38 39 The Sunflower Production Guide DISEASES EARLY SEASON DISEASES Downy Mildew Downy mildew is capable of killing or stunting plants, reducing stands and causing severe yield losses during wet years. Host Crops Sunflower Biology Downy mildew is caused by a soil-borne, wind-borne and seed-borne fungi Plasmopara halstedii, which can survive for up to 10 years in soil. Under cool, water-saturated soil conditions, the spores germinate upon contact with sunflower roots, enter, and spread throughout the entire plant. This is systemic infection. Systemic infection occurs only when the roots are less than 2 inches long when they contact the fungi. Sunflower plants surviving the initial infection will produce white spores on the underside of cholorotic areas on leaves. The white spores are air-borne, and upon landing on sunflower leaves may cause secondary infections. Secondary infections are most common when sunflower leaves remain wet for prolonged periods of time. Plants are susceptible to secondary infections for a much longer period of time versus systemic infections via root infection. As infected plants rot and are tilled into the soil, the fungus forms the resting stage which will germinate during favorable conditions in subsequent years. Figure 38. Seedling Sunflower with downy mildew. Figure 39. Downy Mildew causes stunting and heads to face straight upwards. Diseases | 37
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The Sunflower Production Guide
DISEASESEARLY SEASON DISEASESDowny Mildew
Downymildewiscapableofkillingorstuntingplants,reducingstandsandcausingsevereyieldlossesduringwetyears.
Host Crops
Sunflower
Biology
Downymildewiscausedbyasoil-borne,wind-borneandseed-bornefungiPlasmopara halstedii,whichcansurviveforupto10yearsinsoil.Undercool,water-saturatedsoilconditions,thesporesgerminateuponcontactwithsunflowerroots,enter,andspreadthroughouttheentireplant.Thisissystemicinfection.Systemicinfectionoccursonlywhentherootsarelessthan2incheslongwhentheycontactthefungi.Sunflowerplantssurvivingtheinitialinfectionwillproducewhitesporesontheundersideofcholoroticareasonleaves.Thewhitesporesareair-borne,anduponlandingonsunflowerleavesmaycausesecondaryinfections.Secondaryinfectionsaremostcommonwhensunflowerleavesremainwetforprolongedperiodsoftime.Plantsaresusceptibletosecondaryinfectionsforamuchlongerperiodoftimeversussystemicinfectionsviarootinfection.Asinfectedplantsrotandaretilledintothesoil,thefungusformstherestingstagewhichwillgerminateduringfavorableconditionsinsubsequentyears.
Figure 38. Seedling Sunflower with downy mildew.
Figure 39. Downy Mildew causes stunting and heads to face straight upwards.
Diseases|37
The Sunflower Production Guide
Symptoms
Symptomscanvaryconsiderablydependingonamountofinoculum,theageandhostorganattackedandenvironmentalfactors.Symptomscanbebroadlyclassifiedasthosecausedbysystemicorlocalizedinfections.Rootinfectionofseedlingsoryoungplantsresultsinsystemicinfection.Severelyinfectedplantsmaydiebeforeorshortlyafteremergenceorintheseedlingstage.Typicalsystemicsymptoms(Figure38)inseedlingsincludedwarfing,yellowingoftheleavesandtheappearanceofwhite,cottonymassesonthelowerandsometimesupperleafsurfaceduringperiodsofhighhumidity.Whenseedlingsareinfectedseveralweeksafteremergence,orafungicideseedtreatmentinhibitsratherthanpreventsinfection,theplantsstartshowingsymptomsatthefour-,six-,oreight-leafstage;thisistermed‘delayedsystemicinfection’.Theseplantsmayormaynotdeveloptypicaldownymildewleafsymptoms,butaretypicallystuntedwiththickened,clublikeroots.Theheadsofthefewplantsreachingmaturityfacestraightupwardsandseldomproduceviableseed(Figure39).Airbornedownymildewsporescancauselocalizedsmall,angularfoliarlesionswiththewhitefungalgrowthontheundersideoftheleaf.Theseinfectionsgenerallyhaveminimalimpactonyield.
