information was also givenregarding wind, clouds, temperature ceiling, time firing wouldbegin and the forecast for the following night.Mr. Young’s research showed a major freeze struck SouthernCalifornia about every 10 to 15 years, and he wrote thatthe winter of 1936-37 would be the 15th year without a majorfreeze. Numerous articles were written by him warning growersthat they were not prepared for a major freeze. He knewtheir oil supply was low and warned the growers not to becomplacent. A major freeze did strike and was the worst freezeon record. There were ten nights in January where minimumtemperatures were well below critical levels. Thousands ofgrowers fought to prevent the loss of practically everythingthey owned. Help came from all sectors. The local economywas in jeopardy. Groves were severely damaged, and manytrees were lost. The growers with adequate coverage of orchardheaters did not lose trees in the higher elevated areas.From his Pomona headquarters as founder of the FruitFrost Warning Service, he served for thirty-nine years, retiringon March 15, 1956. Floyd D. Young did more for the citrusindustry than any other person, it has been said, except forCharles C. Teague. On his retirement the editor of The Citrographwrote, “Like the hail to a sentry, the famed ‘voice’ ...will always be remembered as the vigilant guardian of the vastcitrus industry”... of California and Arizona.After his retirement, R. Roy Simpson followed, and theservice was continued later by Roy Rodgers, then by DaleHarris and Ronald S. Hamilton. Because the citrus industrydiminished in California and Arizona during the mid-1990s,the Fruit Frost Warning Service was discontinued. In someareas, the growers hired private forecasters; however, in otherdistricts or areas there was no service available for hire.Richard H. Barker is the founder and president of theCitrus Roots-Preserving Citrus Heritage Foundation. For anumber of years, he has been leading a drive to bring abouta higher awareness of the role citrus played in developingCalifornia. Dick is a retired investment banker and wasa third generation Sunkist grower. He has published fourvolumes on citrus heritage. lIllustrations sourced by R. Barker and provided by Universityof Southern California Library, Pomona Public Library,and the Barker Collection.WOLFSKILL: A correctionin reported historyRichard H. BarkerPrefaceThis is a very positive story, a story which corrects historyand gives credit to a major participant who previouslyhad been totally excluded for approximately 122 years. Itall developed from a compelling letter written 40 years agowhich you will read more about.For the March/April 2011 issue of Citrograph under“More legacies of the Wolfskill family”, I had written a shortdescription under the photo of the Arcade Station about thedonation of 14 acres to the Southern Pacific Railroad Co.to be utilized as its “future” passenger station, and to secureSeptember/October 2011 Citrograph 47

Volume I of IIIIncluding a fold outtime line chart ofby Marie A. Boyd and Richard H. BarkerVolume III of III$ 15 00the position of Los Angeles on the rail company’s main line.This was an inducement pledge made in or around 1872.Joan Hedding related to me the contents of her lateaunt’s letter and expressed the wish to set the gifting correctin accordance to her aunt’s memory. My reply was that theauthoritative writing of Ira H. Wilson is considered to becorrect, so prove it otherwise! Now you will have the opportunityto read of her findings regarding the donors of theland, the year, and the corrected acreage.Here is Joan’s report:“When I was doing research for the Citrus Roots PreservingCitrus Heritage Foundation for its series published in theCitrograph magazine article on my great-great-grandfatherWilliam Wolfskill, I came across a lettersent to me 40 years ago by my Aunt Mary(Mary Weyse Kelleher) referencing aland grant made in 1890 by the Wolfskillfamily to the Southern Pacific Railroad.In her letter, Aunt Mary expressed disappointmentin a newspaper article aboutthe land grant that apparently attributedMaria FranciscaWolfskill Shephardthe land donation to William Wolfskill’seldest son, Jose. 1 Aunt Mary wrote therewas ‘no mention of dear Tia Francisca,whose idea it was and who owned more than half of thatproperty they relinquished.’