Downy mildew in vineyards - Department of Agriculture and Food

Important Disclaimer:The Chief Executive Offi cer of the Department of Agriculture and Food and the State of Western Australia accept no liabilitywhatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.In relying on or using this document or any advice or information expressly or impliedly contained within it, you accept all risksand responsibility for loss, injury, damages, costs and other consequences of any kind whatsoever resulting directly or indirectlyto you or any other person from you doing so. It is for you to obtain your own advice and conduct your own investigations andassessments of any proposals that you may be considering in light of your own circumstances.This material has been written for Western Australian conditions. Its availability does not imply suitability to other areas, and anyinterpretation is the responsibility of the user.The product trade names in this publication are supplied on the understanding that no preference between equivalent products isintended and that the inclusion of a product does not imply endorsement by Department of Agriculture and Food Western Australiaover any other equivalent product from any other manufacturer. Any omission of a trade name is unintentional.Always read the label.Users of agricultural (or veterinary) chemical products must always read the label and any Permit before using the product, andstrictly comply with the directions on the label or the conditions of any Permit. Users are not absolved from compliance with thedirections on the label or the conditions of the Permit by reason of any statement made or not made in this publication. Referto your winery, chemical re-seller, chemical company, Australian Wine Research Institute, Australian Pesticides and VeterinaryMedicines Authority or Department of Agriculture and Food Western Australia if you are unsure about chemical registrations orchemical residues (especially if the grapes are being exported or are being used for export wine).Information in this publication has been produced in association with non-Department of Agriculture and Food individuals, groupsand organisations. The names of these signifi cant contributors have been provided at the beginning of this publication.Recommendations were current at the time of preparation of this material (March 2007).© State of Western Australia, 2007

Bulletin 4708Replaces Bulletin 4439ISSN 1833-7236May 2007DOWNY MILDEWin vineyardsDiana Fisher, Viticulture Development Officer, Andrew Taylor,Plant Pathologist, Colin Gordon, Senior Technical Officer,Department of Agriculture and Food Western Australia andPeter Magarey, Senior Plant Pathologist, Department ofPrimary Industries and Resources, South Australia.

PAGE 4DOWNY MILDEW IN VINEYARDSContentsIntroduction 5Symptoms .... 6Leaves .. 6Other causes of similar symptoms on leaves .............................................................................................................................8Shoots ... 8Infl orescences, bunches and berries .......................... 9Damage and loss ..................................................................................................... 10Varietal susceptibility ......................................................................................... 10Disease cycle .................................................................................................................. 10Overwintering ..................................................................................................... 10Primary infection (soil to vine) ............................................. 10Oilspots .......................................................................................................................... 11Secondary infection (leaf to: leaf, shoot, infl orescence, berries, stalk) ...................................11Spread ............................................................................................................................ 13Monitoring .............................................................................................................................. 13Management options ......................................................................................... 15Cultural ........................................................................................................................... 15Chemical ..................................................................................................................... 15Acknowledgements .............................................................................................. 18Further Reading .......................................................................................................... 18

IntroductionDOWNY MILDEW IN VINEYARDS PAGE 5Downy mildew, caused by Plasmopara viticola, is a major fungal disease of grapevinesthat originates from North America. This disease was fi rst detected in a commercialWestern Australian vineyard in October 1998 and has since been found in mostgrape growing areas within the State. It is found in all other grape growing areas ofAustralia.Plasmopara viticola is specifi c to grapevines (e.g. Vitis vinifera), although not allVitis spp. are susceptible. The American rootstock species and hybrids are lesssusceptible or are resistant (refer to ‘Varietal susceptibility’). Other species of downymildew, such as those found on cucurbits and roses, do not attack grapevines.Although, weather conditions that favour the developmentand spread of grapevine downy mildew alsomay encourage the development andspread of cucurbit and rosedowny mildews.

PAGE 6DOWNY MILDEW IN VINEYARDSSymptomsDowny mildew attacks all green parts ofthe grapevine.LeavesThe symptoms on leaves vary with leafage. On young leaves (i.e. spring),the disease will appear on the uppersurface as small yellow spots referredto as oilspots; they are about 10 mmdiameter often with a chocolate halo.These spots tend to grow to about 50mm diameter as they mature and thehalo fades. As these spots enlarge theymay appear to cover most of the leaf,especially if there is more than one spoton the leaf.Downy mildew oilspot.Rapidly growing oilspots often merge,infecting much of the leaf. Often downymildew is not detected until the diseasespreads and more oilspots are visible (notethe white areas on the leaves are spraydeposits).Downy mildew oilspots are often surroundedby a chocolate halo that fades as theoilspots grow (note the white areas on theleaves are spray deposits).

