target spot and its potential impact on tennessee cotton ...“target spot,” or corynespora leaf...
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TargetSpotandItsPotentialImpactonTennesseeCotton
TysonB.RaperAssistantProfessor
CottonandSmallGrainsSpecialistDepartmentofPlantSciences
HeatherYoung-KellyAssistantProfessor
FieldCropsPlantPathologyDepartmentofPlantSciences
TargetspothasbeennotedinseveralcottonproducingcountiesinTennesseeoverthepastfewyears.Althoughmuchconcerningthediseaseremainsunknown,researchattheUniversityofTennesseeandotherMid-South/Southeasterninstitutionshasbeguntoanswerseveralkeyquestions.
Whatistargetspot?“Targetspot,”orCorynesporaleafspot,isanewdiseasethathasbeenmovingthroughtheSoutheastern
statesoverthepastseveralyears.CausedbythefungusCorynesporacassiicola,thediseasewasoriginallydescribedinMississippiasearlyas1961(Jones,1961)butreportsofthediseasehavebeenuncommonuntilroughly10yearsagowhenDr.KemeraitattheUniversityofGeorgiabegannotingthediseasealongtheGulfCoast(Kemeraitetal.,2011).Sincethen,theCorynesporaleafspotshavebeenreportedinalmosteverycotton-producingstateintheSoutheastandMid-South.
Whatdoesitlooklike?Corynesporaleafspotsaretypicallyfoundonmaturemainstemandsubtendingleavesclosetothebaseof
theplant.Insevereinfestations,lesionsmaybenotedintheuppercanopyandonbractsandbolls.Corynesporaleafspotsarecommonlyreferredtoas“targetspots”duetotheirregularconcentricringscontainedwithinthedarkbrowntolightbrownlesions.Althoughlesionsizeandnumberperleafcanvarysubstantially,lesionsarecommonlylargerthan1/4-inchindiameter,andoftenmultiplelesionsoccuroneachinfectedleaf.Affectedleaveswilltypicallyretaintheirdarkgreencolor,althoughsomewilloccasionallyturnyellow.Affectedplantsmayalsoprematurelyshedmatureleavesstartingfromthebaseoftheplantasthediseaseprogresses.
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Figure1:LeftandTopRight:Lesionstypicallyappearfirstonlowerleavesandslowlyprogressuptheplant,butsizeandnumberofthelesionscanvarysubstantiallyfromleaftoleaf.BottomRight:Corynesporaleafspotsarecommonlyreferredtoas“targetspots”duetotheirregularconcentricringscontainedwithinthedarkbrowntolightbrownlesionscommonlylargerthanaquarterofaninchindiameter.
Whatcausestargetspot?Littleisknownaboutthedevelopmentofthediseaseincottonbutseveralfactorsappeartocontributeto
increasedincidence.IncontrasttoAlternaria,CercosporaandStemphyliumleafspots,whicharetypicallyassociatedwithanutrient(frequentlypotassium)deficiency,Corynesporaleafspotgenerallyisfoundaftercanopyclosureinhigh-yieldingenvironments(two-plusbale)whichundergoprolongedperiodsofexcessivecanopyhumidity.Theseperiodsmaybecausedbyfrequentrainfallevents,irrigationand/orcloudyweather.Anexcessiveamountofavailablenitrogenpromotesrankgrowthandthusmayalsosupportthedevelopmentofthedisease.Duetotheabilityofthefungustosurviveoncottondebris,no-tillorstrip-tilledfieldsthatarecroppedwithcontinuouscottontendtobemoreaffected.
Varietysusceptibilitytothediseaseappearstovaryslightly,withearlier-maturingvarietiesoccasionallydisplayingreduceddiseaseincidenceandpercentdefoliation.Atthispoint,itisnotclearwhetherearliermaturitycausesavarietytobelesssusceptibleoriflatermaturitysimplycorrelateswithalargercanopycreatingamicro-climatethatbettersupportsthespreadofthedisease.Somedifferencesinsusceptibilityhavebeennotedbetweenvarietiesofthesamematurity,withleafmorphologicalfeaturesandplantgrowthcharacteristicssuspectedtoplayarole;however,itisunclearwhetherthesefactorscauseorcorrelatewithareductioninsusceptibility.
Figure2:Severeinfestationsoftargetspotcanleadtotheprematuresheddingofmatureleavesstartingfromthebaseoftheplant.MainstemandsubtendingleavesatalllowerpositionshadbeenshedinthisplotinTennesseeduring2014.
HowcanTargetSpotimpactcottonproduction?Althoughtheyield-reducingmechanismisnotwellunderstood,itisthoughtthatthemajorconcernswith
Corynesporaleafspotaredamagetophotosyntheticallyactivetissuesand/orprematuredefoliation.SubsequentyieldreductionsattributedtothediseaseinSouthAlabamaandGeorgiahavebeenestimatedtoexceed200lb/ac(Haganetal.2013).Theprematuresheddingofmainstemandsubtendingleavesmayimpactyieldifthediseaseoccursearlyduringtheboll-fillperiod,butsheddinglateintheboll-fillperiodmayreducetheincidenceofboll-rotandmakedefoliationeasier.