“The Wolfskill land donation of 12.03 acres to the SouthernPacific Railroad was part of 70 acres in Los Angeles originallyowned by William Wolfskill. This property included his largeadobe home and his first orchard and vineyard. On his death,he bequeathed half of this property to hisson Jose and the other half to his daughterFrancisca.“My interest was piqued. I had respectfor my aunt’s veracity and also a desire tobring to light Francisca Wolfskill’s part inthe donation of the 12 acres. Part of mymotivation was the knowledge that FranciscaWolfskill died without any survivingchildren and had no one to speak for herJose (Joseph)Wolfskill-- unlike her brother Jose Wolfskill, whohad 10 children.“Francisca Wolfskill, the secondCitrus Roots Series...Citrus RootsPreserving Citrus Heritage FoundationKeeping citrus heritage alive in the minds of those living in California through publications, educational exhibits and artistic works48 Citrograph September/October 2011GIFT IDEAS!!Citrus Roots...Our Legacy - Volume IVCitrus Powered the Economy of Orange Countyfor over a half century Induced by a “Romance”All donations are tax deductible for income taxpurposes to the full extent allowed by law.For ordering informationvisit our websitewww.citrusroots.comdaughter of William Wolfskill, was born in 1844 and in 1880she married Charles Shephard, one of the early fruit packersand shippers who worked with the sons of William in thedistribution of the Wolfskill oranges and lemons. My AuntMary was the granddaughter of Juana Wolfskill, WilliamWolfskill’s firstborn child. My aunt’s mother died when shewas very young, and she was raised by her great aunt FranciscaWolfskill Shephard whom she called Tia (aunt).“I found assistance from the Sherman Library and Gardensin Corona Del Mar, the Railroad Museum of Sacramento,and the Los Angeles County Recorder’s Office, and I receivedIn 1888, the Arcade Station in Los Angeles (on Alamedabetween Fourth and Sixth) was completed to serve passengertraffic. The land (12.03 acres) to secure this station, as we nowlearn, was donated by Joseph Wolfskill and his sister, FranciscaShepherd in 1887 in order to establish that the main line ofthe railroad would be directed through the City of Los Angeles.This donation was one of the inducements to secure thisopportunity. The land donation agreement was signed in 1889by the two donors, and accepting on behalf of the SouthernPacific Company was Leland Stanford, president.The wording in the agreement as to the use of the propertywas specific and unambiguous: “CONDITIONS: “The abovedescribedpremises to be used for a general R.R. passengerstation, and for general passenger, baggage and expressbusiness and not otherwise; provided that eating rooms for theaccommodation of travelers may be also maintained therein;and that said premises shall revert to the grantors, their heirsor assigns, upon breach of said conditions...” However, after 26years of serving the passenger traffic related to Los Angeles,the role of the Arcade Station was changed to booking thebusiness of freight.Selling the GOLDHistory ofSunkist ® and Pure Gold ®CITRUS ROOTS . . . OUR LEGACYBy: Rahno Mabel MacCurdy, V.A. Lockabey and others...compiled and edited by R.H. BarkerCitrus Roots...Our Legacy - Volume ISelling the Gold - History of Sunkist®and Pure Gold®Citrus Roots...Our Legacy - Volume IICitriculture to Citrus CultureCitrus Roots...Our Legacy - Volume IIIOur Legacy...Baldy View Entrepreneurs- 25 men & women who left a legacyOur Legacy:Baldy ViewENTREPRENEURSAmerican Business Cycles from 1810 to 1978vs. the Life Span of Twenty-Five EntrepreneursCITRUS ROOTS ... OUR LEGACY(Fed. Tax ID # 43-2102497)

See Results in Citrus!6 Month Growth TrialControlWith MycoApply®Southern Pacific Company, Central Station, Los Angeles wasbuilt in 1914 to service the passenger business of the route.Unless the earlier conditions were amended, this appears to bein conflict with what Joseph and Francisca had stipulated.a copy of the deed which is a six-page handwritten legal document.It describes the 12.03 acres of the land and designatesa place for the signatures of the grantors: J.W. Wolfskill andFrancisca Wolfskill de Shephard, and the grantee: LelandStanford (for Southern Pacific).