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O’Brien, P.A. and Milroy, S.P. (2017) Towards biological control of Spongospora subterranea f. sp. subterranea, the causal agent of powdery

scab in potato. Australasian Plant Pathology, 46 (1). pp. 1-10.    

http://researchrepository.murdoch.edu.au/id/eprint/35687/  

Copyright: © 2017, Australasian Plant Pathology Society Inc.

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160708SssRevv9.1 1

Towards biological control of Spongospora subterranea f. sp. subterranea, 1

the causal agent of powdery scab in potato 2

3

PA O’Brien* & SP Milroy. School of Veterinary & Life Sciences, Murdoch 4

University, Murdoch WA 6150 Australia 5

*Correspondingauthoremail:P.Obrien@murdoch.edu.au6

7

Abstract:8

Powderyscabofpotato,causedbytheobligatebiotrophicprotozoanpathogen9

Spongosporasubterraneaf.sp.subterranea(Sss),isamajorprobleminpotato10

growingareasthroughouttheworld.Itresultsinlesions(scabs)onthesurface11

ofthetuberswhichrendersthemunmarketable.Inrecentyearstherehasbeen12

anincreasingnumberofreportsofthedisease,manyfromnewareas.13

Managementofthediseasehasproveddifficultandreliesontheintegrated14

applicationofarangeofmethods.Biocontrolisnotcurrentlyusedforthe15

managementofpowderyscabalthoughtheresultsofpreliminarystudieshave16

beenencouraging.Thisreviewevaluatesthepotentialfordevelopinga17

biocontrolstrategyforpowderyscab.18

19

20

Introduction21 22

Powderyscabofpotato,causedbythebiotrophicprotozoanpathogen23

Spongosporasubterraneaf.sp.subterranea(Sss),isamajorprobleminmany24

potatogrowingareasthroughouttheworld.Itresultsinlesions(scabs)onthe25

surfaceoftubersthatarefilledwithabrownpowderconsistingofsporosori26

(alsoreferredtoassporeballs),hencethecommonname(Harrisonetal.1997).27

Affectedtubershavelowacceptanceatmarketandaredown‐gradedorrejected28

bytraders,leadingtoreducedreturnstogrowersandincreasedwastewithinthe29

industry.Forproducersofseedpotatoes,thelesionsmayleadtorejectionof30

entireconsignmentsasthediseaseisspreadthroughinfectedtubers(Falloonet31

al.1996;Kirkham1986).Economiclossesinthefreshpotatomarketarevery32

160708SssRevv9.1 2

difficulttoquantify.AnnuallossestotheAustralianprocessingindustryhave33

beenestimatedatA$13.4million(Wilson2016).34

35

Inadditiontoproducinglesionsontubers,thepathogenalsoinfectsroots,36

aprocessthatleadstoreducedwaterandnutrientuptakeandthusimpaired37

shootandtubergrowth(Falloonetal.2016).Historicallythisaspecthas38

receivedlessattentionbecausetuberlesionsaremoreevident,butrootinfection39

isnowconsideredtohavethegreaterdeleteriouseffectoncropproduction.In40

fieldtrials,impairmentofwaterandnutrientutilisationcanleadtoa25%41

reductioninshootdryweight,a26%reductioninthenumberoftubersperplant42

anda42%reductionintuberweightperplant(Falloonetal.2016).43

Susceptibilityofcultivarstorootinfectionisonlylooselyrelatedtopreviously44

determinedsusceptibilitytopowderyscab(i.e.developmentoftuberlesions)45

andhencetheyareconsideredtobeseparatediseaseprocesses(Falloonetal.46

2016;Nitzanetal.2008).Rootinfection,withimpairedwaterandnutrient47

utilisation,canoccurfromearlyinthegrowthofplants,butgallformationand48

tuberlesionsonlybecomeevidentatlaterstagesofplantdevelopment(Falloon49

etal.2016).Gallformation,whensevere,canalsoleadtoimpairedwaterand50

nutrientutilisation(JohnsonandCummings2015).51

52

Inadditiontoitsdirectimpactonthehostcrop,Sssisalsothevectorof53

thepotatomop‐topfurovirus(Arifetal.1995).Mop‐topvirushasbeenreported54

tocauseyieldlossesofbetween30and60%(Carnegieetal.2010)andtubers55

expressingthecharacteristic‘spraing’symptomsareunacceptableatmarket.56

Thevirushasspreadaroundtheworldinthelast40yearsanditcanremain57

infectiveinfieldswithoutcultivationofpotatoesformanyyears(Kirk2008;58

Kalischuketal.2016).Itisspeculatedthatalternativehostsmayprolong59

survivaloftheinfectiousvirusalthoughthehostrangeofthevirusismore60

restrictedthanthatofitsvector(Kirk2008).61

62

ThepathogenSsshasprovedchallengingtocontrol,withnosingle63

methodreliablygivingfullcontrol.Rather,managementreliesontheintegrated64

applicationofarangeoftools(Falloon2008).Inthispaperwewillbrieflyreview65

160708SssRevv9.1 3

thebiologyofthepathogenandcurrentcontrolmethodsasabasisforexploring66

thepotentialtodevelopeffectivebiologicalcontroloptions.67

68

69