Canyieldlossbedeterminedfromlesionsize/frequencyorpercentdefoliation?SeveralscientistsstudyingthediseaseattheUniversityofTennesseeInstituteofAgricultureandotherland-
grantinstitutionshavenotedadisconnectbetweenlesionsize/frequencyandpercentdefoliation,especiallyacrosslocationsintheSouth.Furthermore,recentstudiesinAlabamabyHaganetal.,2013havenotedanadditionaldisconnectbetweenyieldandtargetspotintensity.Specifically,Haganhasnotedthehighest-yieldingvarietiesoccasionallydisplaythehighestlesiondensityandpercentdefoliation.Aswithmostdiseases,theeffecttargetspothasonyieldseemstobeinfluencedbydiseaseonset,whenandatwhatgrowthstageinfectionfirstoccurs,andweatherconditionsinfluencingdiseasedevelopment.
WhatarethepotentialimpactstoTennesseecotton?
TennesseecottonhasinitiallybeenpredictedtobeatlowrisktotargetspotsinceTennessee’senvironmentisnotconduciveforearlyonsetofthedisease,rapiddiseasedevelopmentandsubsequentsubstantialyieldloss.StudiesconductedattheWestTennesseeAgResearchandEducationCenteroverthepastfewyearshavenotednumericallyloweryieldsinplotsimpactedbythedisease,butthesereductionsareoftennotstatisticallysignificant.ThesestudiesalsoindicateTennesseecottoncurrentlyfallswithinalow-riskcategoryforyield-impactinginfectionsoftargetspot.Still,sincethefirstreportoftargetspotinTennesseeduring2013,theonsetofthediseasehasbeenoccurringearlierwithineachyear,diseaseincidencehasbeenincreasing,andincreasedratesofdefoliationhavebeenobserved.ThissuggeststhepathogenisstillbecomingestablishedinTennesseeandyearlyincreasesmaybeduetoincreasinginoculum.Therefore,itisnotcertainwhatimpacttargetspotmayhaveonTennesseecottoninthefuture.
Whatstepscanbetakentoreducetheimpactoftargetspot?
Rotatetoanon-hostsuchascornifpossible.RecentworkinBrazil,pastworkinMississippiandpreliminaryresultsfromGeorgiahaveindicatedpathogenisolatesfromsoybeanalsocausediseaseoncotton(Sumabatetal.,2016;Galbierietal.,2014;Jonesetal.,1961).Therefore,rotationwithsoybeanmaynotreducediseaseincidence.
Plantmultiple,high-yielding,stable-performingvarietiesadaptedtothefieldenvironment.Althoughdifferencesinvarietysusceptibilityhavebeennoted,thehighestyieldingvarietiesinagiventrialhavealsooccasionallybeencharacterizedbythehighestdiseaseratingsandgreatestpercentdefoliation.
Properlyapplynitrogenandplantgrowthregulatorstoproducesufficientvegetativegrowthtosupportfruitingbodieswithoutallowingtheplanttobecomerank.AvoidexcessiveapplicationsofNandaccountforresidualNwheneverpossible.Beginmonitoringplantgrowthatthepinheadsquarestageandconsiderapplyingapre-bloomplantgrowthregulatorapplicationtorapidlygrowing,latermaturingvarieties.
Considerationsforfungicideapplications:Whiletwoapplicationsoffungicidehaveconsistentlyreduceddiseaseanddefoliationduetotargetspot,yieldresponseshavebeenvariableinTennesseesmallplotresearch.FungicideslabeledforcottonincludeHeadline,TwinLine,Topguard,PriaxorandQuadris(includinggenericazoxystrobinproducts).Factorsthatincreasethepotentialbenefitsofafungicideapplicationtoaddresstargetspotincludefieldsthathavebeencroppedincontinuouscotton,no-tillorstrip-tillmanagementinwhichplantresidueremainsonthesoilsurface,highnitrogenlevels,highyieldingvarietiescharacterizedbyexcessivevegetativegrowth,andprolongedperiodsofhighhumiditywithinthecanopyduetoirrigationorfrequentrainfall.OngoingresearchconductedattheUniversityofTennesseeInstituteofAgricultureandatseveralotherMid-SouthandSoutheasternUniversitieswillhelptobetterunderstandtargetspotandmanagementneedsinTennessee.
Galbieri,R.,D.C.E.B.Araujo,L.Kobayasti,L.Girotto,J.N.Matos,M.S.Marangoni,W.P.Almeida,andY.R.Mehta.2014.CorynesporaleafblightofcottoninBrazilanditsmanagement.AmericanJournalofPlantSciences5:3805-3811.
Hagan,A.K,K.Glass,M.Pegues,J.Jones,L.Wells,andS.Nightengale.2013.ReactionofCottonVarietiestoTargetSpotinAlabama.AlabamaCooperativeExtensionSystem:PP-729.
Hagan,A.K.andE.J.Sikora.2011.CorynesporaLeafSpotinAlabamaCotton.AlabamaCooperativeExtensionSystem:PP-706.
Jones,J.P.1961.AleafspotofcottoncausedbyCorynesporacassiicola.Phytopathology51:305-308.Kemerait,R.C.,Jr.,F.H.Sanders,G.H.Harris,J.E.Woodward,S.N.Brown,R.J.Byrne.2011.Assessmentand
managementoffoliardiseasesaffectingcottoninGeorgiaandTexas.2011Proc.BeltwideCottonConferences:287-292.
Sumabat,L.G.,M.T.Brewer,andR.C.Kemerait.2016.HostrangeandphylogeneticdiversityofCorynesporacassiicola,causeofTargetSpotofcottoninthesoutheasternU.S.2016Proc.BeltwideCottonConferences
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W37007/1616-0188Programsinagricultureandnaturalresources,4-Hyouthdevelopment,familyandconsumersciences,andresourcedevelopment.UniversityofTennesseeInstituteofAgriculture,U.S.DepartmentofAgricultureandcountygovernmentscooperating.UTExtensionprovidesequalopportunitiesin
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