“I am pleased that the identity of both Jose and Franciscaas land donors has been cleared up, and I believe that AuntMary and Tia Francisca would be very pleased as well.”1This same attribution to Jose Wolfskill as grantor wasmade in Iris Higbee Wilsons’s book,”William Wolfskill,Frontier Trapper to California Rancher”, p 215, publishedin 1965. lPhotos courtesy of Joan Hedding, Marguerite Oates, andUniversity of Southern California Library.Soil & Crop34284-B Road 196Woodlake, CA 93286(559) 564-3805DISTRIBUTED BYTulare Ag Products, Inc.3233 South “I” StreetTulare, CA 93274(559) 686-5115New Era Farm Service2904 E. Oakdale Ave.Tulare, CA 93274(559) 686-3833Nutri-Phite® KeyPlex®Successful growers’ nutrition programs always stand out!So why aren’t you on a Nutri-Phite® KeyPlex® program?• Increases Brix• Increases Yield• Field Proven• Safe35801 Road 132, Visalia, CA 93292800-868-6446 • 559-635-4784 • 559-625-9255 FAX • www.biagro.comContact: Tom Haught • 661-713-5374 • tom@biagro.com / Gary Morrow • 559-281-4626 • gmorrow@biagro.comSeptember/October 2011 Citrograph 49

CRB Funded Research ReportsResearch Project Progress ReportEvaluation of disinfectants forcitrus packinghousesJoseph L. Smilanick, David Sorenson, Gabriel Verduzco, Zilfina Rubio Ames and Monir MansourEditor’s Note: The work discussed here is part ofongoing research on “New technologies to minimizepostharvest decay of citrus,” with Dr. Smilanick as theprincipal investigator.In California and other arid citrus production areas,early season navel and mandarin oranges are harvestedand treated with ethylene gas for two or more days inhumidified “degreening” rooms at 20°C (68°F) to acceleratethe degradation of chlorophyll to enhance the orange colorof the fruit rind.These environmental conditions are optimal for the developmentof green mold, caused by Penicillium digitatum,and blue mold, caused by P. italicum. These two pathogens,both of which generally require wounds to initiate infections,are responsible for most of the postharvest decay losses inCalifornia.Beginning about 2000, bin drenching with fungicidessuch as thiabendazole or imazalil before degreening wasintroduced in California. A consequence of these treatmentsis that the ethylene-treated fruit are now protected by thesefungicides, but this adds the additional risk that fungicideresistant isolates will develop and contaminate the degreeningrooms, storage facilities, and packing equipment. Particularlywhen degreening is prolonged to three days or longer, decayof the fruit during degreening is common, and contaminationbecomes severe.Periodic disinfection of the rooms by fog or mist applications,when the fruit are absent, is done to decontaminatethem of residual Penicillium spores. Formalin solution, whichcontains formaldehyde as an active ingredient, has long beenused for this purpose, but a replacement is needed.Formaldehyde is used primarily as a fumigant in agriculturalpremises such as poultry and swine farms and processingplants as well as in citrus packing and mushroom houses. It isused as a hard surface disinfectant in commercial premises,industrial premises, and veterinary clinics. Formaldehyde isalso registered as a preservative for consumer products suchas laundry detergents, general-purpose cleaners and wallpaper adhesives.For citrus facility disinfection applications, the rate of useis 16 fl. oz of a product containing 37% to 55% formaldehyde/1000 ft 3 , which is applied through a stationary mountedspray manifold with two compressed air operated nozzles(1/4J-SU5, Spraying Systems, Inc., Wheaton, IL).Formaldehyde cannot contact the fruit or be used morethan twice yearly, and the maximum rates of formaldehydeuse decline progressively as the proximity of inhabited dwellingsor schools approaches these facilities. Treated buildingsmust be left closed, locked and secured against unauthorizedentry for a minimum of 24 hours, then entered when the airconcentration level of formaldehyde is below 0.75 parts permillion.In prior work, we examined ozone gas and thermal treatmentsas alternatives for formaldehyde, and both show somepromise, although these approaches are very different thanthe use of conventional sanitizers.In this study, our objective was to apply candidate sanitizersolutions in a fog or mist mode with generic, uncomplicatedatomizer equipment in commercial degreening rooms andcompare their effectiveness to formaldehyde treatment.Most of the common active ingredients of approvedsanitation agents used in food industries were represented.Materials were selected based on aspects of safety, availability,cost, and their prior registrations. Candidate compounds andtheir rates were selected based on issues of