Soilborne Blight Diseases of Peanut - Texas Peanut Program

Division of Agricultural Sciences and Natural Resources • Oklahoma State University

SouthernblightandSclerotiniablightarethetwomostimportantsoilbornediseasesofpeanutsinOklahoma.Thesediseaseshavethepotentialtocausesevereeconomiclosses,becausetheycauserapidwiltinganddeathofpeanutplantspriortomaturity.Thediseasecyclesandsymptomprogressionofthetwodiseasesaresimilar,butthechemicalsandmanyoftheculturalpracticesusedtomanagethemaredifferent.Sclerotiniablightcanalsobepotentiallymoredestructive,persistent, and difficult to control than southern blight. There-fore, it is critical that each disease be properly identified so thatappropriatecontrolscanbeimplemented.

Southern Blight SouthernblightiscausedbythefungusSclerotium rolfsii andisawidespreaddiseaseofpeanutandothercrops inOklahoma.SouthernblightcausesthegreatestyieldlossesinpeanutofanydiseaseintheUnitedStates.Diseasedevel-opmentisfavoredbyhotandmoistconditionsinthepeanutcanopy.Therefore,thediseaseoccursmainlyinmid-tolate-seasoninOklahoma.Thefungusprimarilyattacksthebaseofstemsnearthesoilline,butanyplantpartincontactwithsoil may be damaged. Infected plants are generally killedpriortomaturity.Pegandpodinfectionsarecommonandresultinpodlossatharvest.Populationsof S. rolfsiiincreasein infested fields cropped to peanut unless control measures aretaken.Highpopulationsofthepathogencombinedwithfavorableconditions forsouthernblightcan result inyieldlossesof25percentormore.

Symptoms The first readily apparent symptom of southern blight is rapidyellowingandwiltingoflimbsorentireplants(Figure1).Affectedlimbsandplantsthenturnbrownanddieasaresultofdecayofthelowerstem.AdvancedsymptomsofsouthernblightareverysimilartothoseofSclerotiniablight.Thebaseofdiseasedplantsmustbeclosely examined for signsofthe fungus todistinguishbetweenthe twodiseases.Earlydetectionofthediseaseisalsopossibleiflowerstemsareexaminedpriortodevelopmentofadvancedsymptoms. Southern blight infection is characterized by a whiteorcream-coloredmoldygrowth(mycelium)covering lowerstemsandimpartingawhite-washedappearancetothebaseofaffectedplants(Figure2).Themyceliumofthesouthernblightfungusiscoarse,rope-like,closelyappressedtostems,

andmayradiateoutoverthesoilsurface(Figure3).Thisisbestobservedwhenthesoilsurfaceismoist.Myceliummayalsobeseengrowingfromdecayingplantmatteronthesoilsurface.Lesions(deadareas)oninfectedstemsandpegsare at first light brown, then become dark brown. Advanced symptomsdevelopwhenlesionscoalescetogirdlethelowerstem.Small,round,mustardseed-likestructuressoonde-veloponthemycelium(Figure4).Thesestructuresarecalledsclerotiaandserveasreproductiveandsurvivalstructuresof the fungus. The sclerotia first appear as white tufts that later become light brown and finally dark brown at maturity. Sclerotiaareuniformlyround,about1/16inchindiameter,andareproducedinlargequantitiesondeadpeanuttissue.

Disease Cycle Initial southern blight infections arise from sclerotiawhich overwinter in peanut fields. Sclerotia of the fungus are similartoseedsofaplantinthattheyremaindormantuntilconditionsbecomefavorableforgermination.Conditionsthatfavorgerminationandinfectionarehighmoisture,warmtohottemperatures,andthepresenceofdeadplantlitteronthesoilsurface.Moisturerequirementscanbemetfromirrigationorrainfall.Conditionsthatfavorgrowthofthefungusnormallybegintodevelopatpeggingwhenlimbsgrowrapidlyalongthesoilsurfaceandthecanopybecomesdense.Leaflitterarisingfromleafspotinfections,naturalleafaging,andweedoroldcropresiduesstimulategerminationofsclerotiaandserveasafoodbaseforthefungus.Thefungusthengrowsoutwardfromthefoodsourcesandinfectsanypartofthepeanutincontactwiththesoilsurface.Planttissuesarekilledbyanacidproducedbythefungus.Diseasedevelopmentbelowthesoilsurfacecanbeextensiveinsandy,well-aeratedsoilsresultinginseverepegandpodrot. Whenplantsarekilledandthefoodsupplyofthefungusbecomes limiting, numerous sclerotia develop that easilydislodgeandfalltothesoilsurface.Sclerotiasurvivewellatthesoilsurfacebutsurvivepoorlywhenburieddeepbecausethefungushasahighdemandforoxygen.Sclerotiaatornearthesoilsurfacecansurvivethreetofouryears,whilesurvivalisayearorlesswhensclerotiaareburieddeeply. Thesouthernblight fungusdoesnotproducesporeswhich move in the air, so the disease is confined to localized areas in a field where sclerotia reside. Centers of disease are numerouswherehighnumbersofsclerotiaresidenearthe

