Unit 6 Plant Diseases Caused by Nematodes.docx
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Unit 6 Plant Diseases Caused by Nematodes.docx
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Unit6PlantDiseasesCausedbyNematodes
Unit6PlantDiseasesCausedbyNematodes
线虫(Nematodes)是一类较低等的动物,属于无脊椎动物中的线虫动物门,雌雄同体或雌雄异体。
据估计,全世界有超过50万种线虫,主要包括寄生性线虫和非寄生性线虫两大类,其中的非寄生性线虫约占总数的52%~65%。
到上世纪90年代为止。
科学报道过的线虫大约有2万种,其中约2千种为植物寄生性线虫,广泛寄生于植物的根、茎、叶、花、芽或种子上,如臭名昭著的马铃薯胞囊线虫、香蕉穿孔线虫和松材线虫等,它们不仅严重危害植物的生长,还会造成巨大的经济损失。
NematodesbelongtothekingdomAnimalia.Nematodesarewormlikeinappearancebutquitedistincttaxonomicallyfromthetrueworms.Mostoftheseveralthousandspeciesofnematodeslivefreelyinfreshorsaltwatersorinthesoil,andfeedonmicroorganismsandmicroscopicplantsandanimals.Numerousspeciesofnematodesattackandparasitizehumansandanimals,inwhichtheycausevariousdiseases.Severalhundredspecies,however,areknowntofeedonlivingplants,obtainingtheirfoodwithspearsorstylets(Fig.6-1)andcausingavarietyofplantdiseasesworldwide.Theannualworldwidelossescausedbynematodesonthelife-sustainingcrops,whichincludeallgrainsandlegumes,banana,cassava,coconut,potato,sugarbeet,sugarcane,sweetpotato,andyam,areestimatedtobeabout11%.Lossesformostothereconomicallyimportantcrops(vegetables,fruits,andnonediblefieldcrops)areabout14%,foratotalofover$80billionannually.
CharacteristicsofPlantPathogenicNematodes
Morphology
Plant-parasiticnematodesaresmall,300tol,000micrometers,withsomeupto4millimeters
long,by15to35micrometerswide(Figs.6-1and6-2).Theirsmalldiametermakestheminvisibletothenakedeye,buttheycanbeobservedeasilyunderthemicroscope.Nematodesare,ingeneral,eelshapedandroundincrosssection,msmooth,unsegmentedbodies,withoutlegsorotherappendages.Thefemaleofsomespecies,however,becomeswollenatmaturityandhavepear-shapedorspheroidbodies(Fig.6-2)
Anatomy
Thenematodebody(Fig.6-1)ismoreorlesstransparent.Itiscoveredbyacolorlesscuticle,
whichisusuallymarkedbystriationsorothermarkings.Thecuticlemoltswhenanematodegoes
throughthesuccessivejuvenilestages.Thecuticleisproducedbythehypodermis,whichconsistsoflivingcellsandextendsintothebodycavityasfourchordsseparatingfourbandsoflongitudinalmuscles.Themusclesenablethenematodetomove.
Thebodycavitycontainsafluidthroughwhichcirculationandrespirationtakeplace.Thedigestivesystemisahollowtubeextendingfromthemouththroughtheesophagus,intestine,rectum,andanus.Lips,usuallysixinnumber,surroundthemouth.Mostplantparasiticnematodeshaveahollowstyletorspear,butafewhaveasolidmodifiedspear.Thespearisusedtopunctureholesinplantcellsandthroughwhichtowithdrawnutrientsfromthecells.
Thereproductivesystemsofnematodesarewelldeveloped.Femaleshaveoneortwoovaries,followedbyanoviductanduterusterminatinginavulva.Themalereproductivestructureissimilartothatofthefemale,butthereisatestis,seminalvesicle,andaterminusinacommonopeningwiththeintestine.Apairofprotrusile,copulatoryspiculesarealsopresentinthemale.Reproductioninplantparasiticnematodesisthrougheggsandmaybesexualorparthenogenetic.Manyspecieslackmales.