DOWNY MILDEW IN VINEYARDS PAGE 7After suitable warm humid nights, adense, raised, white cottony growthdevelops on the underside of theyellow oilspots. This growth is commonlyreferred to as ‘white down’. As the spotsage naturally, or after a sporulation event,or after hot weather, their centres dry outand become a reddish brown leaving anouter ring of yellow. The fungus in thisyellow ring remains active and givenfavourable conditions at night, canproduce a ring of ‘white down’ on thisouter active edge..After favourable conditions this outer activeyellow ring can produce ‘white down’.White down (spores) develop beneathoilspots after a suitably warm, humid night.The centre of the oilspots dry out andbecome a reddish brown leaving an outerring of yellow.

PAGE 8DOWNY MILDEW IN VINEYARDSInfection on mature leaves will be yellowbrownand remain small, confi ned by thefi nest of veins.White down (spores) will also developbeneath infections on mature leaves after asuitably warm, humid night.Later in the growing season (i.e. latesummer and autumn) on mature leaves,leaf infections will appear as small,angular, yellow spots that are limitedin growth by the veins. These form atapestry-like (mosaic) pattern of spotsthat soon turn reddish brown. Defoliationcan occur in severely affected vines.Other causes of similar symptomson leavesYoung downy mildew infections may beconfused with powdery mildew, causedby Erysiphe necator. Downy mildewspots are oily with a chocolate halo anddevelop ‘white down’ on the undersideof the leaf. Powdery mildew spots areoften smaller and yellow-green whichthen develop a thin layer of ash-greypowdery spores that may eventuallycover both sides of the leaf (refer toBulletin 4575 Powdery Mildew in WineGrapes in Western Australia).Yellow spots on leaves may also be dueto spray drift damage from herbicidessuch as paraquat (e.g. Spray.Seed ® )or sucking insect damage. Paraquatdamage is distinguishable from downymildew as it does not grow and it tendsto develop a small brown spot in thecentre of the yellow area. Also, neitherform of damage will produce white downon the underside of the spots after thebag test (see ‘Monitoring’).White growth on the underside of a leafmay also be due to grape leaf blistermite damage. This white growth isdistinguishable from downy mildew as itforms within blister-like green galls thatbulge on the upper side of the leaf.White fungal growth on a leaf may alsobe due to other fungi such as Penicillium,Aspergillus or Rhizopus. These fungalinfections will eventually change togreen, blue, black or brown.ShootsInfection on young shoots, stems andtendrils are seen as oily brown areas.These oily patches may spread into leafstalks, which turn brown and may die.After suitable warm humid nights theseoily patches may also sporulate and becovered with white down.

DOWNY MILDEW IN VINEYARDS PAGE 9Inflorescences, bunches and berriesInfection on inflorescences, youngberries and bunches are seen as oilybrown areas. After suitable warm humidnights they may be covered with whitedown. Infected inflorescences and youngbunches rapidly turn brown and wither.Infected young berries stop growing,harden and may later develop a purplehue. They then turn a dark brown, shriveland fall from bunches.Berries become resistant to infectionwhen they are about pea size (around5-6 mm diameter). However, they maystill be killed if the berry or bunch stemsbecome infected. They may also sunburnand fail to ripen if defoliation occurs fromleaf infection.White spores may develop oninfl orescences after infection.Infection near fl owering can kill the buncheswhich then turn brown.

