Biological control of nematodesprospects and opportunitiesWord下载.docx

Biological control of nematodesprospects and opportunitiesWord下载.docx

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B.Kerry

Introduction

Exploitationofnaturalenemies

Environmentalconcerns

Developmentofabiologicalcontrolagentfornematodes-Anecologicalapproach

Thenext20years

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.

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