Download The Use of Bactericides in Plant Agriculture with Reference to Use in

February2016
TheUseofBactericidesinPlantAgriculturewith
ReferencetoUseinCitrustoMitigateHLB
StephanieL.Slinski,Ph.D.,BactericideProjectManager,CRDF
ThepurposeofthiscommunicationistodiscusstheuseofbactericidesinplantagriculturetocontroldiseaseepidemicsandtheapproachesthathavebeentestedtocontrolHuanglongbing(HLB)incitrustrees.
TheCitrusResearchandDevelopmentFoundation(CRDF)hasmadethistopicapriorityinrespondingto
HLBinFloridacitrus.HLBisadiseasedevastatingthecitrusindustryinFloridaandthroughouttheworld.
Presently,nochemicaltreatmentorresistantplantisavailablethatwillcontrolthedisease.FortheFlorida
citrusindustrytosurvivethisepidemic,achemicalcontrolwillbenecessarytosuppressthedisease,keepingthetreesinproductionuntilgrovescanbereplantedwithresistantortolerantvarieties.Thisissimilar
toapproachestakenduringplantdiseaseepidemicsinothercrops,whichwereeventuallycontrolledbythe
plantingofresistantvarieties.Oxytetracycline,streptomycinsulfateandcopperhavebeenthemainchemicalsavailabletotreatbacterialplantdiseasesintheUS.Theuseofcopperislimitedtofoliardiseasesinregionswherecopperresistanceisnotwidespread.Streptomycinandoxytetracyclineareroutinelyusedon
somecropswhencopperisinadequate.Oxytetracyclinehasalsooftenbeenusedinthepasttohelpmanageplantdiseaseepidemics.Researchusingbactericidesoncitrussuggeststhatchemicalcontrolmayimprovecitrustreehealthandcontributetosustainingthecitrusindustryinthecurrentepidemic.
Introduction
ThediseaseHLBhasbeenknowninmanyregionsofthe
worldforoveracentury.ThisdiseasewasonlyintroducedintoFloridaadecadeago,buthassincereached
epidemicproportions.HLBinFloridaispresumably
causedbythebacteriumCandidatusLiberibacterasiaticus(CLas),whichistransmittedbyaninsect,theAsian
citruspsyllid(ACP,DiaphorinacitriKuwayama).Thebacteriumisinjecteddirectlyintothevascularsystemofthe
treebythevectorandcanspreadsystemicallydownto
therootsandthroughoutthetreecanopy.Thenumber
ofFloridacitrusgrovesstillinproductionhasshrunk
dramaticallybecauseofHLBduetoadecreaseinmarketablefruitandsignificantlyhigherthanpreviousproductioncostsresultingfromtreedecline.Thecitrusindustryemploysaround62,000peopleandhasaneconomicimpactofapproximately10.8billiondollars(1).
TheJanuary2016all-orangecropestimateofonly69
millionboxeswaslessthanhalfofthepre-epidemic
numbers.Withtherisingcoststokeepthediseased
treesinproduction,thesustainabilityoftheindustryis
inquestion.Whatwillultimatelyensurethecontinued
existenceoftheindustryareresistantcitrusvarieties,
butnoresistantvarietyispresentlybeingpropagatedin
nurseries.Evenifaresistantvarietywereidentifiedtoday,itwouldtakemanyyearstoreplantmorethan
500,000acresofcitrusandforthosetreestoreachmaturity.PlantdiseaseepidemicslikeHLBarerelatively
common,andwhileultimatelyepidemicsarecontrolled
usingresistantortolerantplantvarieties,achemical
controlisoftenusedasastopgap.
AgriculturalBactericides
Plantdiseasescausedbybacteriaareverydifficultto
control,especiallywhenabacterialikeCLasresidesin
thephloemoftheplantvascularsystem,andfew
bactericidesareavailabletotreatbacterialdiseases.