safety, cost, availability,their uses in other applications, and the feasibility ofregistration (Table 1).Candidate sanitizers included hydrogen peroxide, peroxyaceticacid, several quaternary ammonium compounds,isopropyl alcohol, ethyl alcohol, sodium ortho-phenyl phenate,sodium hypochlorite, and chlorine dioxide. Productscontaining these compounds that are marketed as sanitizersfor many applications, and those tested in the present workare shown (Table 1).We used commercial degreening facilities in this study. Bydoing so, both the potency and homogeneity of the distributionof the materials within the existing commercial roomscould be assessed under practical conditions in a single testso promising treatments could be identified.Degreening roomsThe rooms that we used in this study were approximately6 m (20 ft.) in height, 6 to 9 m (20-30 ft.) in width, and 20 m(65ft.) in length, with plywood walls and ceiling, a roll-upmetal door, and a concrete floor. There is little variation inthe size and design of these rooms among facilities, all ofwhich used low-pressure, horizontal air delivery, many witha false ceiling. The rooms do not have air-tight seals and are50 Citrograph September/October 2011

designed to have an air-exchange rate of one room volumeper hour to minimize carbon dioxide accumulation duringuse, although these vents were closed during our sanitizertests. Air circulation was provided in each room by threefans of two horsepower each that operated continuouslythroughout the tests. These rooms are already equippedwith humidification systems and heat control used to adjustenvironmental conditions and fruit temperatures to thosethat optimize degreening.Pathogen preparationA dense suspension of conidia from a two-week-old colonyof P. digitatum cultured at 24°C (75°F) on potato dextroseagar was collected on a fine-haired brush and immediatelyapplied onto one side of each of 20 to 24 craft wood sticks, 10cm (4 in.) in length and 0.5 cm (0.2 in.) wide, and they weredried in air one day before use. About 1 hour before thesanitizers were applied, the craft wood sticks were attachedthroughout the degreening room where the sanitizers wereto be applied, and four were placed in an identical untreatedroom. They were distributed at equal intervals along bothwalls and at the back of the room and placed either at lowor high levels from the floor; half were placed about 50 cm(11 in.)in height from the floor, and half were placed about200 cm (80 in.)(in height from the floor.The rooms were humidified for about 1 hour before thesanitizers were applied, and the relative humidity was not lessthan 85% and the temperature was 20° to 22°C (68°F -72°F).The water volume applied varied but was most frequentlywas 6 L per 100 m 3 .Compound applicationThe candidate compounds were applied with either a 50L carboy with 2 compressed air assisted nozzles (1/4J seriesTable 1. Formulations, active ingredients, and sources of the sanitizers evaluated for the control of conidia ofPenicillium digitatum in commercial citrus degreening rooms. The classes of products are: A) aldehyde; B) peroxygencompound or oxidizer; C) chlorine or hypochlorite; D) chlorine dioxide; E) quaternary ammonium; F) phenolic;and G) alcohol.Treatment Class Active ingredient contents SourceFormalin A formaldehyde 37% Sigma-Aldrich Chemical Co., St, Louis, MOMyacide GA 15 A glutaraldehyde 14.3 to 15.8% BASF, Fremont, CACitrisol Ba mineral oxychloride oxidizer 10 to 20% KeyLate Solutions LLC, Corpus Christi TXHydrogen peroxide B hydrogen peroxide 30% Solvay Chemicals, Inc., Houston, TXHDH peroxy B peroxyacetic acid 6% HDH Agri Products, LLC, Tavares, FLhydrogen peroxide 30%Storox B hydrogen peroxide 27% BioSafe Systems, Hartford, CTJetOxide B hydrogen peroxide 26.5% JET Harvest Solutions, Longwood, FLperoxyacetic acid 4.9%Chemchlor C sodium hypochlorite 12.5% Brenntag Pacific, Inc. Santa Fe Springs, CAProOxine D chlorine dioxide 5% Bio-Cide International, Norman, OKSani-T-10 E n-alkyl dimethyl benzyl ammonium chloride 5% Spartan Chemical Co., Inc., Maumee, OHn-alkyl dimethyl ethylbenzyl ammonium chloride 5%Quat 5L E octyl decyl dimethyl ammonium chloride 6.5% 3M, St. Paul, MNdioctyl dimethyl ammonium chloride 2.6%didecyl dimethyl ammonium chloride 3.9%alkyl dimethyl benzyl ammonium chloride 8.7%Acidize DS-50 E dodecylbenzenesulfonic acid 15% Shepard Brothers, Inc., La Habra, CAphosphoric acid 50%Dowicide 1 F ortho-phenyl phenol 99%, diluted to 25% Dow Chemical Company, Wilmington, DEEthyl alcohol G ethyl alcohol 95% Sigma-Aldrich Chemical Co., St, Louis, MOIsopropyl alcohol G isopropyl alcohol 99.9% Sigma Chemical Co., Chicago, ILaCitrisol is produced as a cleaning agent, not a disinfectant or sanitizer.September/October 2011 Citrograph 51