Yieldlossesmaybesubstantial.Ifinfectedplantsaredispersedrandomlythroughoutthefield,yieldlossesprobablywillnotbeobservedunlessinfectionexceeds15percent.Neighboringplantscancompensateforseverelyinfectedplantsbygrowinglargerheads.Whenthediseaseisinalocalizedarea,suchasalowspotinafieldandallplantsareinfected,yieldlosscanbesubstantial.
Scouting Techniques
Infectedplantscanmostoftenbefoundaloneorinstandingwater.
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CONTROL TIPS
Resistantvarietiesareavailable;however,duetothedevelopmentofnewraces,resistancemaynotbeasufficientmanagementtoolinallfields.Fungicideseedtreatmentscanbeaneffectivemanagementtoolfordownymildew,butaswithgeneticresistance,thepathogenhasdevelopedresistancetometalaxylandmefanoxam,twoofthemostcommonlyusedfungicides.Otherseedtreatmentoptionsareavailableandmorewillbelabeledinthefuture.RefertothecurrentissueoftheGuidetoCropProtectionforupdatedinformationonproductsandratesforapplication.Othermanagementproceduresaretocontrolweedhostswhichincludewildandvolunteersunflowerstohelpreduceinoculum.
The Sunflower Production Guide
FOLIAR DISEASESRust
Earlyinfectionofsusceptiblevarietiescandecreaseheadsize,seedsize,oilcontentandyield.Confectionhybridsaremoresusceptiblethanoil-seedtypes.
Hosts
Wildsunflowerandcultivatedsunflower
Biology
SunflowerrustiscausedbyPuccinia helianthi.Infectioncanoccuratanytimeduringthegrowingseason,dependingontheinoculumsourceandenvironmentalconditions.Conditionsfavorableforinfectionarefreewaterontheleaves,eitherfromrainfallordew,andwarmtemperatures.Aminimumoftwohoursofwetleavesissufficientforrustinfection;sixtoeighthoursofleafwetnessresultsinthemaximumamountofinfection.Sunflowerrustsporesoverwinteronthedebrisfrominfectedsunflowerleavesandstemsofpreviouscrops.Inthespringsporesgerminatetoinfectvolunteerseedlings,wildsunflowerornewyoungplantsinnearbyfields.Underfavorableconditions,sporeproductionandinfectioncanoccurcontinuouslywithinayear.The‘repeatingstage’isthemostdamaging,asmultiplewavesofsporesarespreadbywindtootherfieldsasconditionsallow.Asthecropripens,thesporesformtheoverwinteringstagewhichcanthenre-infectfollowingsunflowercrops.
Symptoms
Thefirstsignsofrustusuallyappearwhensunflowersareatorpastbloomasenvironmentalconditionswithinthecropcanopyaremorefavorableforinfection.Theaecialstage(Figure41)ofrustappearslatespringtosummerasclustersoforangecups.Themostcommonstageofrust(uredinia)(Figure 42)isoftenobservedwithintwoweeksoftheaecia.Thesepustulesaresmall(0.1to1mm),cinnamon-brown,canberubbedofeasilyandoccuronboththeupperandundersidesofleaves.Asthediseaseprogresses,urediniamaybefoundontheupperleaves,stemandbractsofthesunflowerplant.Inresponsetotemperature,theurediniaconverttotelia(blackspotsontheuppersurfaceofleaves)attheendoftheseasonwhichdonotruboffandaretheoverwinteringstructures.
Economic Threshold
Rustseverityontheupperfourleavesis3percentorgreater.
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0.5% Leaf Area Infected 1% Leaf Area Infected 2% Leaf Area Infected 5% Leaf Area Infected
Figure 40. Leaf area infected by rust.