JohnP.DamiconeExtensionPlantPathologist

HassanA.MeloukPlantPathologist,USDA/ARS

EPP-7186

OklahomaCooperativeExtensionFactSheetsarealsoavailableonourwebsiteat:

http://osufacts.okstate.edu

Soilborne Blight Diseases of Peanut

Oklahoma Cooperative Extension Service

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soilsurfaceandconditionsarefavorableforsouthernblight.Diseasecentersarelessnumerouswherefewsclerotiaarepresentatthesoilsurfaceorwhenconditionsareunfavorableforsouthernblight.Theoccurrenceandseverityofsouthernblight epidemics are difficult to predict because of interactions betweennumbersofsclerotianearthesoilsurface,moistureandtemperaturelevels,amountofplantresidue,andcanopydensity.

Control Culturalpracticestomanagesouthernblightcanbeveryeffective,becauseactionstakentoreducethepopulationsofsclerotiaatthesoilsurfacecandirectlyreducediseaseincidence. Integration of cultural practices with chemicalapplicationsaswarrantedwillgenerally result in themostsatisfactoryandeconomicalcontrol. Moldboardplowinginthespringturnsundercropresidueandburiessclerotiawheretheycannotcausedisease.Mostaspectsofcultivationduringthegrowingseasonencouragesouthernblightdevelopment. Increasedsoilaerationfromcultivationallowsdeeperpenetrationof the funguswherepegsandpodscanbereadilyattacked.Cultivationisause-fulmeansofweedcontrol;however,cultivation,knownasdirting,thatmovessoilontothelimbsorcrownoftheplantshouldbeavoided.Buriedsclerotiaarebroughttothesoilsurfacebythispracticeanddepositednearthecrownarea

whereinfectionsaremorelikelytooccur.Dirtingalsoformsatroughthatretainsmoistureandplantlitteratthecrownoftheplant. Severalotherpracticescanbeadoptedtoreducemois-tureretentioninthecanopy,thusdiscouragingsouthernblightdevelopment.Plantingonaraisedbedpromotesdrainageanddryingofthecanopy.Plantingonabedalsoreducestheaccumulationofplantlitterinthepeggingzonebypermit-tingwashingoflitterintotherowfurrows.Irrigationsshouldbeproperlytimedtoallowapplicationofadequatebutnotexcessiveamountsofwaterandtomaximizethedryperiodbetweenirrigations.Applicationofsmallamountsofwaterfrequently(every2to3days)shouldbeavoidedascontinu-ouscanopywetnesswillresult. Croprotationisaneffectivepracticeformanagementofsouthernblight,butrotationcropsmustbecarefullyselectedbecausethesouthernblightfungushasawidehostrange.Corn,grainsorghum,sudangrass,andcottonareexcellentrotationcropsforsouthernblightcontrol.Two-yearrotationsarerequiredwheresouthernblighthasbecomesevere.Infields where incidence of southern blight is low, a one-year rotationwillreducechancesofsouthernbecomingsevere.Fallowingbetweenpeanutcropsisonlyeffectiveiflandiskeptfreeofweeds. Fungicide applications are often required in fields where southernblightisamainfactorlimitingproduction.Twoap-

Figure 4. Close-up of the small, uniformly round sclerotia of the southern blight fungus.