SU5, Spraying Systems Co., Wheaton, IL) or a fan atomizerwith a capacity of 75 L (Aquafog ® HRSM - Mobile FinemistHumidifier, Jaybird Manufacturing, Inc., State College,PA). Both methods delivered approximately 75 L per hourper unit, and the manufacturers state the droplet sizes emittedare primarily within 10 to 20 micrometers, a recommendedsize for fogging applications since they are airborne longenough to disperse widely.Thirty-three tests were conducted where 15 sanitizerswere evaluated. Water alone was applied as a control.Concentrations of the sanitizers are expressed as the activeingredient content by weight per m 3 of room volume. Allwere compared to formalin solution, applied at a rate of 2.6g formaldehyde per m 3 .The room was closed, and the atomizer was operated forabout 3 hours until all of the solution was dispensed. The relativehumidity when the materials were applied was not lessthan 85%, and the temperature was 20° to 24°C (68°F-72°F).None of the facilities was in active operation, and the roomswere empty when these tests were conducted. No workerswere present. Placards were placed indicating entry into thefacilities was prohibited.Blue mold and green mold.After 24 hours, the room was opened, aerated for about1 hour, and the craft wood sticks were retrieved. Each wasplaced in a sterile test tube, a small volume of 0.1% wt/volTriton TX-100 was added, it was mixed briefly, and a 20 mLvolume containing 200 to 500 conidia was placed on PDA.After 18 hr at 20°C (68°F), the number of germinated conidiawas counted at a magnification of 200X with a compoundmicroscope.ResultsMost sanitizers were moderately effective or ineffectiveunder the conditions we tested them, either because the rateswe tested were too low or their distribution within the roomswas inadequate (Table 2).Three sanitizer formulations were approximately equalin effectiveness to formalin solution. They were: 1) MyacideGA 15 (BASF) active ingredient of glutaraldehyde; hydrogenperoxide alone or a mixture of 30% hydrogen peroxide(Solvay) + Sani-T-10 (Spartan Chemical Co.) a quaternarysanitizer mixture; and, Citrisol (KeyLate Solutions LLC)proprietary mineral oxidizer.Glutaraldehyde was effective at very low rates, muchlower than the rate commonly employed for formaldehyde inthis application (Table 2). We suspect in some tests its diffusioninto the adjacent control room occurred and reduced thesubsequent germination of conidia in the control treatments.The oxidizing compounds hydrogen peroxide and ‘Citrisol’were very effective in most tests, although with eachcompound there was one test where their effectiveness waslower.The results of this study indicate that several productsmay be promising alternatives to formaldehyde fumigationfor the citrus industry, and this work may have some value inother produce and food processing industries where facilitysanitation is important.The fogging equipment used in this study was not complexor particularly sophisticated. Burfoot et. al. reviewed manyaspects of disinfectant fogging. They stated that once there ishomogeneous distribution of the fog, deposition on upwardfacing surfaces readily occurs, while deposition on vertical ordownward facing surfaces is more difficult. Effective disinfectionof these surfaces requires horizontal air movement or adisinfectant with vapor action. A recommended droplet sizefor most fogging applications was 15 microns because of itsgood dispersion and relatively long settling time of 45-60minutes. The specifications of both foggers we used indicatedthey dispensed a large portion of droplets of this size.Hydrogen peroxide, glutaraldehyde, and a proprietarycleaning agent (“Citrisol”) were effective degreening roomsanitizers. The ineffectiveness of many of the other compoundsis surprising, since all are known to be potent sanitizers.Chlorine dioxide effectiveness was irregular, and the ratesneeded to control germination of the conidia were higher thanany of its other registered uses. Ethyl alcohol and isopropylalcohol were ineffective even at the very high rates applied.Chlorine was ineffective