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CONTROL TIPS
Themosteffectivewaytoavoidlossfromrustisbyplantingrust-resistanthybrids.Mostoilseedandconfectionaryhybridshavegoodtoexcellentresistancetomostracesofrust.Rotationtoothercropsisalsoausefultooltominimizeinfections,assunflowerrustonlyinfectssunflowers.Ifpossible,avoidplantingnexttoafieldthathadsunflowerslastyear;managewildsunflowersorvolunteersunflowersastheyarehostsforthedisease.Earlyseedingandshortseasonhybridscanalsohelpminimizediseaseseverity.Anyculturalpracticewhichfostersadensecanopy,suchashighplantstandsandhighnitrogenfertilization,whichtrapsdew,increaseschancesofasevererustinfectionandshouldbeavoidedifrisksarehigh.Foliarfungicidesareregisteredforrust.ConsultthecurrenteditionofGuidetoCropProtectionfor products.
Figure 41. Aecia cups on the underside of sunflower leaf.
Figure 42. Rust uredinia develop on the under-and upper-leaf surfaces.
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The Sunflower Production Guide
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The Sunflower Production Guide
Alternaria Leaf and Stem Spot
Alternarialeafspotisaubiquitousdiseaseonsenescingleavesandgenerallyoflittleconcern.Underwarmandhumidconditions,itcanhoweverbecomeaseriousdefoliatingandyieldreducingdisease.
Host Crops
Sunflower,safflowerandcocklebur
Biology
Alternariafungioverwinterondiseasedstalksandmaybeseed-borneatlowlevels.SeedlingblightscausedbyAlternariamaydevelopwhensunflowerplantsemergeinrainyweatherinAlternaria-infestedsoil.Alternariasporesarespreadbywindandsplashingwater.Thefungirequirefreewaterfor4hoursforinfectiontooccur.Leafsymptomsaremostfrequentlyobservedafterfloweringasthedensecanopyisconducivetoinfection.Onceestablished,thediseasecandevelopquicklyunderfavorableconditions.InwesternCanada,climateisnottypicallyconduciveforAlternariaepidemicsandgenerallyonlythelowersenescingleavesareaffected.
Symptoms
Darkbrownirregularspotswithdarkbrowntopurplebordersandagraycenterdeveloponleaves(Figure43).Thespotsonyoungplantsmayhaveayellowhalo.Leaflesionsmaycoalesce,causingleavestowither.Stemlesionsbeginasdarkflecksthenenlargetoformlong,narrowlesions.Thestemlesionsoftenjointoformlargeblackenedareaswhichmayresultinstembreakage.Thelesionsarelocatedrandomlyalongthestemandnotassociatedwithapetiole.
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CONTROL TIPS
Croprotationandburyinginfestedcropresidueto hastendecompositionhelpsminimizeAlternaria infection.
Figure 43. Alternaria leaf lesions close up.
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The Sunflower Production Guide
Septoria Leaf Spot
Septoriaiswidelydistributedonsunflowersbutusuallycauseslittledamage.Insevereinstancesitcancausedefoliationofthelowerleaves.
Host Crops
CultivatedandWildSunflower
Biology
SeptoriaiscausedbythefungiSeptoria helianthi.Thefungisurviveoninfectedcropresidueandcanalsobeseed-borne.Thediseasecanappearanytimeduringthegrowingseasonwithinitiationfavoredbymoderatelyhightemperaturesandabundantrainfall.
Symptoms
Septoriadevelopsfirstonthelowerleavesandspreadsupwardsthroughthecanopy.Thelesions(upto15mmindiameter)beginaswater-soakedareaswhicharegreasygreeninappearance.Thespotsbecomeangulartodiamondshaped,withtancentersandbrownmargins.Youngspotsareoftensurroundedbyanarrowhalothatgraduallymergeswiththesurroundinggreentissue(Figure44).Maturespotscontainsmallblackspecksorfruitingbodies.ThisisthebestwaytodistinguishAlternariafromSeptoria.Thelesionsmaycoalesceinlaterstagesandtheleafmaywitheranddie.