Occurrence70 71

PowderyscabwasfirstreportedasadiseaseinGermanyin184172

(Harrisonetal.1997).Subsequentlytherewerereportsonoccurrencesofthe73

diseasefrommanylocationsbetween1846and1992(Harrisonetal.1997).In74

recentyearstherehasbeenaresurgenceinreportsofthedisease,manyfrom75

newareassuchasAustralia,NewZealand,Columbia,Pakistan,Koreaandothers76

(Balendresetal.2016;Harrisonetal.1997;Merz2008).Theresurgenceinthe77

diseasehasbeenattributedtoincreasedcultivationofpotatocultivarswithlong78

growingseasons,useofsusceptiblecultivars,increasedirrigationofcrops,79

inadequatecroprotation,andde‐registrationofmercurybasedfungicides80

(Braithwaiteetal.1994;Harrisonetal.1997;Merz2008).Asthediseaseis81

transmittedthroughinfectedtubers,theincreasedtransportofseedtubers82

aroundtheworldmayhaveexacerbatedthespreadofthedisease(Gauetal.83

2015).Increasedawarenessandincreasedefficiencyindetectionofthedisease84

mayalsohavecontributedtotheincreaseinreporting.85

86

87

Taxonomy88 89

Spongosporasubterranea(Wallr.)Lagerhf.sp.subterraneaTomlinson,the90

causalagentofpotatopowderyscab,isasoil‐borneobligatepathogenanda91

plasmodiophoridcharacterizedashavingcruciformnucleardivision,92

multinucleateplasmodia,biflagellatezoosporesandrestingspores(Hutchison93

andKawchuk1998).Thetaxonomicpositionoftheplasmodiophoridshasbeen94

uncertainforsometime.Traditionallytheywereplacedinthefungialthough95

otherresearchershavearguedforaprotozoalorigin(reviewedin(QuandChrist96

2004)).AnalysisofSSU‐rDNAsequencesinfiveindependentstudiesledtothe97

conflictingviewsthattheyareunrelatedtoanyothereukaryotes(threestudies)98

orarerelatedtotherhizopoda(twostudies).Theresultsofmorerecent99

160708SssRevv9.1 4

analysesnowrobustlyplacethemwithintheeukaryotesupergroupRhizaria,as100

asistergrouptotheomnivorousvampyrellidamoebae(Neuhauseretal.2014).101

Plasmodiophoridsarethebetterknownmembersofthegroupbecausethey102

includeanumberofplantparasitescausingeconomicallysignificantdiseasesof103

cropsincludingbrassicas,potatoes,andgraincrops(e.g.maize,rice,wheat,104

sorghum).Themoststudiedspeciesistheclubroot‐causingPlasmodiophora105

brassicae,aparasiteofcruciferswhichaccountsforupto10%lossofthe106

worldwideproductionofBrassicacrops.Otherwell‐studiedspeciesinclude107

Spongosporasubterranea,whichcausespowderyscabofpotatoandcanserveas108

avectorforPotatoMopTopVirus.Non‐pathogenicspeciessuchasPolymyxa109

graministransmitseconomicallyimportantvirusestoanumberofgrainplants110

whilePolymyxabetaeisthevectorforbeetnecroticyellowveinvirus,thecause111

ofsugarbeet“rhizomania”.112

113

114

LifeCycle115116

Thelifecycle(Fig1)hasbeendescribedintheexcellentreviewsof117

Harrisonetal.(1997),(Merz2008),andBalendresetal.(2016).Intheasexual118

orzoosporangialphase(Fig1innercircle)thehostplantisinfectedbyhaploid119

biflagellatezoosporesthatencystontherootortubersurfacesandpenetratethe120

planttissue.Withintheplantthezoosporesdevelopintomulticellular121

plasmodia.Afterrepeatedcelldivisionstheplasmodiadifferentiateinto122

zoosporangiafromwhichhaploidbiflagellatesecondaryzoosporesarereleased.123

Theseexitthesporangiathroughporesthatextendsthroughthesporangium124

wallsandtherootsurfacesresultinginreleasedirectlyintothesoil.The125

secondaryzoosporescaninitiateinfectionsonotherpartsofthesameplantsuch126

astubers,oronadjacentplants.Thisprovidesthebasisformultiplere‐infection127

inasinglegrowingseasonandthusthepotentialforveryrapidinoculumbuild‐128

up.129

130

Inthesexualphase(Fig1outercircle)thickwalledrestingsporeseach131

germinatetoreleaseaprimarybiflagellatehaploidzoospore.Twohaploid132

160708SssRevv9.1 5

zoosporesmayundergocellfusion(plasmogamy)toformabinucleatezoospore133

thatinfectstheplant(althoughthislaststepisstilldebated).Thisdevelopsinto134

amultinucleateplasmodiumwithbinucleatecells.Eventuallythenucleifuse135

(karyogamy)andundergomeiosistodifferentiateintothickwalledspores136

withinastructureknownasasporosorus.Therestingsporesareveryresistant137

structuresandcanpersistinsoilfor4‐5years.Thesexualstagehasnotbeen138

detectedinSssbuthasbeendescribedinthecloselyrelatedspecies139

Plasmodiophorabrassica(TommerupandIngram1971).140

141

The potential for rapid increases in inoculum level and the persistence of 142

resistant spores in the soil are key considerations in the development of management 143