at the highest rate applied (2000parts per million), and adjustment of the pH downward toimprove its effectiveness was unsuccessful. Ortho-phenylphenol was not effective. Phenol disinfectants generally arebiocidal (kill conidia on contact) at high concentrations, butothers have shown that they exhibit biostatic (inhibit conidialgermination only while present in sufficient concentration)activity that persists even when diluted to low concentrations.The active ingredient in “Citrisol” is described as a mineraloxychloride oxidizer. The intended use of this product isas a cleaner; it is not registered as a sanitizer by the EPA. Thedistributor indicates it is in compliance with the EPA’s greenchemistry program, where the design of chemical productsand processes that reduce or eliminate the use or generationof hazardous substances is promoted.Hydrogen peroxideHydrogen peroxide is an odorless, clear liquid, slightlymore viscous than water. It decomposes to water and oxygen.Low concentrations of hydrogen peroxide applied to foods asan antimicrobial agent are classified as Generally Recognizedas Safe (GRAS) by the FDA (21CFR184.1366). Agriculturaland food contact products usually contain no more than 35%hydrogen peroxide, which is then usually diluted to 1% orless when applied as a spray or a liquid. This is about theminimum concentration of hydrogen peroxide previously52 Citrograph September/October 2011

Table 2. Formulations, chemical classes, water volume applied through foggers into the degreening room, roomvolume, concentration of active ingredients per cubic meter of room volume, date of the application, and subsequentgermination of conidia of Penicillium digitatum, cause of citrus green mold. Control values are the meanof germination of 150 conidia each collected from four locations, while treated germination values are the meanof 150 conidia each collected from 13 to 16 locations.Applied water Room Active ingredient(s) Conidial germination (%)Formulation Class volume (L) volume (m 3 ) applied (g-m 3 ) Date Control TreatedFormalin A 114 708 1.98 Aug 18, 2008 95 2Myacide GA15 A 50 680 0.05 Nov 15, 2010 48 4Myacide GA15 A 50 680 0.10 Oct 20, 2010 65 0Myacide GA15 A 50 680 0.21 Oct 20, 2010 85 0Myacide GA15 A 50 680 0.21 Nov 15, 2010 65 0Citrisol B 606 5700 1.01 Feb 2, 2010 86 1Citrisol B 114 708 1.89 July 14, 2008 95 19Citrisol B 230 765 3.47 Dec 2, 2009 95 0H 2O 2B 300 680 8.36 Sept 17, 2010 85 1H 2O 2B 150 680 8.36 Sept 14, 2010 74 13H 2O 2B 150 708 6.40 Sept 29, 2011 77 2H 2O 2B 300 680 4.18 July 25, 2011 69 14H 2O 2+ Sani-T-10 B 525 5700 1.36 + 0.12 Nov 18, 2010 95 1H 2O 2+ Sani-T-10 B 303 680 8.36 + 1.11 Sept 14, 2010 74 0H 2O 2+ Sani-T-10 B 303 680 8.36 + 1.11 Sept 17, 2010 85 0HDH peroxy B 114 707 4.53 Dec 6, 2007 95 37Storox B 114 707 1.68 Sept 14, 2007 95 24Storox B 114 707 3.36 Nov 13, 2007 95 26Storox B 114 707 0.67 Oct 16, 2007 95 69Jet oxide B 114 707 0.80 Oct 25, 2007 95 43Chemchlor, pH 8.5 C 114 707 0.33 Sept 5, 2008 95 65Chemchlor, pH 8.5 C 114 707 0.33 Sept 24, 2008 95 44Chemchlor, pH 7.0 C 114 707 0.33 Sept 19, 2008 95 69ProOxine D 57 680 0.42 July 2, 2009 86 13ProOxine D 144 960 0.75 Oct 7, 2009 97 40ProOxine D 57 680 0.20 July 15, 2009 95 18ProOxine D 227 765 0.36 Nov 5, 2009 79 12ProOxine D 227 965 0.12 Oct 21, 2009 98 36Sani-T-10 E 114 708 0.38 Oct 12, 2007 95 70Sani-T-10 E 114 708 0.25 Sept 26, 2007 95 91Quat 5L E 151 680 0.80 Sept 2, 2010 86 30Quat 5L E 227 765 1.05 Nov 10, 2009 78 78Acidize DS-50 E 114 708 1.70 Dec 2, 2009 94 49Dowicide 1, pH 10.0 F 102 1415 0.36 Dec 21, 2009 62 41Dowicide 1, pH 11.5 F 102 1415 0.36 Dec 21, 2009 94 88Dowicide 1, pH 11.5 F 68 764 0.44 Jan 5, 2010 89 78Ethyl alcohol G 114 708 36.2 Oct 10, 2007 95 77Isopropyl alcohol G 114 708 42.1 Jul 31, 2008 95 95September/October 2011 Citrograph 53

