Citrus Declines Caused By Nematodes In Florida V. Slow Decline

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Title:
Citrus Declines Caused By Nematodes In Florida V. Slow Decline
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Fact sheet
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Heald, C.M.
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University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
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Gainesville, Fla.
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Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Melanie Mercer.
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"Publication date: July 1987. Revised: May 1994"
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"Circular No. 143"

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NematologyCircularNo.143 May1994CitrusDeclinesCausedbyNematodesinFlorida V.SlowDecline1 C.M.HealdandJ.H.O'Bannon2Thefirstreportofanassociationbetweena nematodeandcitrusappearedin1889;however,it wasnotuntil1912when Tylenchulussemipenetrans Cobbwasdiscoveredontherootsofcitrustreesin Californiathatanematodewasfoundtocausea diseasedconditionofcitrus,latercalled"slow decline."Morethan60yearsago,Dr.N.A.Cobb predictedthatthecitrusnematode,asitcametobe known,wouldsomedaybecomeanimportantpestof citrusinFlorida(1)ifitwerenotcontrolled.His predictionhasbecomeafact.Sincethattimesurveys haveshownthisnematodetobeapestofcitrusinall citrus-growingareasoftheworld.Ninetypercentof grovesinTexasareinfestedwhilelessthan50percent ofFloridagrovesareinfestedwiththenematode. CaliforniaandArizonagroveshaveinfestationsof65 and90percentrespectively.Becausethisnematode doesnotdevastatecitrustrees,itspresencemaynot beimmediatelydetectedandthedamageitdoesmay notbeeasilyrecognized.Intime,however,itseffect ontreeswillbecomenoticeable. Mostcitrusrootstocksusedcommerciallyare attackedbythecitrusnematode.However,thereare noobviousrootsymptomstoindicatethepresenceof thispestonnurserystock,anditoftengoes unnoticed,afactorwhichassuredlyaccountsforits worldwidedistributioninthepast.Themajorsource ofspreadhadbeenmovementofinfectednursery stock.Currentlyquarantineregulationsinmostareas havereducedthismethodofspread.Asecondsource ofinfectionisfromreplantinghealthytreesinold, infestedgrovesites;amajorprobleminthecitrus industry.SOILTEXTURESoiltexturegreatlyinfluencesthenematode's abilitytoinfecttreeroots.Treescanbecomeinfected inalmostanysoil,butearlyinfectioninhighnumbers andsubsequentpopulationincreaseoccursmore rapidlyinfine-texturedorganicsoilsthanincoarsetexturedsands.Abovegroundtreesymptomsdevelop earlyonnematodeinfectedtreesgrowinginorganic orfine-texturedsoils,becausecitrusnematode penetrationisfavoredandnumbersincreaserapidly. Thesepopulationsdamageanddestroyrootsearlyin thelifeofatree,thusreducingitsvigor.Incontrast, abovegroundsymptomsmaynotbeevidentformany yearsoninfectedtreesgrowingindeep,welldrained sands,characteristicoftheridgeareaofcentral Florida.Here,thecitrusnematodepopulation increasesslowly,andtherootsystemgrowsatarate aboutequaltothelossofinfectedroots.Although therootsystemundertheseconditionsismore vigorousthanthatinorganicsoils,damaging populationseventuallydevelopintime.Fruitlosses fromcitrusnematodesinFloridamaybeashighas 140boxesoffruitperacreofinfectedtrees.Other citrusproducingareasoftheU.S.havereported similar,ifnotgreater,lossesfromthisnematode.LIFECYCLEThelifecycleconsistsofseveralstages,andis completedin6to8weeksat25C.Eggsarelaidin agelatinousmatrixbythefemaleontherootsof citrus;thedurationofeggincubationrangesfrom12 to14days.Thenematodesundergofourmolts;the 1.ThisdocumentisNematologyCircularNo.143,aseriesoftheFloridaDepartmentofAgricultureandConsumerServices,Divisionof PlantIndustry,Gainesville,Florida.Publicationdate:July1987.Revised:May1994. 2.C.M.Heald,ResearchNematologist-retired,U.S.DepartmentofAgriculture,AgriculturalResearchService,SubtropicalAgricultural ResearchLaboratory,P.O.Box267,Welasco,Texas78596;J.H.O'Bannon,NematologistEmeritus,BureauofEntomology,Nematology andPlantPathology,FDACS,DivisionofPlantIndustry,P.O.Box147100,GainesvilleFL32614-7100.