Figure 2. Mycelium of the southern blight fungus at the base of an infected plant. Note the sclerotia developing on the leaf litter at the bottom and center of the photo-graph.

Figure 3. Coarse, rope-like mycelium of the southern blight fungus radiating out over the surface of moist soil.

Figure 1. Wilted plant or “flagging” from either southern blight or Sclerotinia blight. The base of this plant must be closely examined to determine which disease is the cause.

proachestochemicalcontrolcanbeconsidered.Fungicidemay be applied at a specific growth stage (eg., pegging) or at the first appearance of disease, whichever occurs first. This preventive approach is useful in fields continuously cropped to peanut and where southern blight is a knownproblem. Secondly, fungicide application may be delayeduntilthediseasebecomesestablished.Thisapproachmaybenecessarybecauseatimelypreventiveapplicationwasnotmadeorbecause thedisease increasedunexpectedlywhereithasnotbeenapriorproblem.Reducedcostsmayresult from delaying the first application long enough to ne-gatetheneedforasecondtreatment,oranytreatmentifthedisease fails to develop. These benefits may be offset by a reductioninthelevelofcontrolachieved.Fieldsshouldbescoutedweeklythroughouttheseasontomonitorthestatusofsouthernblight. Currently registered fungicides provide only 40 to 60percent control of southern blight. New compounds maysoonberegisteredforsouthernblightthathaveimprovedef-fectiveness.Itisessentialthatfungicidesbeappliedproperlyso that maximum benefit can be achieved. Consult the most recentPeanutProductionGuide(OCESCircularE-608)forthelatestinformationonrecommendedfungicidesandmethodsofapplicationforcontrolofsouthernblight. Leafspotcontrolhasavariableeffectonsouthernblight.Controllingleafspotwillmarkedlyreduceleaflitteraccumul-ationnearthecrownareasothatsouthernblightdevelopmentisnotencouraged.However,recentresearchindicatesthatwherepopulationsofsouthernblightsclerotiaarehigh,foliardiseasecontrol increases the levelofsouthernblight.Thehealthycanopyisthoughttoentrapmoistureandmaintainamorefavorableenvironmentfordiseasedevelopment.Wherelevels of sclerotia are low, leafspot control decreases thesouthernblightdevelopment.Leaflitterapparentlyprovidesafoodsourceforthefungustoenhancegrowthandinfectionatlowpopulationlevels.Foliardiseasecontrolisanessentialcomponentofpeanutproduction.Thepotentialforreducedyield from leafspot exceeds any possible benefit from allowing leafspottodevelopinordertopossiblylessensouthernblightwherepopulationsofthepathogenarehigh.Furthermore,leaflitterprovidesanexcellentsubstrateonwhichpopulationsofthesouthernblightfunguscanincreasetopossiblycausemoreinfectionsinfollowingpeanutcrops.

Southern Blight Management Tips

1. Practice crop rotation with non-susceptible cropssuchascorn,sorghum,orcotton.

2. Moldboardplowtoburysclerotiaandcropresidue.3. Plantonaraisedbedtoimprovedrainageandreduce

levelsofmoistureinthecanopy.4. Avoid the practice of dirting peanuts where soil is

movedagainstplantlimbsandcrownsduringculti-vation.

5. Apply fungicides to reduce losses when southernblightisamainfactorlimitingproduction.

6. Timeirrigationstoapplyadequatebutnotexcessiveamountsofwaterandavoidfrequentlyapplyingsmallamountsofwater.

7. Maintainfoliardiseasecontroltoreduceaccumulationofleaflitterinthepeggingzone.

Sclerotinia Blight Sclerotiniablight,causedbythefungusSclerotinia minor,isadestructivemid-tolate-seasondiseaseofpeanuts.Thedisease was first found in 1971 on peanuts in the U.S. in Virginia.IthassincebeenfoundinNorthCarolina,Oklahoma,andTexas.ThediseasehasrecentlybecomemoreseriousinOklahomabecauseofincreasedplantingofrunnervarieties,whichareverysusceptibletoSclerotiniablight.ThediseaseissimilartosouthernblightinthatSclerotiniasurvivesinthesoilbyformingresistantseed-likestructures(sclerotia)andattacksplantsnearthesoilline.Sclerotiniablight,however,ispotentiallymoredamagingbecauseofitsabilitytorapidlyspreadwithinthepeanutcanopy.InOklahoma,yieldlossesinexperimentalplotsinfestedwithSclerotiniahaverangedfrom 7 percent to over 80 percent. The disease usuallybecomessevere late in thegrowingseason,butmayoc-curinOklahomaasearlyasJuly,dependinguponweatherconditions.Becausethediseaseoftenoccurs late, lossesareprimarilyfrompeanutlossatharvestasaresultofstemandpegrot.