Althoughcopperhasbeenusedinagricultureforseveral
centuriesasafoliartreatmenttoprotectplantsfrom
fungalandbacterialdiseases,whenusedsystemicallyat
bactericidallevels,copperistoxicthus,copperhasnot
beendemonstratedtobeaneffectivetreatmentfor
HLB.AtypeofbactericidesthathasbeenfoundtoprovidealevelofcontrolofHLBandmanyotherbacterial
diseasesisantibiotics.Thetwomainantibioticslabeled
foruseinagriculturearestreptomycinsulfateandoxytetracycline.Twoformsofoxytetracyclineareused,oxy-
tetracyclinehydrochlorideandoxytetracyclinecalcium,
butonceinsolutionthemoleculesareidentical.These
materialshavebeenusedsincethe1950sandarealso
importantinhumanandanimalmedicine,butpresently
onlyafractionofonepercentoftotaldomesticantibioticuseisforplantagriculture(2).
Themainusesforbothoxytetracyclineandstreptomycinsulfateinagricultureareforpreventionoffireblight
(Erwiniaamylovera)inapplesandpears.Streptomycinis
thepreferredbactericide,sinceitiskillsthebacteriaon
contact(bactericidal),whereasoxytetracyclineisbacteriostatic,suppressingbacterialgrowth.Bacteriostatic
bactericidesrequirealongerdurationofexposureto
suppressgrowthlongenoughtoreducethebacterial
titer.Alternatingchemicalswithdifferentmodesofactionistheprimarystrategyforresistancemanagement.
Streptomycin-resistantE.amyloverastrainsnowoccur
insomeareasbecausestreptomycinwastheonlychemicalavailableinthepast.Oxytetracyclinehasbeenlabeledforuseagainstfireblighttomanageresistancein
someregionsandrecently,kasugamycinhasbeenapprovedforuseinmostUSstatesforthesamepurpose.
Oxytetracyclineisalsousedagainstbacterialspotof
peachandnectarine(Xanthomonasarboricolapv.pruni)
andinnon-bearingplants,whereitismainlyappliedby
injectiontopreventinfectionortotreatimportantlandscapeorspecimenplants.Oxytetracyclinehasbeen
usedhistoricallytocontrolimportantbacterialandphytoplasmadiseasesinemergencysituations,suchascoconutlethalyellowingdisease,X-diseaseofpeachesand
cherries,peachyellowleafrollandpeardecline.These
diseasesnowaremainlycontrolledusingtolerantor
resistantvarietiescombinedwithgoodmanagement
practices.
BactericideResearchAgainstHLB
Theuseofoxytetracyclinehasbeenevaluatedinthe
pasttocontrolHLBandhasbeenshowntohavesome
efficacy.MostofthestudiesusingtetracyclinesandsimilarbactericidestotreatthesymptomsofHLBwere
conductedinthe1960sthroughthe1980sinareasof
long-establishedinfectionbythedisease.Severalmembersofthetetracyclinegroupwereinvestigatedincludingtetracyclinehydrochloride,chlortetracycline,dimethylchlortetracyclineandoxytetracycline.Thesestudies
mainlyevaluatedperformanceofbactericidesapplied
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byfoliarapplicationorbytrunkinjection,althoughsome
earlierstudiesimmersedbudwoodinabactericidesolutionbeforegraftingorimmersedtherootsbeforetransplanting(3,4).Molecularmethodsfordetectionofthe
HLBbacteriumwerenotavailablewhentheseearly
studieswereperformedandthusthestudiesevaluated
thetimingofvisiblesymptomdevelopmentasameasureofefficacy.Resultsvariedbyexperiment,buttetracyclinewasfoundtobeeffectiveinmostcaseswiththe
suppressionofsymptomslastingfrommonthstoyears
(3-16).Inoneexperimentevaluatingbothinjectionand
foliarapplicationsoftetracycline,moretreesrecovered
andsymptomsweresuppressedforalongerperiodof
timewhentetracyclinewasinjectedcomparedwithfoliarapplications(8).Otherstudiesfocusedoneithertrunk
injectionorfoliarapplications.Someofthetetracyclines
usedintheseexperimentswerefoundtobetoxictothe
plant,causingsymptomssuchasyellowing,smaller
leavesanddiscolorationattrunkinjectionsites.Dimethylchlortetracyclineandoxytetracyclinedidnotcause
thesamesymptoms,possiblybecauseofthegreater
solubilityofthesematerials.Onestudyalsofoundthat
thepHofthetetracyclinesolutioninfluencedthe
amountoftoxicity(15),providingfurtherevidencethat
theformulationchemistrycanbeadjustedtoprevent
toxicity.