strategies for Sss. 144

145

PathogenDiversity146 147

InconsideringthedevelopmentofcontrolstrategiesforSssitisessential148

tounderstandthedegreeofgeneticdiversityin,andthegeneticstructureofthe149

pathogenpopulation.Variousstudieshaveshowntheexistenceofgenotypic150

variation,includingbetweengeographiclocations.AnalysisofITSsequences151

showedthatNorthAmericanisolates,togetherwithAustralasianandsome152

Europeancollectionsformedaclonalpopulation(groupII)whilstSouth153

AmericanandsomeEuropeancollectionsformedaseparategroup(groupI)154

(Bulmanetal.2001;Gauetal.2015;QuandChrist2004).TheSouthAmerican155

collectionsshowedgreatergeneticdiversitycomparedtocollectionsfromthe156

restoftheworld,suggestingthatthisisthecentreoforiginofthespecies(Gauet157

al.2013;Gauetal.2015).158

159

QuandChrist(2004)foundthattheirEuropean(fromIrelandand160

Scotland)collectionswereassociatedwithparticularpotatocultivars.The161

EuropeangroupIcollectionswereallfromcv.Saturnafromdifferentlocationsin162

IrelandandScotlandoverseveralyearswhilsttheEuropeangroupIIcollections163

werefromcvKerrsPinkfromlocationsinIrelandoverseveralyears.Insome164

instancescollectionswereobtainedfrombothcultivarsfromthesameareaby165

160708SssRevv9.1 6

thesamegrowers.SimilarobservationswerereportedbyGauetal.(2013).In166

someAndeanregionsofSouthAmericashortdayrelativesofthewidelygrown167

longdaypotatospeciesSolanumtuberosumssptuberosumarecultivated.These168

areS.tuberosumssp.andigena,andS.tuberosumphureja.SouthAmerican169

samplesofSssfromS.t.phurejarootgallsformedonegroup,whilstsamplesfrom170

tuberlesionsonS.t.tuberosumandS.t.andigenaformedanothergroup.Samples171

fromtherestoftheworldformedathirdgroupwithahighlyclonalstructure.172

173

AnalysisofNorthAmericanisolateswithasuiteofRFLPmarkersshowed174

thattheyclusteredintotwogroups,oneincludedisolatesoriginatingfrom175

westernNorthAmerica.,,andthesecondincludedisolatesoriginatingfrom176

easternNorthAmerica(QuandChrist2006b).177

178

Theresultsofthedifferentstudiesshowthatthereisgenotypicvariation179

acrossgeographicalregions.Theyalsohighlighttheneedforalargescalestudy180

onaworldwidepopulationusingmultiplegeneticlocisuchasmicrosatellite181

markers(Dobrowolskietal.2003)orRFLPmarkers(QuandChrist2006b).The182

greatergeneticdiversityfoundintheSouthAmericanpopulation(Gauetal.183

2015)suggeststhatthismaybethecentreoforiginofthepathogen.Movement184

ofpotatopropagulesfromSouthAmericaneedstobeappropriatelyregulatedto185

minimizetheriskofintroducingnewpathogendiversityintotheworld’s186

productionregions.187

188

189

CurrentControlofPowderyScab190191

Hostresistance192 193

Althoughanumberofpotatocultivarswithdifferentlevelsofresistance194

toSsshavebeenidentified(Falloonetal.2003),nocultivarisknowntobe195

completelyresistanttopowderyscab(Falloon2008;HernandezMaldonadoetal.196

2013).Resistancevariesfromveryresistanttoverysusceptible(quantitative)197

suggestingapolygenicbasis(Falloon2008).Ingeneral,moreresistantcultivars198

havefewergallsandfewerzoosporesintheirrootsalthoughthecorrelationis199

160708SssRevv9.1 7

nottightandexceptionsexist(Falloonetal.2003).HernandezMaldonadoetal.200

(2013)comparedSssaccumulationinrootsandthenumbersofgallsthat201

developedonaresistantandasusceptiblepotatocultivarandfoundthat202

althoughtherewaslittledifferencebetweenthecultivarsatearlystagesof203

infectiontherewasincreasingdivergenceasthediseaseprogressed.This204

suggeststhatalthoughtheprimaryinfectionmaybethesameinbothcultivars,205

secondaryinfectionmayberestrictedintheresistantcultivar.206

207

Chemicalcontrol208 209

Thereareanumberofagro‐chemicalsthatcanbeappliedtoseedtubers210

ortothesoil,atorbeforeplanting,thathavebeenshowntoreducepowdery211

scablevels(reviewedinFalloon(2008)).Braithwaiteetal.(1994)evaluated25212

fungicidetreatmentsappliedtoseverelyinfectedtubersjustbeforeplanting.213

Plantemergencewasassessed,andatmaturitytuberswereharvestedand214

scoredforpowderyscabinfection.Anumberofthetreatmentsreducedthe215

proportionofdiseasedtubersfrom95%atplantingtoanaverageof29%inthe216

subsequentcropcomparedwiththeuntreatedcontrolswhichdeclinedto70%.217

Someoftheothertreatmentsresultedinphytotoxiceffects.Treatmentoftubers218

withzincsulfateandzincoxidesixweeksbeforeplantingalsoreducedinfection219

inthesubsequentcropcomparedwithuntreatedcontrols.220

221

Asubsequentstudyevaluatedchemicalsappliedeitherastuberdressings222

orasin‐furrowapplications.Thefungicidesfluazinam,mancozeb,dichlorophen‐223

Na,wereeffectiveastuberdressingsofinfectedtubers,reducingtheincidenceof224

powderyscabandincreasingtheyieldoftubers(kg/plot)byupto36%(Falloon225

etal.1996).Thesamestudyevaluatedin‐furrowtreatmentsbyplanting226