Symptoms SymptomsofSclerotiniablightmayappearanytimefrommid-to late-seasonwhenweatherconditionsarecooltomoderately warm and wet. The first symptom of Sclerotinia blightisthewiltingandyellowingoflateralormainbranches(Figure1).Affectedbranchesnearthesoillinemustbecloselyexamined for definitive identification of Sclerotinia. A fluffy orcottony,whitemoldygrowth(mycelium)developsarounddiseasedareasoflowerstems(Figure5).ThemyceliumofSclerotiniaismosteasilyobservedinmorningswhendewispresentorafterarain,andmaydisappearlaterinthedaywhen sunlight dries the lower canopy. This is in contrastwiththecoarse,rope-likemyceliumofsouthernblightwhichremainsvisibleundersomewhatdrierconditions.Deadareas(lesions) develop on affected stems that appear sunken,elongated,andlighttantoalmostpalewhite(Figure6).Soonentirelimbswiltandturnyellow.Affectedbranchesdieandturndarkbrownincolorandeventuallywholeplantsmaybekilled. The small seed-like survival structures (sclerotia) ofSclerotinia are black and irregularly shaped and range insizefrom1/24to1/16inch(Figure7).Thesesclerotiamayaggregateintolargermassesthatappearsimilartomousedroppings.Theseareclearlydifferentfromthetantobrownandroundsclerotiaofthesouthernblightfungus.Sclerotiamaynotbeevidentduringtheearlystagesofdiseasede-velopment.Sclerotiaareformedinandoninfectedstems,leaflets, pegs, pods, and roots. A characteristic symptom of Sclerotiniablightisstemshredding(Figure8).Diseasedstemtissuewillshredeasilywhenrolledbetweenthethumbandfingers. Shredding is the result of the rotting of all stem tis-suesexceptthevascularstrands.Shreddingofpegsresultsinlossofpodsinthesoilatharvest.

Disease Cycle Sclerotinia overwinters and survives between peanutcrops in the soil as sclerotia. Viable sclerotia, capable ofinfectingpeanuts,havebeenfoundintheplowlayerofin-festedsoilsforuptofouryearsafterapeanutcrop.Sclerotiarapidlygerminateandinfectplantsunderconditionsofcool

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tomoderateaveragedailytemperatures(65to70oF),highsoil moisture, and high relative humidity (95 to 100%). Stems, leaves,andpegslyingadjacenttogerminatingsclerotiabe-comeinfected.Thediseasecanspreadrapidlyunderfavorableconditionsasinfectioncanoccurin24hoursandsymptomsappearin72hours.Thefungusdirectlyinfectshealthyplantparts,butmechanicallydamagedvinesaremorepronetoinfection. Sclerotia are formed in large numbers after thefungusconsumesallavailablenutrientsfrominfectedplantparts.Sclerotiabecomedislodgedfromplantsandfalltothesoilorremainininfectedplantpartsleftafterharvest. SpreadofSclerotiniablightfromplantsinfectedbyscle-rotiaislimitedbecausethefungusdoesnotproduceairbornesporesduringthegrowingseason.Sclerotiamaygerminateandformasmall,cup-shapedmushroomwhichproducesairbornespores.InOklahoma,thesesporesareproducedintheearlyspring,severalweeksbeforepeanutsareplanted.Forthisreasonairbornesporesarenotconsideredanimpor-tantpartofthediseasecycleorameansofdiseasespread.Spreadofthediseasetohealthypartsofthesameplantortoanadjacenthealthyplantoccursonlywhenmyceliumofthefunguscomesincontactwithhealthyparts.Conditionsfavor-ingactivegrowthofthefungusalsofavorspreadtohealthyplants. Centers of diseased plants occur in fields because of thelimiteddistancewhichsecondaryspreadoccurs.Centersofblightedplantsbecomemorenumerousasconcentrations