Penicillinwasincludedinafewoftheearlystudies,and
resultsrangedfromhavinglittletonoeffectonsymptoms(5,10,13),havingalesser,butmoresustainedeffectovertimewhencomparedtooxytetracycline(9),to
havingasimilareffecttotetracyclineHCl(6).Amore
recentstudyconductedinFloridatestedacombination
ofahighdoseofpenicillinGandstreptomycinalong
withseveralotherchemicalsincludingahighdoseof
oxytetracyclineandkasugamycin,usingabudwoodimmersionfollowedbygraftingtechnique(17).Theresults
wereinconclusiveforoxytetracyclinebecauseitwas
phytotoxicatthehighrateandthegraftswereunsuccessful.Thecombinationofpenicillinandstreptomycin
successfullyreducedthenumberofbacteriatoalow,
butdetectablelevel,buthowthiscomparedwithoxytetracyclinewasnotdetermined,norwastheefficacyof
theindividualcompounds.Thisstudydemonstratedthat
kasugamycinwasineffectiveagainstCLas.InastudyexaminingthesetreatmentsappliedtothefoliageofperiwinkleplantsinfectedwithCLas,bothoxytetracycline
andthecombinationtreatmentreducedtheamountof
thebacteriatoastatisticallysimilarlevel(18).Penicillin
doesnotcurrentlyhaveanyregistereduseinplantagri-
culture.Thetimelineforregisteringthisnewactiveingredientwouldbemanyyearsaswithanynewactive
ingredient.This,alongwiththehealthrisksandlackof
evidenceofsuperioritytootherchemicalscurrently
usedinplantagriculture,suggestthatpenicillinisnota
viabletreatmentforHLB.
Discussion
Despiteshowingsomeefficacy,oxytetracyclinewasnot
widelyusedinthepasttotreatHLB.Thisispartiallybecauseofthephytotoxicityofearlyformulationsandalso
becausethemostefficientmethodforintroducingthe
materialintotheplantwasbytrunkinjection.Whileeffectivetosomedegree,trunkinjectionhistoricallyhas
notbeenfoundtobecosteffectivefortreatingmany
acresofcitrustrees.AuthorsofareviewofcitrusgreeninginSouthAfricaexplainedthattrunkinjectionsoftetracyclineinSouthAfricadidnotbecomecommonbecauseofphytotoxicity,residuesinthefruit,production
ofsmallfruitandbecausediseasesuppressionwasnot
sustained(19).Anadditionalexplanationfornotrelying
heavilyonantibioticsthatwasnotdiscussedinthereviewwasthattheLiberibacterspeciesfoundinAfrica
(CandidatusLiberibacterafricanus),alongwiththevectorpsyllidspecies,haveagreatersensitivitytoheat
thanCLasandACP,andshiftingtheindustrytoamore
tropicalregioninSouthAfricahasreducedthedisease
incidence.Thismaybewhymethodsfortrunkinjection
treatmentswereneveroptimized.InothercitrusgrowingregionseffectedbyHLB,relocatingthecitrusproductiontoareaswithperiodsofcoolerand/ordrier
weatherthatallowforadormancyperiod,orgeographicallyisolatedareas,havehelpedtoreducethepsyllid
pressureandslowthespreadofthedisease.
