Hands-onidentificationofvegetablediseases:Cucurbits

Theme:Howtodiagnoseaspecificdiseasefromdiseasesordisorderswithsimilarsymptoms.

MathewsParet,Nicholas Dufault, Gary Vallad, Shouan Zhang Fanny Iriarte, Susannah Da Silva, Blake Thaxton, Steve Olson, Josh Freeman

NorthFloridaResearchandEducationCenter,Quincy,FLDepartmentofPlantPathology,UniversityofFlorida,Gainesville,FLUniversityofFloridaCooperativeExtension

2014,FL(Watermelon)19,700acresharvested,#2inU.S.(109,050acresintheU.S)>$80millionFarmValue,#1inU.S.($431millionintheU.S)(USDA-NASS,2015)

Cucurbits

OthercucurbitsCucumbers:9,400acres,#1inU.S.Squash:6,800acres,#1inU.S

•  Scenario1:Spots•  Scenario2:Wilts•  Scenario3:Leaf,fruitdistortions•  Scenario4:Fruitdamage•  Scenario5:Stunting

Cucurbitssectiondiscussionlayout

Scenario1:MattOrwatduringaregularcantaloupefieldvisitinWashingtoncountyfindseverythingok.Justafewdayslater,hegetsacallfromthesamegrowersayingthatthereareprogressingleafspotsinthefieldandneedshishelpinidentification.Mattunderstandstheseriousnessandgetsonhistrucktovisitthefarmagain.Onthewayhethinksabout.

Possiblereasons:1.  Downymildew:Oomycete(O)2.  Gummystemblight:F3.  Anthracnose:F4.  Cercosporaleafspot:F5.  Angularleafspot:B6.  Alternarialeafspot:F7.  Powderymildew:F

Angular,yellowleafspotsisanindicationforDownymildewcausedbyPseudoperonosporacubensis

InanothersectionofthefieldMattnoticesthetransitionfromyellowangularspotstonecroticspotsandheisevenmorepositivethatthediseaseisDownymildew

ClusterofsporesattheundersideoftheleavesisadefinitiveconfirmationforDownymildew.Mattchecksthegrowerssprayregimeandplacesarecommendation.

LesionsonthemarginoftheleavesisaverydistinctoneofGummystemblight

IfthesymptomslookdifferentthantheonesforDownymildew,MattcheckswhetheritisGummystemblightcausedbyDidymellabryoniae

SevereinfectionofGummystemblightonleaves

Crackingofthestemwithgummyoozeonaninfectedvine

NotdefinitivesymptomsforGummystemblight

Gummyoozefromaninfectedvine

Notadefinitivesymptom.Therecanbeotherreasonsforooze

Watersoakingonaninfectedleafandsmallblackfruitingbodiesembeddedonthetissue

Numeroustinyblackfruitingbodiesonaninfectedvine

Fruitingbodiesonaninfectedleaf

Ifitisnotgummystemblight,thismaybeAnthracnosecausedbyColletotrichumorbiculare.Theuniquesymptomisirregularshapedspotsscatteredontheleaves.

ComparisonoflesionsofAnthracnosevs.Gummystemblight

CharacteristiccrackinginthelesionscausedbyAnthracnose

IfitisnotAnthracnose,thismaybeCercosporaleafspotcausedbyCercosporacucumerina.Theuniquesymptomiscircularblackspotsscatteredontheleaves.

Atlaterstages,thecenterofthelesionsbecomeswhite

Cercosporaleafspotcancausesevereleafyellowing

AngularleafspotisabacterialdiseasecausedbyPseudomonassyringaepv.lachrymansorrelatedspecies.Howcanyouconfirmit?

AlternarialeafspotcausedbyAlternariacucumerinacanalsocausesimilarkindofsymptomsasAngularleafspot.

Adefinitiveconfirmationcanonlybedonebymicroscopicanalysisofsporemorphology.