ofsclerotiainsoilincrease.Concentrationsofsclerotiahavebeenfoundtoexceed30perpintofsoil.Aconcentrationas low as five sclerotia per pint of soil is sufficient to cause severeblightwhenenvironmentalconditionsarefavorable. SclerotiaarealsotheprimarymeansoflongdistancespreadofSclerotiniablight.Sclerotiaareeasilymovedfromfield to field with soil or infested peanut straw. Movement of farm implements from infested to clean fields is an important meansofdiseasespread.ResearchinOklahomahasalsoshownthatsclerotiafedtocattleininfestedpeanutstrawcanpass through thedigestive tractofcattleand remainviable.Sclerotiamaybethendepositedwithdungincleanfields. Sclerotia are also small and light enough to be moved withwind-drivensoil.WatermovingoversoilinfestedwithSclerotinia may also move sclerotia within fields or to nearby fields. Contaminated water could arise from irrigation or rainfall runoff from infested fields. Tillage practices help move sclerotia throughout the plow zone, but probably have aminimaleffectonincreasingthesizeofcontaminatedareas.Plowingburiessclerotiaat thesurfaceand in turnbringsburiedsclerotiatothesurface.Expansionofinfestedareasfromplowingaloneoccurs,butataslowrate. Sclerotiniaisseedborneandhasbeenfoundasacon-taminantofseedfromresearchplotsatlevelsashighas12percent.Transmissionofthediseasefromcontaminatedseedlotstoresultingplantsrangedfromzeroto36percent,depend-

ingonthevariety,whenseedsweregrowninthegreenhouseunderconditionsfavorableforthedisease.However,over400commercialseedlotsfromdifferentareasofOklahomahavebeentestedtodateforthepresenceofSclerotiniaandonlyfourwerepositive.Thepercentcontaminationinthosefourseedlotsaveragedonly0.5percent.Thisresearchindicatesthat although introduction of Sclerotinia into clean fields by seedispossible,itmaynotbeanimportantmeans. Sclerotiniahasawidehostrangewhichincludesbothcultivatedandweedhosts.Susceptiblecropsincludealfalfa,rape, canola, sunflower, bean, sweet potato, tobacco, cole crops,andsoybean.However,someofthesecropssuchassoybeanmaynotbecomeseverelydiseasedwhengrownininfested fields in Oklahoma. Alternate hosts may aid in the establishment of Sclerotinia in clean fields or the build-up of Sclerotinia in fields already infested.

Control BecauseSclerotiniablightcanbeverydamaginganddifficult to control, steps should be taken to limit the spread of the fungus and its entry into clean fields. Tractors, plows, combines, and other farm implements leaving infested fields shouldbecarefullycleanedandwashedpriortoenteringcleanfields. Soil and peanut straw should be completely removed fromthese implements.Cattlefedwithpeanutstrawfromfields infested with Sclerotinia should not be pastured on clean fields. Peanut hay taken from infested fields should not be fed to cattle grazing on clean fields. To eliminate the potential for importing the disease into fields on peanut seed, seedshouldbe treatedwitha fungicideknown to reduceSclerotinia. A combination of cultural and chemical managementpracticeswillprovidethemostsatisfactorycontrolofScler-otiniablight.Practicesthatreducemoistureretentioninthecanopy and promote aeration should be adopted. Theseincludeplantingonaraisedbed,andavoidanceofnarrowrowspacingsandoverlyheavyseedingrates.Irrigationtimingisalsoimportant.Enoughwatershouldbeappliedateachirrigationtopermitthoroughandprolongeddryingofthesoilsurfacebeforethenextapplication. Excessive vine damage should be avoided becauseSclerotiniaeasilyinfectswoundedplantparts.Vineinjurycanbeminimizedbycultivatingearlyornotatallandbyusingtractorswithnarrowtires. TheuprightgrowthhabitofSpanishvarieties formsamoreopencanopythanrunnervarietieswhichhaveadenseandprostratecanopy.Thisopencanopyislessfavorabletoblight development. Research has shown that in a field se-verelyinfestedwithSclerotinia,thethree-yearaverageyieldreductionforaSpanishvarietywas16percentcomparedto62percentforarunnervariety.ASpanishvarietywithresis-tance to Sclerotinia blight, Tamspan 90, was recently released. The resistance in Tamspan 90 is related to canopy structure. FewerdiseasedplantsandsclerotiaperplantdeveloponthisvarietycomparedtootherSpanishorrunnervarieties.Spanishvarieties,preferablyonewithresistance,shouldbeplantedtoreducelossesfromSclerotiniainseverelyinfestedfields.