Nothavingtheoptionsavailabletootherregionswith
thisdisease,theFloridacitrusindustryhasreliedon
otheraspectsofcitruscultureincludingnutrition,soil
chemistryandpruninginordertomanageHLB.While
thesemethodscancontributetokeepingtreesineconomicalproduction,theydonotreducetheamountof
infectiononceatreedevelopsthedisease.Goodpsyllid
controlwillalwaysbeanimportantaspectofdisease
management.Psyllidcontrolwillonlyhelptoprevent
newinfections,butifachemicalisfoundtosuppress
thebacteriainthetree,psyllidcontrolwillbeimportant
tomaintainthissuppression.Fewmethodsareavailable
tosuppressoreliminatethebacteriuminthetree.Heat
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therapyisoneoftheearliestmethodsusedinthehistoryofplantpathologytoeliminatepathogensfromplant
material.Thismethodhasbeenadaptedforlarger-scale
useincitrusgrovesandprovidessomediseasesuppression,butisstillinexperimentaltrials.
Recently,industryinteresthasgrownforthepotential
useofantibioticstotreatHLB.ThisisbasedonpastresearchandalsofromtheSection18foremergencyuse
ofstreptomycinsulfatetotreatanotherimportantdiseaseincitrus,citruscanker(Xanthomonascitrisubsp.
citri).Fromthisinterest,severalstudiesoftheefficacy
offoliarapplicationsofoxytetracyclineandstreptomycinsulfateagainstHLBhavebeenfundedbyCRDFsince
mid-2014.Resultsfromthesestudies,alongwithresults
producedindependentlybytheregistrants,arepartof
the2015petitiontoEPAforaSection18emergency
exemptionallowinguseofthesematerialsoncitrusin
Florida.Thedecisiononthispetitionisforthcoming.ApprovaloftheSection18petitionwillallowgrowersto
applyfoliarapplicationsofoxytetracyclineandstreptomycinsulfate.
Foliarapplicationisthemostcommonmethodofapplyingpesticidestocitrusgroves.Thisisamainreasonwhy
theresearchfortheSection18applicationhasinitially
focusedonthismethod.Adjuvantsareavailablethat
increasetheabilityofthebactericidestomovethrough
theleafsurface.However,theremaybemoreefficient
methodstointroducethematerialintothetreeand
haveagreaterimpactonthebacterialpopulationinthe
phloem.Trunkinjectionisanexampleofanalternative
method.Trunkinjectionmaybeeconomicallyviableif
equipmentandmethodsweretobedevelopedtoallow
growerstoinjectalargenumberoftreesinashortperiodoftimesimilartohowinjectionsareconductedto
preventinfectionofavocadobythelaurelwiltpathogen
insouthernFlorida.Aproof-of-conceptexperimentis
currentlyinplace,fundedbyCRDF,toevaluatetheeffectoftrunkinjectionsofoxytetracyclineandstreptomycinontreehealthandfruitquality.Ifsuccessful,this
studymayleadtothedevelopmentofcost-effective
injectiontechniquesforgrowers.Althoughtheworkhas
beeninitiatedtomakethistechniqueanoptionfor
growers,expansionofcurrentlabelswillberequiredto
includetrunkapplication.
Theuseofchemicalsalonetocontrolplantdiseasesis
notsustainable,butastopgapsolution.Whenconfrontinganepidemic,chemicalcontrolisoftenthefirstline
ofdefense,followedbythedevelopmentofresistant
plants.Citrusplantbreedersareworkingtowardsdevelopingtolerantandresistantcitrusvarietiestosustain
theindustryoverthelongterm,butuntilthesevarieties
areavailable,short-termsolutionsarenecessary.The
useofoxytetracyclineandstreptomycinwitheffective
adjuvantsmaysuppressthebacteriainthetree,allowingthetreetoproduceastablecropofadequatequantityandqualitytosustaincurrentproduction.Further
advancesinapplication,coupledwithadditionalbactericides,maybeavitalsteptowardsmanagingtheHLB
epidemicinFlorida.
References
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McManus,P.S.,Stockwell,V.O.,Sundin,G.W.,&
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daGraça,J.V.,Douhan,G.W.,Halbert,S.E.,Keremane,M.L.,Lee,R.F.,Vidalakis,G.,&Zhao,H.(2015).
Huanglongbing:Anoverviewofacomplexpathosystemravagingtheworld'scitrus.Journalofintegrativeplantbiology.
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