uninfectedtubersofcvsRuaandAgriaintoheavilyinfestedsoil.Treatments227

withfluazinamreducedpowderyscabincidence,andincreasedyieldof228

marketable'Rua'tubersbyupto55%,and'Agria'tubersby140%.Highratesof229

mancozebalsoreducedincidenceofthediseaseandincreasedmarketableyield230

of'Rua'by34%,andof'Agria'by68%.In‐furrowtreatmentwithzincoxideand231

foliartreatmentswithphosphorousaciddidnotcontrolthedisease.232

160708SssRevv9.1 8

233

Croprotation234 235

Longrotationswithnon‐hostspeciesarerecommendedastheresting236

sporesofSsscanpersistformanyyearsinthesoil(Falloon2008).However,the237

hostrangeofSssismuchwiderthanwaspreviouslythoughtandincludesmany238

non‐solanaceousspecies(QuandChrist2006a).Of26specieswithin10families239

frommonocotyledonsanddicotyledonstested,16specieswerefoundtobe240

susceptibletoSss.TwelvespecieswerenewlyrecordedhostsforSss.However241

QuandChrist(2006a)alsoobservedthatalthoughsomespecieswereinfected,242

theydidnotproducesporosori.Itwassuggestedthatthesecouldpossiblybe243

usedtoreducetheinoculumlevelsinsoil.Sparrowetal.(2015)monitored244

pathogenDNAlevelsinsoilsofSouthEasternAustraliaoveraneightyears245

periodandsuggestedaminimumoffiveyearsbetweenpotatocrops.246

247

LarkinandGriffin(2007)evaluatedthepotentialofrotationwithbrassica248

cropstodecreasethelevelofpathogeninoculuminsoilandsoreducedisease249

severityinsubsequentpotatocrops.Brassicasproducesulphurcompounds,250

glucosinolates,whichbreakdowntoisothiocyanatesthataretoxictoawide251

rangeofphytopathogens.Inafieldtrial,Indianmustardsignificantlyreduced252

Sssinoculumlevelsinthesoilasmeasuredbyabioassay.Cropsofrapeseedor253

yellowmustardwerelesseffective.Allthreebrassicaspeciesdecreasedthe254

severityofpowderyscabby25‐39%relativetoastandardoatsrotation.The255

brassicasalsoreducedtheincidenceoftuberswithscabsby19‐40%withIndian256

mustardagainbeingthemosteffective.Indianmustardproduceshighlevelsof257

glucosinolates,someofwhichconverttothemostbiologicallyactiveformsof258

isothiocyanatesproducedbybrassicas(CharronandSams1999).Inthesame259

study,rotationwith‘Lemtal’ryegrassledtosimilarreductionsindisease260

severityandincidenceoftuberswithlesionsaswereobservedafterrotation261

withthebrassicas.Thissuggeststhatfactorsotherthantheproductionof262

isothiocyanatesmayhavebeenresponsibleforthereductioninincidenceand263

severity.264

265

160708SssRevv9.1 9

Asanalternativetodirectpathogeninhibition,rotationcropsmayalso266

affectpathogensindirectlybyinfluencingchangesinthesoilmicrobial267

community.Thismayincreasediseasesuppressionbythesoil.Inastudyto268

examinetheeffectsofrotationsystemsonsoilmicrobialcommunities,Larkin269

andHoneycutt(2006)demonstrateddistinctiveeffectsofspecificrotationcrops270

andcroppingsequencesonthesecommunities.Theyalsofoundhigher271

populationsofmicroorganismsthataregenerallybeneficialtoplants,suchas272

Pseudomonasspp.andTrichodermaspp.,afterplantingbarley,canola,andsweet273

corncrops.274

275

Agronomicfactors276 277

Agronomicfactors,particularlysoilnitrogenstatusandsoilwatercontent278

werefoundtoaffecttheseverityofdisease(Shahetal.2014).Theincidence279

and/orseverityofpowderyscabwereincreasedbynitrogen(nitrateand280

ammonium)applications.Nitrogenapplicationresultedinagreateramountof281

SssDNAinthesoilandthiseffectwasobservedfortwoyearsafterthetrial.A282

positivecorrelationbetweensoilSssDNAanddiseaseseveritywasobservedby283

Brierleyetal.(2013)andbyNakayamaandSayama(2013).However,Shahetal.284

(2012)reportedthatintheirstudytherewasnoconsistentlystrongrelationship285

betweentheamountofinoculumattimeofplantingasmeasuredbythenumber286

ofsporosoripergsoilanddiseaseincidenceorseverityatharvest.Irrigation287

treatmentalsoaffecteddiseaseseverity.Anirrigationregimeoptimalforpotato288

growthresultedingreaterseverity,butnotgreaterincidence,ofpowderyscab289

thanaconstrainedirrigationinput(Shahetal.2014).Thisisconsistentwith290

highersoilmoisturecontentfacilitatingthemovementofzoosporesthroughthe291

soilandincreasingthechancesofplantinfection(Merz2008;Balendresetal.292

2016).293

294

295

PotentialforBiocontrol296297

Biocontrol,basedontheuseofanorganismtorestricttheabilityofa298

pathogentocausediseaseisanaspectofplantdiseasecontrolthathasreceived299

160708SssRevv9.1 10

muchattentioninrecentyears.Thisisfuelledbytheemergenceoffungicide300

resistanceinpathogenpopulations,deregistrationoffungicides,andgrowing301

concernsabouttheuseofchemicalsinfoodproduction.Incontrastbiocontrolis302

seenasasafe,non‐toxic,renewablealternative.Insomecasesofbiocontrolthe303

levelofcontrolrivalsthatachievedwithchemicalsalthoughalackofconsistency304

ofcontrolisoftenaconcern.305