Anotherimportantaspectofvarietyselectionismaturitylength.Spanishandothershort-seasonvarietieshavetheadvantageofearlierharvestdates.BecauseSclerotiniablightinOklahomaisnormallymostsevereinSeptemberandOcto-ber,long-seasonrunnervarietiesareparticularlyvulnerable.Ashort-seasonvariety thatmatures20 to30daysearlierthanalong-seasonvarietywillreducelossesbylesseningtheexposureofplantstoblight-favorableweather. PlantingandharvestdatescanalsobeadjustedtoreducelossesfromSclerotiniablight.Researchhasshownthatinfields where levels of infection are high and fungicide are not applied,yieldsoftherunnervarietyOkrundeclinesteadilywithtimeafter135daysafterplanting.ThenormalmaturityforOkruninOklahomaisabout160days.ThisdiffersfromtheSpanishvarietySpancowhereyieldsincreasewithtimefrom115to135daysafterplantinganddeclinethereafter.ThenormalmaturityforSpancoisabout135days.Therefore,diggingrunnerpeanutsearlywillreducelossesfromSclerotiniablight.Spanishvarietiesmaybedugatnormalmaturity,butnolater,toavoidlateseasonlosses. AfungicideprogramisusuallyrequiredwhereSclerotiniaisaproblem.Propertiminganduseofanappropriatemethodof application are essential for maximum disease control.Fungicides should be applied in a preventive manner toachievethoroughcoverageofbasalstems.Fungicidesonlyprotecthealthyplantpartsfrominfection.Fungicidesdonotcure established infections or kill sclerotia. Therefore, thefirst fungicide application should be made when symptoms are first noticed or preferably before when conditions first becomefavorablefordiseaseafterpegging.Fieldsshouldbecarefullyscoutedthroughouttheseasontomonitortheactivity of Sclerotinia blight. Even when properly applied,currentlyavailablefungicideshaveonlybeenmoderatelyef-fectiveinreducinglossesfromSclerotiniablight.Researchoverseveralyearshasdemonstratedthatfungicideusageresultsinabouta45percentreductioninthepotentialyieldlossfromthedisease.ConsultthemostrecentPeanutPro-ductionGuide(OCESCircularE-608)forthelatestinformationonrecommendedfungicidesandmethodsofapplicationforcontrolofSclerotiniablight. Most foliar fungicidesused toachieveahigh levelofleafspotcontrol increase theseverityofSclerotiniablight.Thiseffectisthoughttobecausedbyleafretentionwhichprovides a dense canopy that retains moisture. However,useofchlorothalonil(Bravo)ona14-dayschedulehasbeenlinked to increased Sclerotinia blight in fields infested with thefungus.ThiseffectcanbeminimizedbyapplyingBravoatintervalsnocloserthat21days.Forgrowersona14-dayleafspotprogram,thismeansalternatingBravoapplicationswithanotherleafspotfungicide. BecausesclerotiaofSclerotiniaarepersistentinsoil,itisunlikelythatcroprotationwillresultinsatisfactorydiseasecontrol. Conversely, continuous cropping of susceptiblerunnervarietieswillresultinarapidbuild-upofSclerotiniablight making profitable peanut culture difficult. A carefully selected rotation should significantly reduce the rate of blight increase in infested fields. Corn, sorghum, cotton, and small grainsareexcellentrotationcropsforreducingbuild-upofSclerotinia.

Figure 5. White, fluffy mycelium of Sclerotinia growing on infected stems in a wet canopy.

Figure 6. Vines killed by Sclerotinia blight are pale white or straw-colored.

Figure 7. Small, irregularly shaped, black sclerotia of Sclerotinia.

Figure 8. Stems infected with Sclerotinia shred easily when rolled between thumb and forefinger.

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