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44 CONTROL TIPS
Croprotation,incorporationofsunflowerresidueanduseofcleanseedarethebestmanagementpracticestomanageSeptoria.
Figure 44. Septoria lesions contain small black fruit bodies.
STALK AND ROOT-INFECTING DISEASESSclerotinia Wilt and Basal Rot
Infectedplantsdierapidly,andiftheplantdiespriortoseedmaturityitresultsinyieldloss,lowertestweight,andloweroilcontent.
Host Crops
Sclerotiniahasaverywidehostrangeofover360species,whichincludessunflowers,canola,mustard,drybeans,fieldpeas,lentilsandpotatoes.
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The Sunflower Production Guide
CONTROL TIPS
Managementofsclerotiniaisdifficultbecauseofthewidehostrange,butrotationtocerealsandcornisthemosteffectivetominimizesclerotiniainthesoil.Sunflowerhybridswithsomelevelsofresistanceareavailable.TheuseofamycoparasiteswhichfeeduponotherfungihasshowntoattackSclerotinia.OnecommerciallyavailableisConiothryium minitans(Contans).Thismycoparasitecankillsclerotiainseveralmonthsratherthanyears.
Figure 45. Sclerotinia wilt.
Figure 46. Sclerotinia Basal rot lesion.
Biology
SclerotiniaoverwintersasSclerotinia sclerotioruminthesoiloronplantdebris.Sclerotiaareirregularlyshapedstructureswhichrangeinsizeandshapefromsphericaland1/8inchindiametertoY-shapedandupto1inchinlength.Thesclerotiabodiescansurviveinthesoilfor5ormoreyears.Assunflowerrootsgrownearsclerotiniainthesoil,thesclerotiaarestimulatedtogerminateproducingmyceliumwhichinfectsthelateralroots.Sclerotiaforminthedecayedstempithandontherootsastheplantdies.Thesclerotiaarereturnedtothesoilduringtillageoperationsandserveassourcesofinoculumforthenextsusceptiblecrop.
Soilmoistureandtemperatureduringthegrowingseasonarenotcriticalfactorsaffectingtherateofincidenceofsclerotiniawilt.
Symptoms
Wilt(Figure45)canappearatanytimebetweenemergenceandmaturity,butismoreprevalentaroundflowering.Suddenwiltoftheplantoccurswheninfectedrootscannotuptakeadequatewatertomeetthedemandsoftheplant.Examinationofthestem-rootareawillrevealatan-brown,watersoakedlesionatthesoilsurface(Figure46).Thestalksandrootsmaybecomecoveredwithwhitemyceliaandhardsclerotiabodiesdevelopunderverywetsoilconditions.
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The Sunflower Production Guide
Sclerotinia Mid-Stalk Rot
Mid-stalkrotisthediseaseleastoftencausedbySclerotinia.Lodgingcancausecompleteyieldlossonaperplantbasis.
Biology
Underwetsoilconditions,thesclerotiainthesoilcangerminatetoformsmallmushroomscalledapotheciathatproduceair-bornespores.Thesporescanoriginateinthesunflowerfieldorcanbeblowninfromadjacentfields.Sporescanmovebyrainsplash,variousinsectcarriersandupto1kmbyair.Thesporesrequirefreewaterandsenescingplanttissuetogerminateandinfectplants.Mid-stalkinfectionmayresultfromleafinfectionorinfectionattheleafaxil.