306

Identifyingbiocontrolagents307 308

Therearefewreportsofstudiesaimedtoidentifypotentialbiological309

controlagents(BCAs)forSssinpotato.Themostextensivestudyisthatof310

NakayamaandSayama(2013).Theyreported54–70%suppressionofdisease311

developmentoverthreeyearsbyapplicationofanisolateofAspergillus312

versicolourtotubers.Thesevaluescomparedto77to93%suppressionbythe313

syntheticfungicidefluazinamappliedinfurrow.WhiletheBCAwaslesseffective314

thanthesyntheticfungicide,thesuppressionwasstatisticallysignificant.315

316

AnotherapproachwastakenbyNielsenandLarsen(2004)whoused317

tomatoasamodeltoexaminetheefficacyofcommerciallyavailablebiocontrol318

productstoreducetheinfectionofroothairsbyprimaryzoosporesofSss.Each319

agentwasassessedintwoexperiments.Thecommerciallyproducedbiocontrol320

productsTRI002andBinabTF(bothcontainingTrichodermaharzianum)gavea321

statisticallysignificantreductioninrootcolonizationinbothexperiments.By322

contrastthebiocontrolproductTRI003(alsocontainingTrichoderma323

harzianum)gavenosignificantreductionineitherexperiment.Theproduct324

FZB24,containingBacillussubtilis,gaveareducedinfectioninonlyoneofthe325

twoexperiments.Ineachexperiment,allplantgrowthparametersexamined326

weremarkedlylowerintheinfectedcontrolsthantheuninfectedcontrols.While327

someofthecontrolagentsincreasedtheplantgrowthparametersabovethatin328

theinfectedcontrol,theseincreaseswerenotconsistent.329

330

IntheexperimentsofNielsenandLarsen(2004)averyhighinoculum331

concentrationofthepathogenwasused;muchgreaterthancouldbeexpectedin332

160708SssRevv9.1 11

fieldsoilorinsoiladheringtothesurfaceofseedtubers(e.g.0‐148sporosori/g333

ofsoil;(Brierleyetal.2013)).Itispossiblethatmoreeffectiveandmore334

consistentcontrolcouldbeobtainedwithmoretypicaldiseasepressures;335

biologicalcontrolofPlasmodiophorabrassicaehasbeenfoundtobemore336

effectiveatlowerpathogenpressure(Narisawaetal.2005;Pengetal.2011).It337

wasnotknownwhethertheBCAstestedbyNielsenandLarsen(2004)were338

inhibitorytoSss.Incontrast,inthestudybyNakayamaandSayama(2013)soil339

fungiwereinitiallyscreenedforinhibitionofSssinfectivityusingabioassay.340

341

AlthoughresearchonbiocontrolofSssislimited,biocontrolof342

Plasmodiophorabrassicae,hasbeenstudiedmoreextensively.Anumberof343

studieshaveidentifiedendophyteswiththeabilitytosuppressthispathogen.344

TheseincludeBacillussubtilis,Gliocladiumcatenulatum,Heteroconium345

chaetospira,Microbisporaroseassprosea,Streptomycesgriseoruber,S.346

griseoviridis,S.lydicus,S.olivochromogenesandTrichodermaatroviride,T.347

harzianumandT.viride.(Lahlalietal.2014;Leeetal.2008;Narisawaetal.1998;348

Pengetal.2014;Pengetal.2011;Wangetal.2012;Wangetal.2011).Insome349

studiesanumberofthesepotentialbiocontrolagentshaveshownlevelsof350

suppressionofclubrootgreaterthan85%andapproachingthelevelof351

effectivenessobservedfromsyntheticfungicides.352

353

Theevidencefromthelimitedstudiesthathavebeencarriedoutsuggests354

thatdevelopingbiocontrolstrategiesforplasmodiophoriddiseasesispossible.355

Howeveraproblemcommontoallbiocontrolstrategiesisthelackofconsistency356

ofdiseasereduction.Severalresearchershavereportedthatusingmixturesof357

BCAshasincreasedtheconsistencyofbiocontrolacrosssiteswithdifferent358

conditions.Sliningeretal.(2001)intheirinvestigationintopostharvestdryrot359

ofpotatofoundthatformulationsofmixedBCAsperformedmoreconsistently360

across32storageenvironmentsvaryingincultivar,washingprocedure,361

temperature,harvestyear,andstoragetime.Enhancedbiocontrolusing362

mixturesofBCAshasbeenreportedforcontroloflateblightinpotato(Slininger363

etal.2007),diseasesofpoplar(Gyenisetal.2003),chilli(Muthukumaretal.364

2011),andcucumber(RaupachandKloepper1998;Robertsetal.2005).Itis365

160708SssRevv9.1 12

alsopossiblethatdifferentmixturesmayneedtobeusedindifferentclimatic366

areas.Thusthereisaneedtoidentifyanumberofpotentialbiocontrolagents.367

Mixturesdonotalwaysgiveincreasedcontrol.Insomecasestheremaybe368

antagonismbetweentheBCAsthatresultsinreducedcontrolcomparedtosingle369

strains.Inevaluatingagentsforcontroloffireblightinpear,Stockwelletal.370

(2011)foundthatmixturesofPseudomonasfluorescensA506,PantoeavagusC9‐371

1andPantoeaagglomeransEh252werelesseffectivethantheindividualstrains.372

ThePantoeastrainsexerttheireffectsthroughtheproductionofpeptide373

antibiotics.Inthemixturetheseweredegradedbyanextracellularprotease374

producedbyP.fluorescensA506.Robertsetal.(2005)alsoreportedantagonism375

betweenBCAstrains.TheyobservedthatpopulationsofTrichodermavirensGL3376

orGL321werebothsubstantiallyreducedafterco‐incubationwithBacillus377

cepaciaBC‐1orSerratiamarcescensisolatesN1‐14orN2‐4incucumber378