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CitrusDeclinesCausedbyNematodesinFlorida V.SlowDecline Page2firstoccurringintheegg,andemergeassecond-stage juveniles.Malesneitherfeedonnorinfectrootsand completetheirlifecyclein7to10days.Onlythe femalebecomesembeddedintherootwhereitfeeds onthecorticalcellsanddevelopstomaturity.About one-fourthoftheanteriorportionofthefemalebody iswithintheroot,usually4to5cellsdeep,butnever beyondthecortex.Afterafeedingsiteisestablished, thenematode'sbodybecomesimmobile,withits posteriorportionexteriortotheroot.Thisportion, outsidetheroot,becomesgreatlyenlargedatmaturity (Figure1). Figure1. Scanningelectronphotomicrographofthe posteriorportionsofanadultcitrusnematodefemaleand juvenilesinvadingacitrusroot.SYMPTOMS RootsCitrusnematodesdonotcausegallingorknotson theroots.Femalesexudeagelatinousmatrixthat servestoprotectthenematodeeggs.Soilparticles adheringtothismatrixcausetheinfectedrootsto appeartohaveincreaseddiameters(Figure2). Actually,infectedrootsareslightlyenlargedandhave averyirregularsurfaceirrespectiveoftheadhering soil.Theouterportionofsuchrootsoftenseparates readilyfromtheaxialportion,exposingthecentral cylinder. Figure2. Left-Citrusrootsheavilyinfestedwithcitrus nematodes.Right-Rootsfromatreetowhichnematode controlmeasureswereapplied.AbovegroundNotallcitrustreeshavingahighcitrusnematode populationontherootsshowabovegroundsymptoms. Symptomexpressionmaynotbenoticeablefor5to 10yearsafterpeaknematodepopulationlevelsare reached.However,abovegroundsymptoms,when theyfinallyappear,consistofgeneralsymptomsof malnutrition,suchasyellowingofleaves,sparse foliage,small,nonuniformfruit,anddefoliatedbranch ends.Infectedtreesshedmoreleavesthando uninfectedtrees.Thisisparticularlytrueduring periodsofenvironmentalstress,suchasthose occurringinFloridaduringthelow-rainfallseasonin thespring.Thedegreeofdeclinemayvary considerablyfromtreetotree.Generally,the correlationbetweennematodepopulationsand

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CitrusDeclinesCausedbyNematodesinFlorida V.SlowDecline Page3declinesymptomsisnegative.Largepopulationsof theparasitebuildupontheextensiverootsof growingtrees.Thesepopulationsdecreaserapidlyas rootsaredamaged,becomedecayed,anddecreasein number.Thus,treesinstagesofdeclinemayhave lowerpopulationsofcitrusnematodesthanthosewith anabundanceofrootsonwhichthenematodescan feed.Underoptimumconditionsofcultivation,trees canliveandproducewithrelativenormalityinspite ofthepresenceofthenematode.However,ifan imbalancebetweenthebeloworaboveground portionofatreeoccurs,thenthenematodebecomes afactorwhichaugmentsthisimbalance.Eventually, treesdecline,duetoadeterioratedrootsystem, resultinginareductioninfruityieldandquality. Treesplantedinoldgrovesiteswhichwere previouslyinfestedwiththecitrusnematodeoften showsymptomsofslowdeclineearlierthaninfected treesplantedincleansoil.ROOTSTOCKREACTIONTherearemanyreportsofcitrusnematode resistantrootstocksworldwide.IntheU.S.trifoliate orangeandotherplantsbotanicallyclosetocitrusare highlyresistantorimmunetothecitrusnematode. Severalselectionsof Poncirustrifoliata haveshowna highdegreeofresistance.Swinglecitrumelohasbeen identifiedasresistantandisbeingdeveloped commercially.However,noneoftheaboverootstocks aresuitableforallcitrusgrowingareasoftheU.S. andcitrusnematodebiotypeshavebeenfoundin variousstatesthatreproducedifferentlyonthese resistantrootstocks.Therefore,thereisstillaneed fordevelopmentofrootstocksthatarenematode, diseaseandinsectresistant.Twobiotypesonce identifiedascitrusnematodesarefoundonmonocots anddicotsinFlorida.Thesebiotypesdonotattack citrus,andrecentstudiesindicatetheyaredifferent speciesfrom Tylenchulussemipenetrans (3). Itismostimportanttostartwithseedlingsthat arefreeofallnematodes.Withthedevelopmentof containergrownnurserystockthatisnematodefree, andstringentsanitationprograms,manyproblemscan beavoided.However,theseyoungtreesmustbe plantedingrovesthathavebeenproperlytreatedfor nematodesorinsitesthathavenoprevioushistoryof citrus.CROPRESPONSEEstimatesoflosseshavebeenattainedby measuringyieldincreasesasaresultofdisease eliminationthroughsuccessfulnematodecontrol,and bycomparingtheperformanceofinfectedtreeswith thatofuninfectedtrees.Dataavailableonyield increasesfromcitrusnematodecontrolinvarious citrus-growingcountriessuggestaworldaveragerange of20to30%increaseincitrusyield.Becauseall infectedcitrustreesarenoteconomicallydamagedby thisnematode,croplossestimatesonaworldwide basisduetothecitrusnematodehavebeenestimated tobebetween8.7and12.2%(2).LITERATURECITED1.Cobb,N.A.1914.Citrus-rootnematode.J. Agric.Res.2:217-230. 2.Cohn,E.1972.Nematodediseaseofcitrus.In J. M.Webster(ed.),EconomicNematology,pp. 215-244.AcademicPress,London. 3.Inserra,R.N.,N.Vovlas,J.H.O'Bannon,andK. R.Langdon.1987.Biologyof Tylenchulus semipenetrans bushraceandconcomitant infectionwith Meloidogyneincognita on Fraxinus caroliniana.Proc.Soil&CropSci.Soc.Florida 46:(inpress).