LifeCycles
Thelifehistoriesofmostplantparasiticnematodesare,ingeneral,quitesimilar.Eggshatchintojuveniles,whoseappearanceandstructureareusuallysimilartothoseoftheadultnematodes.Juvenilesgrowinsize,andeachjuvenilestageisterminatedbyamolt.Allnematodeshavefourjuvenilestages,withthefirstmoltusuallyoccurringintheegg.Afterthefinalmoltthenematodesdifferentiateintomalesandfemales.Thefemalecanthenproducefertileeggseitheraftermatingwithamaleor,intheabsenceofmales,parthenogenetically.
Alifecyclefromeggtoeggmaybecompletedwithin2to4weeksunderoptimumenvironmental,especiallytemperature,conditionsbutwilltakelongerincoolertemperatures.Insomespeciesofnematodesthefirstorsecondjuvenilestagescannotinfectplantsanddependontheenergystoredintheeggfortheirmetabolicfunctions.Whentheinfectivestagesareproduced,however,theymustfeedonasusceptiblehostorstarvetodeath.Anabsenceofsuitablehostsmayresultinthedeathofallindividualsofcertainnematodespecieswithinafewmonths,butinotherspeciesthejuvenilestagesmaydryupandremainquiescentortheeggsmayremaindormantinthesoilforyears.
EcologyandSpread
Almostallplantpathogenicnematodeslivepartoftheirfivesinthesoil.Manylivefreelyinthesoil,feedingsuperficiallyonrootsandundergroundstems,andinall,eveninthespecializedsedentaryparasites,theeggs,thepreparasiticjuvenilestages,andthemalesarefoundinthesoilforallorpartoftheirfives.Soiltemperature,moisture,andaerationaffectsurvivalandmovementofnematodesinthesoil.Nematodesoccuringreatestabundanceinthetop15to30centimetersofsoil.Thedistributionofnematodesincultivatedsoilisusuallyirregularandisgreatestinoraroundtherootsofsusceptibleplants,whichtheyfollowsometimestoconsiderabledepths(30to150centimetersormore).Thegreaterconcentrationofnematodesintheregionofhostplantrootsisdueprimarilytotheirmorerapidreproductiononthefoodsupplyavailableandalsotoattractionofnematodesbysubstancesreleasedintotherhizosphere.TothesemustbeaddedtheSO-calledhatchingfactoreffectofsubstancesoriginatingfromtherootthatdiffuseintothesurroundingsoil,markedlystimulatingthehatchingofeggsofcertainspecies.Mostnematodeeggs,however,hatchfreelyinwaterintheabsenceofanyspecialstimulus.
Nematodesspreadthroughthesoilslowlyundertheirownpower.Theoveralldistancetraveledbyanematodeprobablydoesnotexceedafewmetersperseason.Nematodesmovefasterinthesoilwhentheporesalefinedwithathinfilmofwater(afewmicrometersthick)thanwhenthesoiliswater-logged.Inadditiontotheirownmovement,however,nematodesCanbespreadeasilybyanythingthatmovesandcancarryparticlesofsoil.Farmequipment,irrigation,floodordrainagewater,animalfeet,birds,andduststormsspreadnematodesinlocalareas,whereasoverlongdistancesnematodesarespreadprimarilywithfarmproduceandnurseryplants.Afewnematodesthatattacktheabovegroundpartsofplantsnotonlyspreadthroughthesoilasdescribedearlier,buttheyarealsosplashedtotheplantsbyfallingrainoroverheadwatering.Somespeciesascendwetplantstemorleafsurfacesontheirownpower.FurtherspreadtakesplaceoncontactofinfectedplantpartswithadjacenttwogeneralofthefamilyAphelenchoididae,namelyAphelenchoides(budandleafnematodes)andBursaphelechus(thepinewiltandre-ringnematodes),seldom,ifever,enterthesoil.Theysurviveinsteadinthetissuesoftheplantstheyinfectand,forthelatter,initsinsectvectors.