rhizospheres.Thesereportshighlighttheimportanceofconsideringpossible379

antagonismbetweenstrainswhendevelopingbiocontrolformulations.Co‐380

cultivationinvitrocansometimesrevealinhibitoryeffects(Robertsetal.2005)381

butnotalways.InthestudybyStockwelletal.(2011)theantagonisticeffects382

wouldnothavebeendetectedbyco‐cultivationastheBCAsthemselveswerenot383

affected,onlytheirpotentialactiononthepathogenwasdisrupted.384

385

BCAsaretypicallyidentifiedbyscreeningrhizosphericorendophytic386

bacteriaandfungiforinhibitionofpathogengrowthinvitro,followedby387

greenhousetrialsofgrowthinhibitingisolatesandfinallyfieldtrials.Theuseof388

invitroscreeningasaninitialstephasanumberofsignificantlimitations;most389

notably,itonlyidentifiesorganismwhichactonthepathogenthroughaspecific390

subsetofmechanisms.ControlofdiseasebyaBCAoccursthroughavarietyof391

mechanismsandinmanycasesdirectcontactwiththepathogenisnot392

necessary.Mechanismsofcontrolinclude:detoxificationoftoxinsproducedby393

thepathogen(Newmanetal.2008),degradationofthepathogencellwall394

leadingtolysis(mycoparasitism)(Janetal.2011),productionofantibiotics395

(Raaijmakersetal.2002),antimicrobialsurfactants(Raaijmakersetal.2010),396

siderophoresandvolatiles(Santoyoetal.2012)bytheBCA,inductionofplant397

defenses(Tingetal.2012),stimulationofplantgrowth(vanderLelieetal.398

160708SssRevv9.1 13

2009),physicalocclusionofthepathogenbyoccupyingsitesonrootsurfaces399

thatthepathogenwoulduseforentry(Blumensteinetal.2015),andbiofilm400

formation(Newmanetal.2008).AgivenBCAmayusemorethanone401

mechanismofinhibition,anddifferentBCAsmayexerttheireffectsatdifferent402

timesduringthecropgrowingseason.Invitroscreeningonlyidentifiesthose403

BCAsactingthroughtheproductionofantibioticsorcelllysis.Further,inthe404

specificcaseofSss,invitroscreeningisnotpossibleasthespeciesisanobligate405

biotroph.Thusinitialscreeningwouldneedtoemployinplantamethodssimilar406

tothoseusedbyNakayamaandSayama(2013).407

408

Invitroinhibitionorinhibitionofdiseasedevelopmentingreenhouse409

trialsdoesnotalwaystranslatetoeffectivediseasemanagementinthefield410

whereweather,soilandbiologicalvariabilityarelikelytobemuchgreater.411

Ultimately,theonlyrealisticevaluationisbyfieldtrials.Giventherangeof412

edaphicandenvironmentalfactorsthatcaninfluencetheeffectivenessofBCAs413

(asnotedinthefollowingsection),thenumberandlocationofevaluationsites414

needstobeselectedtoappropriatelyreflecttheanticipatedrangeofusage.As415

theproductionofpotatoexpandsgeographicallyintoMediterranean,sub‐416

tropicalandevenhighaltitudetropicalareas,thisaspectbecomesmore417

significant.Further,giventhatsoilmicrobialcommunitiescanvarydramatically418

withsoiltypeandlandmanagement,variationinthesefactorsneedstobe419

capturedalso.420

421

Sss,impactspotatoveryearlyincropdevelopment(Hughes1980;Taylor422

etal.1986).Screeningmethodologymustreflecttheneedforprotectiontobe423

establishedbythetimeofplantemergence.Withafocussolelyonreducingtuber424

symptoms,protectionwouldneedtopersisttobeyondtuberinitiation;although425

longerprotectionwouldbepreferableasrootdamagecontinuestooccurbeyond426

thatstage(Hughes1980),impactinghostnutrientandwateruptake.427

428

Stabilityacrossproductionenvironments429 430

Studiesontheinfluenceofenvironmentontheefficacyofbiological431

160708SssRevv9.1 14

controlofsoil‐bornediseasesdemonstratetheinfluenceoftemperature(Janget432

al.2011;Landaetal.2001;Landaetal.2004;Schmidtetal.2004),soilwater433

status(Schmidtetal.2004)andsoilphysicalandchemicalcharacteristics434

(Ownleyetal.2003)onthelevelsofcontrolachieved.Giventheincreasingly435

diverseconditionsandgeographiclocationsunderwhichpotatoisnow436

produced(Birchetal.2012;Devauxetal.2014),thiswillbeanimportant437

consideration.438

439

Noresearchhasbeenreportedthatevaluatestheinfluenceof440

environmentalvariables(eithersoilorweather)ontheeffectivenessofBCAsin441

supressingSssandlittlehasbeenreportedinrelationtoP.brassicae.Studying442

thecontrolofP.brassicaebyHeterconiumchaetospira,Narisawaetal.(2005)443

foundeffectivesuppressionunderlowtomoderatemoistureconditionsbutnot444

athighwaterstatus.Ontheotherhand,Pengetal.(2011)foundthatperiodsof445

drysoilimpededtheeffectivenessofarangeofBCAstovaryingdegrees.Bacillus446

subtilisandthesyntheticfungicide,fluazinamwereparticularlysensitive.447

Significantly,whiletheresultsofZhouetal.(2014)showedvariationinthe448

effectivenessofbacterialisolatesinsuppressingP.brassicaeinChinesecabbage,449

theeffectivenessofthesyntheticfungicidealsovaried.450

451

EffectiveformulationofBCA452 453

TheforminwhichaBCAisappliedtoacropmayaffectitspersistenceand454