Misters in action.shown by Cerioni and coworkers to kill conidia of P. digitatum.In our tests, good disinfection of degreening roomsoccurred with hydrogen peroxide at rates close to those approvedfor food-contact applications, although our objectivewas to sanitize empty degreening rooms. Presumably, hydrogenperoxide could be used repeatedly, a significant advantageover formaldehyde, which can only be used twice yearly.The most effective concentration of hydrogen peroxidewe applied was 8.36 g per cubic meter, dispensed either forsolutions containing 2% hydrogen peroxide or 4% hydrogenperoxide. The 2% solution was most effective, probably becausetwice as much solution volume was applied. When theconcentration was further diluted to 1% hydrogen peroxide,and applied at a rate of 4.18 g per cubic meter, its effectivenesswas still relatively high.In these tests, locations in the room that were not wettedafter the hydrogen peroxide was applied indicated wheresome survival of the conidia could be expected, indicatingthorough coverage of the room is needed for the best effectiveness.It should be used with the fruit absent until thoroughevaluation of its phytotoxicity is done. We have previouslyreported that it caused no injury to lemons after immersion in5, 10, or 15% hydrogen peroxide for 10 or 30 seconds, and allpartially reduced green mold but harmed lemons immersedfor 90 seconds.Hydrogen peroxide is not considered highly corrosive, butit can harm some metals and alloys under certain conditions.It’s used for many chemical and industrial processes wherea strong oxidizer is needed, and its use for disinfection ofmedical equipment, food packaging, and fresh produce hasbeen reported previously by McWatters and colleagues aswell as Sapers and Simmons.For example, Afek and coworkers applied a hydrogenperoxide formulation to stored potatoes by fogging andreduced subsequent soft rot, caused by Erwinia carotovora,by more than 90%. The addition of a quaternary ammoniumsanitizer (Sani-T-10) improved hydrogen peroxide performance(Table 2). Sani-T-10 is a sanitizer and algaecide foruse in food processing operations, schools, nursing homes,hospitals, restaurants and other institutions, and no rinse isrequired when it is used in these applications. However, noquaternary ammonium sanitizers are approved at this timefor food contact applications.Users of hydrogen peroxide must be aware of its safe useand storage. Its vapor is a primary irritant, primarily affectingthe eyes and respiratory system, and long-term exposureto low ppm concentrations is also hazardous and can resultin permanent lung damage. Permissible Exposure Limit(PEL) established by the EPA for an eight-hour workdayfor hydrogen peroxide is 1.0 ppm (29 CFR 1910.1000(a)(2)).Certified devices to monitor and ensure compliance with thePEL are available.GlutaraldehydeGlutaraldehyde is a pungent, colorless, oily liquid that iswidely used in the health care industry as a cold sterilant ofmedical and surgical instruments, where OSHA introducedit to replace formaldehyde formerly used for this purpose.We found very low rates of glutaraldehyde were needed todisinfect citrus degreening rooms. It is not corrosive to metals.Glutaraldehyde vapor in the air can cause tearing of theeyes, burning nose, sore throat, cough, nausea, and headache.Ventilation is used to minimize worker exposure to it.There is no PEL established by the EPA for glutaral-54 Citrograph September/October 2011

dehyde. In California, Cal/OSHA has established a PEL of0.05 ppm. The American Congress of Government IndustrialHygienists established a Threshhold Limit Value (TLV),which is the maximum concentration of exposure permittedto glutaraldehyde for a 15-minute period, of 0.05 ppm. Certifieddevices to monitor and ensure compliance with the PELor TLV are available.Specific elements defining the safe use and regulatoryapprovals for glutaraldehyde and hydrogen peroxide forpackinghouse sanitation use have not been developed. Someof their current applications share aspects with their conceivableuse in packinghouses.The objective of this work was limited to disinfection offacilities when the fruit are absent. If applied when fruit arepresent, many issues (effectiveness to control decay, risk ofinjury to the fruit, disinfectant residues, impact of the disinfectanton fungicide residues) must be addressed. Becauseneither glutaraldehyde nor hydrogen peroxide contain anyof the substances listed on Proposition 65 of the State ofCalifornia that are suspected to cause cancer, birth defects,or other reproductive harm, they do not require a warningunder this statute.This project was made possible by a grant from the CaliforniaCitrus Research Board CRB (Project 5600-106). Weacknowledge the technical assistance of Bruce Adams, JulieDoctor and Marty Coert of Fruit Growers Supply PackingServices of Exeter, California, and Dennis Margosan of theUSDA-ARS in Parlier.We thank Lynn Loken of Activon, Inc.; Richard Varleyof Anterra Group, Inc.; Dr. Joe Fitzgeorge of AllotropeEnvironmental LLC; Scott