Symptoms
Infectioncanoccuranytimefromseedlingtomaturitydependingonthepresenceofinfectingsporesandfavorableenvironmentalconditions.Mid-stalkrotbeginswithinfectionoftheleaf,andthefungusprogressesinternallythroughthepetioleuntilitreachesthestem.Theleaflesionsarenotuniqueenoughtoidentifythefungus,butthestemlesionsareidenticaltothatformedbyrootinfection;tancoloredandwater-soaked(Figure47).Thesclerotiacandevelopwithinthestemorontheexteriorofthestem.Leavesabovethelesionwilt,andthestalkeventuallydisintegratesbecomingshreddedasonlyvascularcomponentsofthestemremain(Figure48).
CONTROL TIPS
ThecommentsmadeaboutSclerotiniawiltalsoapplytomid-stalkrotmanagement.Culturalpracticestoavoidhighplantdensitiesbyreducingpopulationsandhighnitrogenfertilizationhelplowertheincidenceofinfection.Shalloworzero-tillagepracticesmayaidinfasterdeteriorationassclerotiaareleftonthegroundsurfaceandsubjecttoincreasedenvironmentaldeterioration.Resistancegeneshavebeenthemosteffectiveapproach,andbreedingforhighertolerancecontinues.
Figure 47. Sclerotinia mid-stalk rot lesion
Figure 48. Sclerotia bodies develop within the stem tissue or on the surface of the stem.
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The Sunflower Production Guide
Phoma Black Stem
PhomaisthemostwidespreadstalkdiseaseinthenorthernGreatPlains,but yieldlossesareconsideredminimal.
Host Crops
Cultivatedsunflower
Biology
Thefungus(Phoma macdonaldii )overwintersininfecteddebris.Phomainfectionoccursthroughoutthegrowingseason;however,itisnotusuallynoticeduntilthestemlesionsbecomeobviouslaterinthesummer.Leavescanbeinfectedbywindcarriedspores,rainsplashedsporesorsporestransmittedbyinsects.Stemweevilsaresuspectedintransmittingthefungiinternallyandexternally.Adultweevilsfeedingontheleavescauseleaflesionswhereascontaminatedlarvaespreadthefungusastheytunnelthroughoutthestem.LeaflesionsarenotdistinctiveandcanbeconfusedforAlternaria.Theleafinfectionsprogressdownthepetioletothestalk.Underfavorableconditions,theleafwilts,thepetioleturnsuniformlyblack,andthestemlesionsexpand.
Symptoms
Large,jetblacklesionsdeveloponthestem(Figure49),sometimesreachingabout4to5cminlength.Thelesionsareuniformlyblackandshinywithdefiniteborders.Smallcircularfruitingbodiesofthefungusareproducedonthesurfaceofthestembuttheserequireahandlenstosee.Thefungusmayalsoproducelesionsonthebackofthehead,ontheleaves,andatthebaseofthestalk.Stemlesionsdonotresultinpithdamageorlodgingandaregenerallyregardedassuperficiallesions.IfstemweevillarvatunnelingspreadsPhomasporeswithinthepith,extensivepithdegenerationcanoccur.
CONTROL TIPS
CulturalpracticestominimizePhomaincludesafouryearrotationtominimizetheinoculumloadinthesoil,delayedplantingandavoidinghighplantpopulationsandhighnitrogenfertilization.Controlofstemweevilscanhelpreducetransmissionofthefungusbutinsecticideapplicationisrarelyeconomicallyjustified.Somehybridsaremoretolerantthanothers,butnoneareimmunetothe disease.
Figure 49. Phoma black stem lesion
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The Sunflower Production Guide
Phomopsis Stem Canker
InrecentyearsPhomopsishasbecomeaveryprevalentdisease.Yieldlossesresultfromsmallerheads,lighterseedandlodgingduetoweakenedstems.