AleafwithsevereblightingduetoAlternarialeafspot.Thedarksectionsonthenecroticregionisduetotheconidiaofthefungus

Close-upoftheblightedleafindicatingnumerousdarkconidia.Aleafhairisshownforcomparison

MattalsonoticesPowderymildewcausedbySphaerothecafuliginea.Itisaneasydiseasetoidentifywithwhitetalcum-likeformationonuppersideofleaves

Powderytalcum-likeformationcanbenoticedontheundersideofleavesandsometimesonfruitsalso.

Clusteroflongwhitechainsofconidiaofpowderymildew

Disease Managementoptions

1 Downymildew a.  Weatherforecasts,anduseIPMPipesentinelplotmappingforprogressionofthediseaseinFlorida

b.  Fungicideapplicationbasedonthepredictedmodels

2 Gummystemblight a.  Fungicideapplication(Alwaysneedstobeaheadofthedisease)

b.  Rotatechemistriesasfungalresistancetofungicidesisprevalent.

3 Anthracnose(Races1,2)

a.  ResistantvarietiesforRace1b.  Fungicideapplication(Alwaysneedstobeaheadofthe

disease)

3 Angularleafspot a.  Cleantransplants,Bactericideapplication4 Cercosporaleafspot a.  Fungicideapplication5 Alternarialeafspot a.  Fungicideapplication6 Powderymildew a.  Fungicideapplicationatthefirstonsetofthedisease

Floridaweatherconditionsareidealforoccurrenceoffoliardiseasesoncucurbitsroundtheyear.Thisnecessitatesthejudicioususeofprotectiveandsystemicfungicidesandrotatingfungicidechemistriesforeffectivediseasemanagement.

Scenario2:DonnaDemorest,duringoneofherwatermelonfieldvisitscameacrossplantswiltinginafieldinColumbiacounty.Fromliteraturesheknowsofatleast3differentcausesforplantwilting.

Possiblereasons:1.  Bacterialwilt:B2.  Fusariumwilt:F3.  Lackofwater:P4.  Excessivesoil

moisture:P

WiltingofwatermelonvinemaybeduetoFusariumwiltcausedbyFusariumoxysporumf.sp.niveum

SeverevasculardiscolorationandwhitemycelialformationisapositiveconfirmationofFusariumwilt.Pathogenishostspecifictowatermelon.

BacterialwiltcausedbyErwiniatracheiphiliavectoredbystripedcucumberbeetleswillnotcauseamycelialformation.AbacterialoozefromthecutstemisapositiveID.

Disease Managementoptions

1 Bacterialwilt a.  Cucumberbeetlemanagement

2 Fusariumwilt(4races)

a.  Resistantvarietiesb.  Graftingwithsquashrootstocksc.  Fungicidedrenchapplicationsat

transplanting

Scenario3a:Whatisgoingonwiththecucurbits.Leafandfruitdistortions!.DavidNistler,regularlycomesacrossnumeroustypesofissuesattheClaycountyonallkindofcucurbits.Kevinknowsthatthismaybeduetoviruses,orsomephysiologicalissues.

Possiblereasons:1.  ZucchiniYellowMosaicVirus(ZYMV)potyvirus:Viral(V)-aphids2.  PapayaRingspotVirus-W(PRSV-W)/WMV-1(potyvirus)V-

aphids3.  WatermelonMosaicVirus-2(WMV-2)potyvirus:V-aphids4.  SquashVeinYellowingVirus(SqVYV):V(Watermelonvine

decline;whitefly)5.  CucumberLeafCrumpleVirus(CuLCrV):V-whitefly

Distortedappearanceoftheleaves;inwardcurling

Potyviruscausesbulgingontheleaves

Uniquepatternsonthefruits.ItisdifficulttoseparatebetweenZYMV,PRSV,andWMV-2basedonfieldsymptoms!.

LeafcrumplingoncucurbitsisonlyknowntobecausedbyCuLCrV,vectoredbywhiteflies.

