Biological control of nematodesprospects and opportunities.docx
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Biological control of nematodesprospects and opportunities.docx
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Biologicalcontrolofnematodesprospectsandopportunities
Biologicalcontrolofnematodes:
prospectsandopportunities
B.Kerry
Introduction
Exploitationofnaturalenemies
Environmentalconcerns
Developmentofabiologicalcontrolagentfornematodes-Anecologicalapproach
Thenext20years
Introduction
Inthepast20yearsthreedevelopmentshaveoccurredwhichhavehadsignificanteffectsontheprospectsandopportunitiesforthebiologicalcontrolofplant-parasiticnematodes.First,severalnematicideshavebeenwithdrawnfromthemarketbecauseofhealthandenvironmentalproblemsassociatedwiththeirproductionanduse(Thomason,1987).Asaresultofthis,andincreasingpublicconcernovertheuseofpesticidesinfoodproduction,therehasbeenincreasedinterestinthedevelopmentofalternativemethodsofcontrol,includingtheuseofbiologicalagents.Second,ithasbeendemonstratedinseveralsoilsthatnematophagousfungiandbacteriaincreaseundersomeperennialcrops,andunderthosegrowninmonocultures,andsomaycontrolsomenematodepests,includingcystandroot-knotnematodes(Stirling,1991).Suchnematode-suppressivesoilshavebeenreportedfromaroundtheworldandincludesomeofthebestdocumentedcasesofeffectivebiologicalcontrolofnematodepests.Finally,anumberofcommercialproductsbasedonnematophagousfungiandbacteriahavebeendeveloped,butallsofarhavehadonlylimitedsuccess.Theirusehasbeenbasedonempiricalresearch,anditisinstructivetoconsiderwhatmightbethekeyfactorsforasuccessfulbiologicalcontrolagentfornematodesinordertoidentifythereasonsforthegeneralfailureoftheproductsthathavebeendeveloped.
Biologicalcontrolismoreinconsistent,lesseffectiveandsloweractingthancontrolnormallyachievedwithchemicals.Althoughimprovementsinperformancemightbeexpectedfrommoreresearchonindividualagents,itseemslikelythattheselimitationsareinherentinmostbiologicalcontrolagentsandthattheirsuccessfulapplicationwilldependonintegrationwithothercontrolmeasures.
Themoststudiedexampleofnaturalcontrolofaplant-parasiticnematodeconcernsthedeclineofpopulationsofthecereal-cystnematode,Heteroderaavenae,undermonoculturesofsusceptiblecerealsinmanysoilsthroughoutnorthernEurope(Kerry,1982).Thisisanexampleofaninducedsuppressioninwhichthenematodeincreasestodamagingpopulationdensitiesinthesecondandthirdcerealcrops,butusuallydeclinesthereaftertoinfestationsof<5eggs/gsoil,whichcauselittlelossofyieldinnorthernEuropeanconditions;itisessentialthatthenematodeisabundantintheearlyyearsofthemonoculturetosupportthebuild-upofthemicrobialparasites(Kerry,1988).Thedeclineinnematodepopulationsismainlycausedbytwoparasiticfungi,NematophthoragynophilaandVerticilliumchlamydosporium,whichattackthedevelopingfemaleontherootsurface;insuppressivesoils95to97percentofthefemalesandeggsaredestroyed(Kerry,CrumpandMullen,1982).Thusthenaturalcontrolofcereal-cystnematodeinarangeofsoilsispredictableandeffective,butslowacting.Researchonthemanipulationofnaturalcontrolhasbeenlimitedandattentionhasconcentratedontheintroductionofspecificagentstoprovidemorerapidcontrolthatmightbecommerciallyexploitable.
Ithasproveddifficulttodevelopabiologicalcontrolagentthatiseffectiveworldwideforanysoil-bornedisease.Despitemuchresearcheffortonlytwoagentshavehadwidespreadsuccess:
PhlebiagiganteaforthecontrolofHeterobasidionannosum,whichspreadsfromtreestumpstotherootsofadjacenttrees,andAgrobacteriumradiobacterthatisappliedasarootdiptotransplantsforcontrolofcrowngallcausedbyA.tumefasciens.Forboth,theagentsareappliedinhighconcentrationsasinundativetreatmentstoareadilyaccessiblesiteofaction(thecutsurfaceofatreestumporthebarerootsofatransplant)andprotectionfromthediseasesforarelativelyshortperiodprovideslong-termcontrol(Deacon,1991).Inmostsituationswherenematodecontrolisrequired,thereisaneedtoprovidelong-termprotectiontoarelativelyinaccessi-bleandgrowingrootsystemwithouttheuseofinundativetreatmentsthatarelikelytobeimpracticalanduneconomic.Hencethedevelopmentofbiologicalcontrolagentsforplant-parasiticnematodesislikelytobedifficultandtorequireadetailedunderstandingofthebiologyandecologyoftheagentandthenematodetarget.