thusitseffectiveness.Resistantpropagulessuchasbacterialendospores,fungal455

conidia,chlamydosporesoroosporesaremorepersistentthanvegetative456

bacterialcellsorfungalhyphalfragments(Schisleretal.2004).Forspore‐457

formingbacteria,yeasts,andfungi,theproductionsystemcanbeoptimisedfor458

theproductionofspores.Fermentationenvironmentsandcultureageinfluence459

theefficacy,stabilityanddesiccationtoleranceofmanyBCAsincludingfungi,460

yeasts,andbacteria(Schisleretal.2004).Thefungalbiocontrolagent461

Trichodermaharzianumdevelopedmyceliumafterfourdaysat280Cand462

chlamydosporesafter10daysinliquidmedium(potatodextrosebroth)at28°C463

whileonsolidmedia(e.g.,PDA,grains,wheatbran,)itproducedconidia(Mishra464

160708SssRevv9.1 15

etal.2012).465

466

PersistencecanbeenhancedbymixingtheBCAwithadditivesthatsupply467

nutrients,protectitfromdesiccation,fromUV,andantagonisticorganisms,and468

increaseitsabilitytosticktothesurfaceofplants.Thedifferenttypesof469

additiveswerereviewedbySchisleretal.(2004).Theadditionofcalcium470

carbonatetoricegrainculturesofTrichodermamartialestimulatedconidia471

productionandenhancedthepersistenceoftheBCAinthefieldandduring472

storage(Hanadaetal.2009).FormulationofstrainsoffluorescentPseudomonas473

withtalchasbeenreportedtoenhanceefficacyincontrolofFusariumwiltin474

banana(Saravananetal.2004)andtomato(Sarmaetal.2011).Inclusionof475

chitininthegrowthmediumfortheproductionofchitinolyticstrainsofSerratia476

hasbeenfoundtobeeffectiveanditispostulatedthatthechitinmayalso477

stimulatethegrowthofotherchitinolyticspeciesinrhizosphereswhichwould478

helptopreventfungalinfection(Kimetal.2008).479

480

EncapsulationoftheBCAwithinabiodegradablematrixofprotein(whey,481

poly‐Lysine)polysaccharide(e.g.,cellulose,alginates,chitosan,pectin),orother482

polymerssuchaslignin,protectstheBCAfrombioticandabioticstressfactors483

andpromotesshelflifeandpersistenceintheenvironment.Overall484

encapsulationresultsingreaterdiseasecontrol.Thetopicofencapsulation,485

includingthetypesofmaterialsandhowtomakecapsules,isreviewedby486

VemmerandPatel(2013).487

488

MethodsofapplicationofBCA489 490

Amajorfactorintheefficacyofbiocontrolagentsisthemethodof491

application.BCAscanbeappliedasseed(tuber)dressings,assoil‐drenchesoras492

afoliarspray.Forrhizosphericorganismsthatwillencounterthepathogen493

beforeitentersthehostplant,andpreventsentryeitherbyocclusionorkilling494

thepathogenitisessentialthattheBCAisabletocolonizerootsastheydevelop.495

496

Forsoil‐bornediseases,soildrenchesaremosteffectiveastheyallow497

160708SssRevv9.1 16

extensivecolonizationofroots(Gossenetal.2013).In‐furrowapplicationsor498

seeddressingsarealternativesalthoughtheymaynotbeaseffectivein499

providinghighlevelsofinoculation.McLeanetal.(2005)comparedmethodsof500

introducingTrichodermaatroviridaeasaBCAforSclerotiumcepivorum(white501

rot)inonions.Inaglasshousetrial,theyfoundthatapelletformulation502

maintainedgreaterlevelsoftheBCAinthesoilcomparedtosolid‐substrateor503

seed‐coatingformulations.Pelletsalsogavemoreextensivecolonizationofroot504

systemsoftheonionseedlings.Inasubsequentfieldtrialapelletformulation505

resultedinagreaternumberofCFU/gofsoilthanasolidsubstrateform.The506

pelletformulationalsogaveamorepersistentinoculum.507

508

Timingoftheapplicationmayalsohaveanimportantimpact.Inatrialof509

suppressionoftherelatedclubrootpathogenP.brassicabythefungalBCAH.510

chaetospira,itwasfoundthatsoilapplicationofagranularformulationofthe511

fungusreducedclubrootseverityby>80%relativetothecontrol.However,512

applyingH.chaetospiraatseedingwasmuchlesseffectivethanearliersoil513

application.MoreextensiverootcolonizationbyH.chaetospiraresultedin514

greatersuppressionofP.brassicaeinfectionandsubsequentclubroot515

development(Lahlalietal.2014)Thus,thegreatersuppressionbytheearlier516

applicationmayhaveresultedfrommoreextensiverootcolonization.517

518

InthecaseofSss,protectionisrequiredfromaboutthetimeofplant519

emergence(Hughes1980;Tayloretal.1986).Thus,foranyBCAtobeeffective,520

earlycolonizationofthebelowgroundplantpartswillbecritical.521

522

Incorporationintodiseasemanagementsystems523 524

Falloon(2008)reviewedthedevelopmentofintegratedmanagement525

systemsforSss.Thereiscurrentlynosinglemanagementpracticeavailablethat526

providesfullandreliablecontrolofthedisease.Effectivemanagementrelieson527

thecoordinateduseofanumberofapproaches.Themaincomponentslistedby528

Falloon(2008)werecroprotation,fieldselection,resistantcultivars,pathogen‐529

freeplantingmaterial,appropriatepesticideuseandsoundcropmanagement.530

160708SssRevv9.1 17

Theuseofbiologicalcontroliscompatiblewiththemajorityofthese531