Hosts
Sunflower
Biology
Thefungi(Phomopsis helianthii)overwinteroninfectedplantdebrisandsporesarerainsplashedorwindblownontoleaves.Theinfectioninitiatesatleafmarginsoflowerleaves,developingintoabrownnecroticareaborderedbyachloroticmargin.Theinfectionspreadsdownthroughtheveinstothepetioleandfinallytothestem.ThesymptomsaresimilartothoseofVerticilliumleafmottle;however,withVerticilliumveinsremaingreen.Stemlesionsusuallydonotappearuntilflowering.Thediseaseismostsevereunderconditionsofprolongedhightemperaturesandhighrainfall.
Symptoms
Firstsymptomsinitiateonlowertomiddleleavesafterfloweringasnecroticspotswithachloroticborderonleafmargins.Thestemlesionsbeginasasmallbrownsunkenspotbutenlargesrapidlybecomingalargetantolightbrownlesionorcankercenteredonaleafpetiole.LesionsaremuchlargerthanPhomablackstem,sometimesreaching6inchesandbrownratherthanblack.Blackfruitingbodies(pycnia)formoninfectedtissue.Phomopsiscausesextensivepithdegradationandthestalkcanusuallybecrushedundermoderatethumbpressure.PhomopsisinfectedplantsaremorepronetolodgingthanPhomainfectedplants.
50CONTROL TIPS
Thoroughincorporationofthesunflowerstalksintothesoiltoburyresidueandcroprotationcanreducediseaseincidenceandseverity.CropresidueleftonthesoilsurfacefostersdevelopmentofPhomopsis.HybridsresistanttoPhomopsis,developedusingparentallinesfromEuropewherethediseaseisparticularlysevere,arecommerciallyavailable.
Figure 50. Phomopsis Stem Canker
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The Sunflower Production Guide
Verticillium Leaf Mottle
Verticilliumcansignificantlyreducesunfloweryieldespeciallyonlightersoils.Qualitymayalsobeaffectedthroughdecreasedoilcontentandseedsize.
Host Crops
Variousplantspeciescanbeinfectedincludingsunflowers,potatoes,alfalfaandsweetclover
Biology
Thefungus(Verticillium dahliae)isseed-andsoil-borne.Itsurvivesasverysmall,black,restingbodies(microsclerotia)indiseasedsunflowerdebrisandbroadleavedweeds,persistingfor5to10years.Themicrosclerotiagerminateinresponsetorootexudates.Theroottipsofsunflowerplantsareinvadedand,eventually,allpartsoftheplantbecomeinfected.Thefungusproducestoxinswhicharetranslocatedthroughouttheplantcausingthechloroticandnecroticareasbetweentheveins.Whentheplantdies,thefungusproducesblackfungalbodiesthatreturntothesoilwiththeplantdebris.
Symptoms
Symptomsusuallyarenotobserveduntilflowering,butmayappearatthesix-leafstageundersevereconditions.Initialsymptomsbeginastinychloroticflecksthatincreaseinsizewithtimedevelopingintoextensiveinter-veinalyellowing(Figure51).Symptomsbeginonthelowerleavesandprogressslowlyuptheplantandmayeventuallyinfectallleaves.Affectedleavesquicklybecomedry.Thevascularsystemmaybediscoloredbrown,apparentasaringaroundthepithincrosssection.Inseverelyinfectedplants,thepithisblackenedwithalayeroftinyblackfruitingbodies.
CONTROL TIPS
Mostoil-seedhybridscontainresistancetoVerticilliumwhereasconfectionhybridsaregenerallymoresusceptible.However,anewstrainhasrecentlybeenidentifiedinCanadaandtheU.S.whichisabletoovercomethesourceofresistanceinthesevarieties.Thediseaseismoresevereonlighterlandwithahistoryofsunflowercroppingthanonheavy,claysoil.Rotationisimportantonlightersoilwithahistoryofsunflowercroppingtoreduceinoculumloadinthe soil.
Figure 51. Verticillium Leaf Mottle
Diseases|47
The Sunflower Production Guide
HEAD ROT AND DISEASES OF MATURE PLANTSSclerotinia Head Rot
Head-rotisconsideredthemostimportantdiseaseaffectingsunflowerproduction,causingyieldandqualityloss.