Thefarmingsysteminwhichbiologicalcontrolisappliedhasamarkedeffectonthewaytheagentisused(Davies,deLeijandKerry,1991).Ingeneral,growersindevelopedagriculturerequireanagentthatcanbeappliedtocropsgrowninmonoculturesoverlargeareasusingstandardapplicationmachinery,soaformulatedproductwithagoodshelf-lifethatcanbeappliedatlowdosagesisrequired;seedtreatmentsarepreferredformostarablecrops.Littleresearchhasbeendoneonthemassproductionandformulationofbiologicalcontrolagentsfornematodes.Someorganisms,suchasrhizospherebacteria,canbeappliedasseedtreatments(OostendorpandSikora,1989),butsuchapplicationstendtoprovideshort-termcontrolandareonlyusefulinreducingtheinvasionofrootsbynematodesthathaveasinglegenerationinthegrowingseason.Insubsistencefarmingsystems,cropstendtobegrowninmixedstandsinrelativelysmallareas(oftenlessthanonehectare)andlabourinputsareoftenlarge.Asaconsequence,relativelylargeapplicationrates(uptoonetonneperhectare)ofanunformulatedagentcouldbemixedintothesoilbyhand,aslongastheorganismcouldbeproducedcheaplyandlocally.Thus,theinitialexploitationofbiologicalcontrolagentsfornematodesmaybeindevelopingcountries(Hussey,1990).However,ifagentsareonlyeffectiveagainstspecificnematodepests,andtheirefficacyisdependentonpestdensities,thentheireffectiveusewillrequireexpertadvicethatmaynotbeavailableinmanydevelopingcountries.
Exploitationofnaturalenemies
Thebiology,ecologyandpotentialofbiologicalcontrolagentsfornematodeshavebeenextensivelyreviewedinrecentyears(Kerry,1987;Stirling,1991;SayreandWalter,1991;Sikora,1992).Nematologistshaveidentifiednaturalenemieswitharangeofmodesofactionsimilartothosecurrentlystudiedbyplantpathologistsforthecontrolofsoil-bornediseases.Commentsontheadvantagesandlimitationsofthemajorgroupsoforganismswithpotentialasbiologicalcontrolagentsaresummarized(Table3).Itmustbestressedthatseveralorganismsthatareeffectivenaturalenemiesofnematodesinthefield,mayhavelimitedpotentialasbiologicalcontrolagentsforapplicationbygrowers.ForexampleNematophthoragynophilaisacausalagentofcereal-cystnematodedeclineinmanysoils,butitslimitedhostrange,complexrequirementsforinvitroculture,andneedforsoilmoisturelevelstobeatfieldcapacitytoensureinfection,meanthatitspotentialuseistoolimitedtowarrantfurtherdevelopmentasabiologicalcontrolagent.
Thestageinthenematodelifecycleattackedbythecontrolagenthasaprofoundeffectonthedamagetothecropandthelevelofpopulationcontrol.Therefore,trappingfungiandrhizospherebacteriathatattackthesecond-stagejuvenilesofcystandroot-knotnematodesmaysignificantlyimprovecropgrowthbutisunlikelytopreventnematodepopulationsincreasing,especiallythosespeciesthathavemorethanonegenerationinagrowingseason.Incontrast,thoseparasitesthatattackdevelopingfemalesandeggsactlikeapartiallyresistantcultivarinthatinitialnematodeinvasionandplantdamageisnotpreventedbutmultiplicationofthenematodeissignificantlyreduced.RootcolonizingfungisuchasthemycorrhizaeandendophyticspeciessuchasFusariumspp.mayreducebothnematodeinvasionanddevelopment.
Sikora(1992)reviewedarangeofcontrolmeasures,includingcroprotation,partialsoilsterilization,soilamendmentsandnematicides,thatcouldbecombinedtoincreasetheactivityofnaturallyoccurringbiologicalcontrolagents.Suchmeasurescouldalsobeusedtoimprovetheperformanceofagentsaddedtosoil.Partialsoilsterilizationbymethodssuchassolarizationreducesnematodeinfestationsandalsoreducesthecompetitionfromtheresidualsoilmicroflora,enablingthebiologicalcontrolagenttoestablishmorereadily.Soilamendmentsmayalsoreducenematodeinfestationsand,byprovidinganenergysource,helptoincreasenumbersoffacultativeparasites;pre-colonizedsubstratesaremosteffectiveinestablishingnematophagousfungiinsoil.Soilsthatarenaturallysuppressivetosomenematodesmaybeusedtoshortenrotationsofsusceptiblecropsandimprovetheperformanceofnematicidesandresistantcultivars(Kerry,1990).Theuseofcropcultivarswhicharetolerantofnematodeattackislikelytobeveryimportantforthesuccessfuldeploymentofbiologicalcontrolagents,thatprovidelesseffectivenematodecontrolthanmostnematicides.Agentssuchasrhizospherebacteriaandrhizospherecompetentfungidependonrootexudatesfortheirproliferationonroots;exudationfromrootsdiffersmarkedlybetweenplantspeciesandcultivarandaffectstheefficacyoftheseagents(KerryanddeLeij,1992;Sikora,1992).
TABLE3
Theadvantagesandlimitationsofpotentialbiologicalcontrolagentswithdifferentmodesofactionagainstplant-parasiticnematodes
Typeofagent
Modeofaction
Comments
FacultativeparasitesTrappingfungi
Trapsproducedonmodifiedmyceliumgiverisetoinfectivetrophichyphae
Advantages:
easilyproducedinvitro;somespeciesrhizo
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