Lawhon of EQ Ag Solutions ofTexas; Tarcisio Ruiz of Pace International; Dr. Art Dawsonof the Dawson Company; Gene Taylor, Jeff Sheets, TerryOrr, Jim Williams and Joe Martinez of the Exeter CitrusAssociation; Brock Bovetti and Tony Flores of the SierraCitrus Association; Bob Johnson and Kevin Severns of theOrange Cove-Sanger Citrus Association; John Kalendar andJohn Clower of the Visalia Citrus Packing Group; and, DonDames of Ventura Pacific, Montalvo facility.Dr. Joseph L. Smilanick is a Research Plant Pathologistwith the USDA Agricultural Research Service, basedat the San Joaquin Valley Agricultural Sciences Center inParlier, CA. David Sorenson is Operations and TechnicalManager, Fruit Growers Supply Company Packing Services,Exeter. Gabriel Verduzco is a technician with USDA-ARS,Parlier. Zilfina Rubio Ames is a University of Californiajunior specialist working with USDA in Parlier. The lateMonir Mansour was employed as a Microbiologist, USDA-ARS, Parlier.CRB research project reference number 5600-106 (5400-106in FY2011-2012).Additional readingAfek, U., Orenstein, J., and Nuriel, E. 1999. Fogging disinfectantsinside storage rooms against pathogens of potatoesand sweet potatoes. Crop Prot. 18:111-114.American Congress of Government Industrial Hygienists.1999. Documentation of the Threshold Limit Values andBiological Exposure Indices, 6th Ed. American Conferenceof Governmental Industrial Hygienists; Publication 0206,Cincinnati, OH.Anon. 2006. Best practices for the safe use of glutaraldehydein health care. OSHA publication 3258-08N.Block, S. S. 2001. Disinfection, Sterilization, and Preservation.Lippincott Williams, and Wilkins, New York.Burfoot, D., Hall, K., Brown, K., and Xu, Y. 1999. Foggingfor the disinfection of food processing factories and equipment.Trends in Food Science & Technology 10: 205-210.Cerioni, L., Rapisarda, V.A., Hilal, M., Prado, F.E., andRodriguez-Montelongo, L. 2009. Synergistic antifungal activityof sodium hypochlorite, hydrogen peroxide, and cupric sulfateagainst Penicillium digitatum. J. Food Prot. 72:1660-1665.Goddard, P. A., and McCue, K. A. 2001. Phenolic compounds.Pp. 255-281 in: Disinfection, Sterilization, and Preservation.Fifth edition. S. S. Block (eds). Lippincott, Williams,and Wilkins, New York.Ladaniya, M. 2008. Preparation of fresh fruit for market.I. Degreening. Pp. 230-245 in: Citrus Fruit. Biology, Technology,and evaluation. Academic Press. New York.McWatters, L.H., Chinnan, M.S., Walker, S.L., Doyle,M.P. and Lin, C.M. (2002) Consumer acceptance of fresh-cuticeberg lettuce treated with 2% hydrogen peroxide and mildheat. J Food Prot 65, 1221–1226.Ozkan, R., Smilanick, J.L., and Karabulut, O.A. 2011. Toxicityof ozone gas to conidia of Penicillium digitatum, Penicilliumitalicum, and Botrytis cinerea and control of gray mold on tablegrapes. Postharvest Biol. Technol. 60:47–51.Ritenour, M.A., Miller, W.M., and Wardowski, W.F. 2003.Recommendations for Degreening Florida Fresh Citrus Fruits.Cir 1170, Horticultural Sciences Department, Florida CooperativeExtension Service, Institute of Food and AgriculturalSciences, University of Florida.Sapers, G.H., and Simmons, G. F. 1998. Hydrogen peroxidedisinfection of minimally processed fruits and vegetables. FoodTechnology 52:48-52.Smilanick, J. L., and Mansour, M. F. 2007. Influence oftemperature and humidity on survival of Penicillium digitatumand Geotrichum citri-aurantii. Plant Disease 91:990-996.Smilanick, J. L., Margosan, D. A., and Henson, D. J. 1995.Evaluation of heated solutions of sulfur dioxide, ethanol,and hydrogen peroxide to control postharvest green mold oflemons. Plant Disease 79: 742-747.Sperber, W. H., and Doyle, M. P. 2010. Compendium of theMicrobiological Spoilage of Foods and Beverages. Springer,New York.Wardowski, W.F., Miller, W.M., and Grierson, W. 2006.Degreening. Pp. 277 to 298 in: Fresh Citrus Fruits, 2nd Edition.Wardowski, W.F., Miller, W.M., Hall, D.J., and Grierson, W.(eds.). Florida Science Source, Inc. Longboat Key, Florida. lCLASSIFIEDWanted: Used Tropic Breeze ground-powered wind machinesand late model EOT (engine on top) machine. Call Vern in Exeterat (559) 799-1053.Classified Advertising will become a regular feature in CitrographMagazine. To place a classified ad or for questions, please contactthe Citrus Research Board by phone at (559) 738-0246 or by faxto (559) 738-0607.Classified Advertising…..It Pays!September/October 2011 Citrograph 55