PAGE 10DOWNY MILDEW IN VINEYARDSDamage and lossSevere infection will cause leaves to fallprematurely, reducing yield and berrysugar content and will expose remainingbunches to sunburn. Total crop loss mayoccur if severe infection is not managed,especially near flowering. Severe leaf fallalso can cause yield loss in the followingseason due to the inability of the vine tostore reserves.Varietal susceptibilityAll varieties of Vitis vinifera aresusceptible to downy mildew infection.Vitis vinifera hybrids, V. aestivalis and V.lambrusca are less susceptible while V.rupestris, V. cordifolia and V. rotundifoliaare generally resistant.Disease cycleDowny mildew is an obligate parasite(meaning it requires a living host) andtherefore it grows on all green parts of thevine. However, there is one overwinteringstage of downy mildew development thatis not found on green tissue.OverwinteringOospores (resting bodies) are formed inlate summer or autumn from the fungalgrowth within leaves, shoots or berries.These resting bodies fall to the groundwhen leaves and bunch parts fall inautumn. There they overwinter in infectedleaves and litter in the soil for 3 to 5 years(and possibly up to 10 years).Oospores are the sexual structures ofdowny mildew. An oospore has a thickwall that makes it less susceptibleto fungicides and adverse weatherconditions, such as exposure to thesun, than zoospores or sporangia (seebelow).Primary infection (soil to vine)The 10:10:24 ‘rule of thumb’ refers tothe conditions required for primary (first)infection to occur. At least 10 mm rainfall(and irrigation) is required while thetemperature is 10°C or more over a 24-hour period. Not all 10:10:24 conditionsare suitable for a primary infection butthis ‘rule of thumb’ provides a goodguide to monitor for favourable primaryinfection conditions.10:10:24 ‘rule of thumb’At least 10 mm rainfall(and irrigation);10°C or more;over a 24 hour periodMore specifically the conditionsrequired for oospores to germinate are:• soil needs to be wet for at least 16hours• usually achieved by 3-5 mm rainfall(and/or irrigation).• temperature also needs to remainabove 10°C.The germinated oospores then releasezoospores (that swim in free water)which then need to be splashed by rainor irrigation to the vine canopy before theend of the 24 hour period. This process

DOWNY MILDEW IN VINEYARDS PAGE 11usually requires another 3-5 mm of rain(and/or irrigation) to ensure suffi cientsplash and leaf wetness for infection onthe underside of the leaves. For this, thefoliage must remain wet for at least 2-3hours at 20°C (or 4-5 hours at 10°C) forthe spores to infect the leaf and completethe primary infection cycle.Irrigation alone is not normally enoughto induce a primary infection. However,rainfall and irrigation combined can besuffi cient for this to occur.OilspotsThe zoospores released during primaryinfection that establish on the undersideof the leaf begin to grow fungal hyphae.These hyphae grow inside the leaves toform oilspots that appear 5 to 17 days (butmore often 5 to 10 days) after infectionhas occurred. The development ofoilspots is quickest in warm weather (18-27°C). At warmer or cooler temperaturesthe incubation period is longer.Primary infection levels are usuallylow with only 1-3 oilspots developingper 50 m of vine row. Hence, primaryinfections are very diffi cult to fi nd anddo not themselves cause crop loss.However they are very important in thatthey allow the disease to spread rapidlyif secondary infection events occur.Secondary infection (leaf to: leaf,shoot, inflorescence, berries, stalk)At least 98% humidity;13°C or more;at least 4 hours darkness;leaf wetness 2-3 hours near dawnDowny mildew (Plasmopara viticola)Figure 1. Downy mildew ‘white down’ underthe microscope. (Diagram courtesy RosPilbeam, formerly Horticulture Pathologist,Department of Agriculture and Food).Active oilspots need to be present beforesecondary infections can occur. Theseoilspots (and surfaces of other diseasedtissue) produce sporangia (seen as whitedown) on suitable warm wet nights.Sporulation requires at least 4 hours ofdarkness to develop, during which timethe temperature is 13°C or more andhumidity is 98% or more.To then cause infection the foliagemust also be wet for at least 2-3 hoursonce sporulation has occurred. Thewet foliage can be the result of rainfall,overhead irrigation or occasionally fromheavy dew.Secondary infections can occur anytimeduring the growing season wheneveroilspots are present and conditions arefavourable.Sporangia are the asexual structures ofdowny mildew. Sporangia are producedon tiny treelike structures known assporangiophores (see Figure 1). A singlesporangium can in turn produce between1 to 10 zoospores. Zoospores are ableto move (swim) through water but arespread mainly by wind and rainsplash.

PAGE 12DOWNY MILDEW IN VINEYARDSFigure 2. The life cycle of downy mildew(reproduced with permission from PeterMagarey).