LeafcrumblingandyellowingisadefinitiveconfirmationforCuLCrV

AleafwithsevereCuLCrV

Scenario3b:Thislookslikenormalleafpatternsonwatermelon.Maybenot!KeithWynnatHamiltoncountywasnotsurewhenagroweraskedthisquestion.Kendecidedtotakeacloselook.

Keithchecksahealthyleafandnoticesthedifference.

Thereisclearsilveringoftheleaves,andiscausedbyfeedingofsilverleafwhitefly.Thewhiteflyreleasesatoxinwhichcausesthesesymptoms.

Disease Managementoptions

1 ZYMVPRSWWMV-2

a.  Aphidmanagementb.  Removingvolunteercucurbitsandhostplants

2 SqVYV a.  Whiteflymanagementb.  Removevolunteercucurbitsandhostplants

3 CuLCrV a.  Whiteflymanagementb.  Removingvolunteercucurbits,hostplantsandplantdebrisfor

whiteflycontrol

4 Silverleaf a.  Whiteflymanagement

Scenario4a:InLeoncounty,MollyJamesonusuallycomesacrossnumeroustypesoffruitrotsoncucurbits.Mollywouldliketotellthegroweraboutthespecificcausesothatamanagementoptioncanbeplacedforfutureproduction.

Possiblereasons:1.  Blossomendrot:P2.  Phytophthoracrownandfruitrot:O3.  Sclerotiumrot:F4.  Bacterialfruitblotch:B5.  Wetrot:F

BlossomEndRot

Phytophthoracrownandfruitrot

BlossomEndRotMustardlikesclerotiaisadefinitiveconfirmationofSclerotiumrotcausedbySclerotiumrolfsii

Sclerotiumrotonwintersquash

Wetrotisadiseaseofflowerpartsandfruitsandhasacharacteristicappearance.WetrotiscausedbythefungusChoaenphoracucurbitarum.Itisaweakpathogenandgenerallyisasecondarytomechanicalorinsectdamage.

BacterialfruitblotchcausesacharacteristicblotchingonthefruitandiscausedbythebacteriumAcidovoraxavenaesubsp.citrulli

Scenario4b:LibbieJohnsongetsacallfromagroweratEscambiacounty.Hehas~200acresofwatermelon.Thecroplooksgreatthefruitslooksgood,andtheleafcoverageisexcellent.However,beforethefirstpickthegrowercutopensomefruitsandfoundnecroticsectionsintherind.Libbieandthegrowerstartedcheckingeachofthevarieties(triploids)plantedandtheycouldfindnecrosisinallofthosevarieties.Theestimateddiseaseincidenceison30-50%ofthefruits.Libbiehasnoideawhatistheissue,butshecameacrossinformationregardingadiseasewithnoconfirmedcausalagenttillnow.

Watermelonrindnecrosisonlyaffectstherind.However,thefruitsarenotmarketable.

SqVYVcanalsocausethesesymptomsinthefruit.SqVYVwillalsoshowvinedeclineandveinyellowingwhichisabsentincaseofwatermelonrindnecrosis.

Scenario5:ShepEubanksgetsacallfromasquashgrowerinGadsdencounty.Plantswerestuntedinsectionsofthefield,butnoclearsignsofsymptomswerenoted.Guesswhat?

Root-knotcausedstuntingoftheplants

CanRoot-knotnematodescausewiltingoftheplants?

Sure,Yes!

Guesswhat?.

•  Fieldidentificationbasedonkeysymptomscausedbyvariouspathogenscanbeaneffectivetoolforrapidrecommendationsatthecountylevel.

•  However,ifyouarenotsure,usediagnosticfacilitiesatUFfordefinitiveconfirmation.

ContactforadditionalinformationMathewsL.ParetAssociateProfessorofPlantPathologyUniversityofFlorida,NFRECparet@ufl.edu,850-875-7154