AllplantparasiticnematodesbelongtothephylumNematoda.Mostoftheimportantparasitic
generabelongtotheorderTylenchida,butafewbelongtotheorderDorylaimida.
Order:
Tylenchida
Superfamily:
Tylenchoidea
Family:
Anguinidae
Genus:
Anguina,wheatorseed-gallnematode
lenchus,stemorbulbnematodeofalfalfa,onion,narcissus,etc.
Family:
Belonolaimidae
Genus:
Belonolaimus。
stingnematodeofcereals,legumes,cucurbits,etc.
Tylenchorhynchus,stuntnematodeoftobacco,corn,cotton,etc.
Family:
Pratylenchidae
Genus:
Pratylenchus,lesionnematodeofalmostallcropplantsandflees
Radopholus,burrowingnematodeofbanana,citrus,coffee,sugarcane,etc.
Nacobbus,falseroot-knotnematode
Family:
Hoplolaimidae
Genus:
Hoplolaimus,lancenematodeofcorn,sugarcane,cotton,alfalfa,etc.
Rotylenchus,spiralnematodeofvariousplants
Heliocotylenchus,spiralnematodeofvariousplants
Rot),lenchulus。
reniformnematodeofcotton,papaya,tea,tomato,etc.
Scuwllonema,dryrotnematodeofyam,cassava,etc.
Family:
Heteroderidae
Genus:
Globodera,roundcystnematodeofpotato
Heterodera,cystnematodeoftobacco,soybean,sugarbeets,cereals
Meloidogyne,root—knotnematodeofalmostallcropplants
Superfamily:
Criconematoidea
Family:
Criconematidae
Genus:
Criconemella,formerlyCriconernaandCriconemoides,ringnematodeof
woodyplants,causeofpeachtreeshortlife
Hemicycliophora,sheathnematodeofvariousplants
Family:
Paratylenchidae
Genus:
Paratylenchus,pinnematodeofvariousplants
Family:
Tylenchulidae
Genus:
Tylenchulus,citrusnematodeofcitrus,grapes,olive,lilac,etc.
Suborder:
Aphelenchina
Family:
Aphelenchoididae
Genus:
Aphelenchoides,foliarnematodeofchrysanthemum,strawberry,begonia,
rice,coconut,etc.
Bursaphelenchus,thepinewiltandthecoconutpalmorredringnematodes
Order:
Dorylaimida
Family:
Longidoridae
Genus:
Longidorus,needlenematodeofsomeplants
Xiphinema,daggernematodeoftrees,woodyvines,andmanyannuals
Family:
Trichodoridae
Genus:
Paratrichodorus,stubby—rootnematodeofcereals,vegetables,cranberry,
andapple
Tnchodorus,stubby—rootnematodeofsugarbeet,potato,cereals,
andapple
Intermsofhabitat,pathogenicnematodesareeitherectoparasites,i.e.,speciesthatdonot
normallyenterroottissuebutfeedonlyfromtheoutsideonthecellsneartherootsurfaces,orendoparasites,i.e.,speciesthatenterthehostandfeedformwithin.BothoftheseCanbeeithermigratory,i.e.,theylivefreelyinthesoilandfeedonplantswithoutbecomingattachedormovearoundinsidetheplant,orsedentary,i.e.,speciesthat,oncewithinaroot,donotmoveaboutEctoparasiticnematodesincludetheringnematodes(sedentary)andthedagger,stubbyroot,
andstingnematodes(allmigratory).Endoparasiticnematodesincludetherootknot,cyst,andcitrusnematodes(allsedentary),andthelesion,stemandbulb,burrowing,leaf,stunt,lance,andspiralnematodes(allsomewhatmigratory).Ofthese,thecyst,lance,andspira
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