DOWNY MILDEW IN VINEYARDS PAGE 13SpreadPrimary infections begin the diseasecycle by providing a source ofoilspots.Secondary infections produce sporesthat can be spread by wind and rainto establish new infection sites.Secondary infections are the majorsource of disease spread.MonitoringGood management of downy mildewis dependent on good monitoring offavourable weather conditions for primaryand secondary infection events and ofdisease progress in the vineyard.Automatic weather stations can beused for monitoring and predictingweather events. Weather stations collectinformation on temperature, rainfall,leaf wetness and humidity (suitablefor downy mildew) and process thedata for the likelihood of a primary orsecondary infection event (e.g. Ausvit).Alternatively a maximum/minimumthermometer and a rain gauge can beused as a guide.Monitoring in the vineyard is importantto confirm possible infections fromweather events. Physical examinationcan confi rm whether a pre-infection orpost-infection fungicide is required andwill also indicate if spray coverage waseffective after an event.Monitoring in the vineyard shouldoccur every 7 - 14 days when weatherconditions are favourable from 3 to 4weeks after budburst (i.e. when shootsare approximately 10 cm long).Some general guidelines on vineyardmonitoring include:• Vines to be monitored should berepresentative of the block beingassessed.• Monitor for possible source areasof downy mildew infection such aswetter more sheltered parts of thevineyard (e.g. near windbreaks andsheds), vines with dense canopiesor areas that have previously beeninfected.• Inspect both sides of 200 vines byscanning the foliage between midmorningand mid-afternoon.• Spend about 30 seconds per vine.Less time will be required early inthe season when vine canopies aresmall.• Focus on the canopy near theground in lower lying areas wherethe soil may remain wet for extendedperiods.• The disease is diffi cult to detect indense shaded canopies. The foliagemay need to be parted to scan theinner leaves.

PAGE 14DOWNY MILDEW IN VINEYARDS• Use a hand lens to check suspectspots on leaves for evidence of thefungus.• Take samples of suspect leaves andbunches and conduct a bag test(see below).• Tag suspect sample sites so theymay become the reference points ifthe sample is confi rmed positive.Monitoring for downy mildew can becombined with monitoring for other pests(e.g. diseases and insects).Note: Other fungi may infect leaves andbunches. Downy mildew spores are whiteand raised. Powdery mildew spores areash-grey and are a uniform thin layer.Botrytis spores are cream to grey-brown.Penicillium, Aspergillus or Rhizopus arewhite and raised but change to green,blue, black or brown. If there is anyuncertainty suspect samples may besubmitted to a pathology laboratory forconfi rmation.AGWEST Plant LaboratoriesDepartment of Agriculture and FoodWestern AustraliaLocked Bag No. 4Bentley Delivery Centre WA 6983Ph: (08) 9368 3721Fax: (08) 9474 2658Oilspots that have already developed‘white down’ on their underside (whetherthis fungal growth is fresh or old) maynot develop any more ‘white down’ evenafter a bag test is conducted.Bag testThe bag test is used to indicate active downy mildew.• Pour clean water into an empty sealable plastic bag; shake the contents,then empty the bag leaving it lightly moistened.• Seal fresh, suspect leaves or bunches in the bag overnight in the dark at20°C to 25°C, but at least greater than 13°C.• A kitchen cupboard is usually ideal. It is best to place the leaves facingdownwards in the bag. Do not place leaves or bunches on top of eachother.• Next morning, fresh white down will have developed on young bunches oron the undersides of oil spots if downy mildew is present and active.

Management optionsCulturalOospores may spread from propertyto property and region to region by themovement of infected leaves and litterin the soil or on vines late summer andautumn. Avoid distribution of infectedsoil and plant matter by equipment andmachinery (e.g. mechanical harvesters,leaf pluckers, trimmers and utilities), bysoil still adhered to rootlings, or by pottedvines from nurseries.Sporangia may spread across propertyboundaries by wind. It is very diffi cult toprevent the spread of these spores. Itis believed that sporangia rarely spreadmore than 200 m by wind.Canopy management practices thatencourage air movement will help todry out leaves and improve sunlightand spray penetration. This will helpto prevent infection. Such practicesinclude:• lower planting density• trellising and pruning to open thecanopy• shoot training to open the canopy• vine trimming and hedging• lateral shoot thinning• leaf plucking.Vegetative growth may also bemanaged by the selection of appropriaterootstocks prior to planting and by carefulapplication of fertilisers (e.g. nitrogen).Excessive growth leads to denseshaded canopies that may encouragethe development of downy mildew.ChemicalPre-infection fungicides (appliedas close as possible but prior to aninfection event)Pre-infection (protectant) fungicides helpto prevent downy mildew zoospores fromentering the green vine tissue. Spraycoverage needs to be excellent toadequately protect all of this green tissue.In particular they need to be applied tothe underside of leaves and the backof bunches. It is important to time theirapplication as close as possible but priorto the possible infection event (e.g. whenpossible primary or secondary weatherevents are forecast).DOWNY MILDEW IN VINEYARDS PAGE 15A pre-infection spray program tends tobe used where downy mildew is wellestablished in a region or vineyard andoccurs frequently. Growers who areunable to conduct careful monitoringtend to use a pre-infection sprayprogram. Large vineyards and tablegrape growers tend also to use preinfectionspray programs or those withsoils which make access limited after arain event, delaying post-infection sprayapplication.Pre-infection fungicides are not systemicand any new growth after the spray hasbeen applied will not be protected.Rain and overhead irrigation will diluteor may wash the protectant sprays offthe vine. Hence, further applications willbe required before the next possibleinfection event.A pre-infection spray program oftenrequires application on a 7 to 14 dayschedule. This may be expanded toa 21 day program later in the seasonas shoot growth slows and possibleinfection events are less. As fl oweringis the critical period to prevent croploss, the spray program may need to betightened to every 5 to 7 days to coincidewith possible infection events.