Hosts
Sunflower,canola,mustard,drybeans,fieldpeas,lentils,potatoes
Biology
Sclerotiniahead-rot(Figure52)isquitevariable,usuallyoccurringlateintheseasonandinfluencedprimarilybytheamountofrainfallfromfloweringthroughtoharvesting.Aswithsclerotiniamidstalkrot,head-rotiscausedbyair-bornesporesproducedbyapothecia(smallmushrooms)eitherwithinthefieldorblownfromaneighboringfield.Thesporesrequirefreewaterandafoodbasesuchasdeadorsenescingplanttissuetogerminateandinfect.Ascosporescolonizethedeadfloretsandpollenonthefaceofthehead.Afterinfectionittakesseveralweeksuntiltheappearanceofbrownlesionsonthebackofthehead.
Symptoms
Thefirstsymptomsofheadrotusuallyaretheappearanceofwater-soakedspotsorbleachedareasonthefleshybackofthehead.Thefunguscandecaytheentirehead,withtheseedlayerfallingawaycompletely,leavingonlyableached,shreddedvascularsysteminterspersedwithlargesclerotia.Thebleached,skeletonizedheadsresemblestrawbroomsandareveryobviousinthefieldevenfromadistance.
Yieldlossfromheadrotonanindividualplantcanrangefromminimaltototallossiftheheaddisintegratesanddropsalltheseedtothegroundpriortoharvest.Intactbutdiseasedheadswillhavelightandfewerseeds,withloweroilcontentandwillshatterduringharvest.
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CONTROL TIPS
Sunflowerhybridsexhibitvariablerangeofsusceptibilitytoheadrot,butnohybridsarecommerciallyavailablewithadequateresistance.Fungicidesmayhelpreducetheincidenceofheadrot,withapplicationsmadepreventivelysinceseveralweekslapsefrominfectiontosymptomdevelopment.ConsultthecurrenteditionoftheGuidetoCropProtectionforproductinformation.
Figure 52. Sclerotinia Head Rot
The Sunflower Production Guide
Rhizopus Head Rot
RhizopusisaverywidespreaddiseaseinthecentralGreatPlains.Affectedheadswillhavealightertestweight,loweroilcontentandreducedseedyields.
Host Crops
ManyhostsforRhizopusincludingsunflower,beetsandtobacco
Biology
Susceptibilityoftheheadincreasesfromthebudstageuptothefullbloomandripeningstages.Sporesaredisseminatedbywind,rainandinsects.Rhizopusenterstheheadthroughwoundscausedbyhail,birds,insectsandhasbeenassociatedwithsunflowermidgedamage.Rapiddiseasedevelopmentoccursinwarm,humidweather.Oncetheheadisfullycolonizedandalltissuekilled,theheaddriesupandbecomes‘mummified’.
Symptoms
Initialsymptomsaresimilartootherheadrotdiseases;brown,sunken,watersoakedlesionsonthebackofthehead.Asthelesionsenlarge,theinterioroftheheadbecomesmushy(Figure53).Theinterioroftheheadbecomesfilledbymyceliuminterspersedwithblackfruitingbodiesresemblingpeppergrains.Oncetheheaddriesupitischaracterizedbyadarkbrown,pepperyappearanceoftissuesinthereceptacleandbecomesveryhard.
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CONTROL TIPS
ControllingheadinsectsisthemostefficientmethodtominimizeincidencesofRhizopusheadrot.Geneticresistancehasbeenidentified,butseverityofthediseasehasnotwarrantedintensiveresearch.RotationdoesnothelpreducetheincidenceofRhizopusduetothenumberofRhizopusspecies.NofungicidesareregisteredforcontrolofRhizopusheadrot.
Figure 53. Rhizopus Head Rot
Diseases|49
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