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,[email protected],850-875-7154