PAGE 16DOWNY MILDEW IN VINEYARDS(Note: In table grapes pre-infectionfungicides should only be used providedit is prior to 10 mm berry size as sprayresidues on berries may occur).Post-infection fungicides (applied assoon as possible after an infectionevent)Post-infection (eradicant) fungicidesare systemic and penetrate the vinetissue killing the downy mildew fungusfrom within the vine tissue. Use of thesefungicides involves withholding spraysuntil an infection event has occurred.Relying on post-infection fungicidesrequires careful monitoring and has agreater risk of downy mildew becomingestablished. However, if downy mildew isnot established in the region or vineyardand few possible infection events occur ithas the advantage of using fewer spraysthat have greater effectiveness.Post-infection fungicides work bestwhen applied as soon as possible afteran infection event - within fi ve days ofinfection and before oilspots appear. Noadditional spraying should be requireduntil weather conditions favour anotherpossible infection event. In this situation,pre-infection fungicides may be usedonce again.Once the fungus is visible it is diffi cultto kill. A single post-infection spray isusually not effective, although it mayreduce the number of spores and limitspread of the disease. Hence, follow upLarge oilspots killed by well-timed sprays ofpost-infection fungicides.

DOWNY MILDEW IN VINEYARDS PAGE 17sprays of post-infection fungicides maybe required after the initial post-infectionfungicide spray.(Note: In table grapes not all postinfectionfungicides can be used after10 mm berry size for the control of downymildew. Please contact your nearestDepartment of Agriculture and FoodOffi ce for further information).Did the post-infection fungicide work?Use the bag test to check whether thepost-infection fungicide was effective.Conduct the tests 24 hours and then,if required, another three days afterspraying with samples of healthybunches, limp and browning bunchesand leaves with oilspots. Keep eachsample in a separate moist bag.Consider immediate repeat spraying witha post-infection fungicide if fresh whitedown is evident on the underside of theleaf or on bunches of these sampleswhen inspected the next morning.Post-infection products should be usedafter an infection event if there is anyconcern that the pre-infection fungicidesapplied were not adequate. Browningbunches may be too badly infected forthe post-infection fungicide to preventcrop loss and only normal lookingbunches will benefi t from the spray.Refer to CropLife Australia www.croplifeaustralia.org.au for resistancemanagement of downy mildewfungicides.

PAGE 18DOWNY MILDEW IN VINEYARDSAcknowledgementsPeter Magarey, Plant Pathologist,Primary Industries and Resources,South Australia, Loxton is gratefullyacknowledged for granting permissionto reproduce Figure 2 ‘The lifecycle ofgrapevine downy mildew’ from page 8of Nicholas, P., Magarey, P. and Watchel,M. (eds) (1994) Diseases and Pests– Grape Production Series Number 1,Winetitles, Adelaide.Peter Magarey is also acknowledged forsupplying the last three photos.Ros Pilbeam, formerly Plant PathologistDepartment of Agriculture and FoodWestern Australia is also gratefullyacknowledged for supplying Figure 1.The first six photos were suppliedby Andrew Taylor and Diana Fisher,Department of Agriculture and FoodWestern Australia.Further ReadingBell, S-J. and Daniel, C.G. (2006)Agrochemicals Registered for Use inAustralian Viticulture 2006/2007. TheAustralian Wine Research Institute, 17 May2006.Braybrook, D., Shanks, A., Aitken, D. andHogan, G. (2002) Viticulture Research toPractice 2003 Integrated Pest Management.Cooperative Research Centre for Viticulture,Adelaide.Fisher, D. and Wicks, T. (2003) PowderyMildew in Wine Grapes in Western Australia.Department of Agriculture WesternAustralia, Bulletin 4575.Nicholas, P., Magarey, P. and Watchel, M.(eds) (1994) Diseases and Pests – GrapeProduction Series Number 1. Winetitles,Adelaide.Pearson, R.C. and Goheen, A.C. (eds)(1988) Compendium of Grape Diseases.American Phytopathological Society,Minnesota.

DOWNY MILDEW IN VINEYARDS PAGE 19

1915-05/07-ID7704