components;howevercompatibilitywithagro‐chemicals(fungicidesandor532

pesticides)isanobviouschallenge.533

534

Potatoproducersuseawiderangeofagro‐chemicalsasdressingsforseed535

tubers,appliedeitherbeforeorafterstorage,orasin‐furrowapplicationsat536

plantingsinbandsupto20cmwide.Fungicideswhichareusedagainstarange537

ofpossiblepathogens,areaparticularissue.Thesevaryfrombroadspectrum538

treatmentstoproductstargetingspecificdiseaseorganisms.Theirmobility539

withinpotatoplantsalsorangesfromcontacttofullysystemic.Thiswillbea540

majorconsiderationfortheincorporationofBCAsintotheproductionsystem.541

Clearly,thesignificancewilldependontheparticularchemicalinuseandits542

modeofapplicationontheonehandandtheidentityoftheBCAandthemodeof543

inoculationontheother.Forexample,in‐furrowapplicationoffungicidesmay544

interferewiththesuccessfulestablishmentofBCAsappliedin‐furrowortothe545

seedpiecedependingontheidentityoftheBCAandthefungicideapplied.Onthe546

otherhandbroad‐spectrumsoilfumigantssuchasmetam‐sodium(sodiumsalt547

ofmethyldithiocarbamate)arestillinuseinsomeregionsasameansof548

reducingsoil‐bornediseasespriortoplantingpotatoes.Althoughthese549

treatmentsarebroadspectrum(Xieetal.2015),thereislikelytobelimited550

interferencewithBCAsappliedtotheseedpotatooratplantingasthefumigants551

arereleasedfromthesoilsomeweekspriortoplanting.However,questions552

havebeenraisedregardingtheimpactofsuchfumigantsonthesizeand553

structureofsoilmicrobialcommunities(DeCaletal.2005;Macaladyetal.1998).554

Significantalterationsincommunitystructuremayhaveunforeseen555

consequencesforthesuccessoftheBCA,thepathogen,orsoilfunctions.556

557

Approachestocombiningbiocontrolagentsandagrochemicalsforuse558

againstthetargetpathogenareimportantforIPM.Levelsofdiseasesuppression559

achievedbycombiningthecontrolmethodsaretypicallyequalorsuperiortothe560

useofBCAalone(Deberdtetal.2008;Hidalgoetal.2003)(seediscussionin561

Hanadaetal.(2009)).562

563

160708SssRevv9.1 18

Giventhatpotatoisvegetativelypropagated,introducinganendophyte564

intopotatotubersviatheparentseedcropmayprovidealevelofprotection565

fromnon‐systemicagrochemicals.Thismayprovideanovelmethodof566

combiningbiocontrolwithseed‐tuberdressingsorin‐furrowpesticide567

applicationsinanintegratedsystem.Iffeasible,thisapproachwouldalso568

provideadegreeofprotectionfortheBCAfromenvironmentalandsoil569

variability,aswellasprovidingtheearliestpossibleintroductionoftheBCA570

whichisimportantgiventheearlyimpactofSssonpotatoplantsandtuber571

quality.Astructuredexperimentalinvestigationisrequired.572

573

574

IsSuccessfulBiologicalControlofSssPossible?575 576

Thelimitedworkavailablesuggeststhateffortstoidentifyorganisms577

capableofsuccessfullysuppressingdiseasedevelopmentbySssarelikelytobe578

successful.Toachievethis,ahighthroughputinplantaassaywillberequiredfor579

screeningendophyticandrhizosphericorganisms.Themethoddescribedby580

NakayamaandSayama(2013)couldbeusedforthispurposeascouldthe581

procedureofNielsenandLarsen(2004).InthisregardAndreaRamirezetal.582

(2013)havedescribedastemcuttingassaytoscreencultivarsforresistanceto583

Sssthatcouldbeadaptedtoscreenfordiseasecontrolbymicrobialisolates.584

AnotherpossibilitydescribedbyMerzetal.(2004)isaproceduretoscreen585

potatocultivarsforresistanceusingtissuecultureplantlets.586

587

However,identifyingbiologicalantagonistswouldappeartobethemore588

straightforwardaspectoftheresearch.Thegreaterchallengeliesindeveloping589

aformulationandmethodofapplicationthatwillprovideadequateprotection590

consistentlyacrossproductionsystemsandgeographiclocationswithinthe591

intendedrangeofuse.MixturesofBCAsmayprovevaluableinthisrespect.592

Importantly,theformulationandmodeofusewillneedtoallowintegrationwith593

otherdiseasecontrolmethods,especiallytheuseoffungicidesandpesticides.594

595

GiventhecommercialsignificanceofthediseaseprocessescausedbySss596

160708SssRevv9.1 19

andthepersistentchallengeofeffectivecontrol,afocusedefforttoexplorethe597

potentialtodeveloppracticalbiologicalcontrolsystemsforthispathogenwould598

appearwarranted. 599

600

601

Acknowledgements602603

ThankstoProf.RichardFalloon(LincolnNZ)forhelpfulcommentsonanearly604

versionofthemanuscript.ThankstoProf.UMerz(Zurich)forpermissionto605

includeFigure1.606

607

608

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Figure1.TentativelifecycleofSpongosporasubterraneaf.sp.subterranea.The929

outerpathrepresentssexualreproductionandtheinnerpathrepresentsasexual930

reproduction.(ReproducedfromMerz(2008)withpermissionofSpringer).931

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Fig1:934935

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