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Suggestedcitation:Chai,Y.,Kriticos,D.J.,Beddow,J.B.,Ota,N.,Yonow,T.,andCuddy,W.S.(2016).Pucciniatriticina.HarvestChoicePestGeography.St.Paul,MN:InSTePP‐HarvestChoice.

BackgroundInformationCommonNames:

WheatLeafRust,BrownRust

Scienti icName: Pucciniatriticina

Taxonomy: Kingdom:Fungi;Phylum:Basidiomycota; Class:Pucciniomycetes;Order:Pucciniales;Family:Pucciniaceae

CropHosts: Wheat(Triticumsp.),barley(Hordeumvulgare),rye(Secalecereale),triticale(XTriticosecale)

Pucciniatriticina(WheatLeafRust)

YuanChai1,DarrenJ.Kriticos1,2,JasonM.Beddow1,NoboruOta3,TaniaYonow1,2,andWilliamS.Cuddy41HarvestChoice,InSTePP,UniversityofMinnesota,St.Paul,MN,USA

2CSIRO,BiosecurityandAgricultureFlagships,Canberra,ACT,Australia3CSIRO,AgricultureFlagship,Perth,WA,Australia

4SouthWalesDepartmentofPrimaryIndustries,Menangle,NSW,Australia

IntroductionAmong the cereal rust diseases,wheat leaf rust, causedbyPuccinia triticina,occursmostcommonlyandhasthewidest distribution (Kolmer et al. 2009). Leaf rust pri‐marily infects the leaf blades, causing red‐orange pus‐tules to erupt from the leaves. These pustules containthousands of urediniospores that can disperse to infectotherplants.Yieldlossesinexcessof50percentcanre‐sultifinfectionoccursearlyinthecrop’slifecycle(Huerta‐Espinoetal.2011).Thefrequencyandwidegeographicdistributionofleafrustinfectionsleadsome(e.g.,Huerta‐Espinoetal.2011;Samborski1985)toconcludethatleafrust is responsible for more crop damage worldwidethantheotherwheatrusts.

KnownDistributionWheat leaf rust is distributed in all wheat‐growing re‐gionsoftheworld(Fig.2).Leafrustoccurrednearlyeve‐ryyearintheUnitedStates(Fig.3),CanadaandMexico,causing serious losses in wheat production (Huerta‐Espino et al. 2011;Roelfs 1989; Singh et al. 2004a). InSouthAmerica,wheatleafrustcausesmajoryieldlossesinArgentina,Bolivia,Brazil,Chile,Paraguay,andUruguay(German et al. 2004). InEast and SouthAsia, high riskregionsforwheatleafrustincludeChina,India,Pakistan,Bangladesh,andNepal,whileinCentralAsiamost(morethan 90 percent) of the wheat crop is planted in areaspronetothedisease(Singhetal.2004b). InRussia,leafrust results in yield losses for both winter and springwheat (Huerta‐Espinoetal.2011). InNorthAfrica, leafrust causes severe yield losses in Egypt and Tunisia(Huerta‐Espino et al. 2011). In South Africa, leaf rustepidemicsfrequentlyoccuronthespringwheatinWest‐ernCape,winterwheatinOrangeFreeStateandirrigat‐ed wheat in other provinces (Pretorius et al., 1987).Though leaf rust is considered a disease of concern inSouthAfrica,fungicideapplication,hostresistanceanda“non‐conducive” environment have limited levels of in‐fection(Huerta‐Espinoetal.2011;Terefeetal.2009).InAustralia, leaf rust is widely dispersed, occurring in allwheatgrowingregions(MurrayandBrennan2009).

Figure1.Wheatleafrustinfection.Source:USDA‐ARSCerealrustimagegalleryhttp://www.ars.usda.gov/SP2UserFiles/ad_hoc/36400500Cerealrusts/wrl_gnhse4.jpg

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mycelial or uredinial infections onwinterwheat in cer‐tainareaswithasuitabletemperaturepro ile.Overwin‐tering mycelium or uredinia can provide the source ofinoculum for subsequent infections. Under favorableconditions, urediniospores are disseminated regionallybywind,anddepositedbyrain.

HostCropsandOtherPlantsThe Puccinia triticina is a fungal disease of wheat(Triticum sp.), barley (Hordeum vulgare), rye (Secalecereale), goatgrasses (Aegilops sp.) and triticale (XTriticosecale). Secondary hosts include dusty meadowrue(Thalictrum speciosissimum),Anchusa,ClematisandIsopyrumfumaroides.

PotentialDistribution

CLIMEX is a lexiblemodelling andmapping tool to de‐scribe a species’ potential geographical distributionbasedonsuitabilitytoclimate.CLIMEXhasbeenusedformodellingtheclimaticnicheofnumerouspestsandplantpathogens.TheCLIMEXmodelforP. triticina was itted

DescriptionandBiologyPuccinia triticina hasacomplexlifecycle,with ivesporestages(Fig.4).Theprimaryhostsincludewheat,tritica‐le, and some grasses; the alternate hosts (sexual stage)include several speciesofThalictrum,Anchusa,Clematisand Isopyrum fumarioides. Puccina triticina producesurediniospores on wheat hosts, and this asexual stagecan cycle inde initely when conditions are favorable.Uredinialinfectionsdevelopteliosporesasthehostplantreaches later stages of its lifecycle. These teliosporesenableoversummeringinareasthatwouldotherwisebetoohotanddry(e.g.,Mediterraneanclimates).Volunteerwheatprovidesareservoirforinoculuminareassuchasthe U.S. southern plains, providing inoculum for winterwheat planted in the fall. Leaf rust can overwinter as

Figure3.WheatleafrustobservationsintheU.S.bycounty,2007‐2012(basedonUSDA‐ARSCerealRustSituationReportsandCerealRustBulletins).

Figure2.Wheatareasoftheworldwhereleafrusthashistoricallybeenaproblem(reproducedbasedonRoelfsetal.1992).

Figure4.Wheatleafrustlifecycle(reproducedbasedonUSDA‐ARScerealrustimagegalleryhttp://www.ars.usda.gov/SP2UserFiles/ad_hoc/36400500Cerealrusts/prt‐cycl.jpg).

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  usingtheCompareLocations(1species)model(Sutherstet al. 2007) using the CliMond1975Hhistorical climatedataset(Kriticosetal.2012).InCLIMEX,thestressfunc‐tions limit thepotential rangeof themodelled taxa,andthe temperature andmoisture growth indices drive theclimate suitability within the geographical range. Thestress functions are therefore generally itted to theknown distribution of the organism, with reference toexperimental data and phenologywhere applicable andavailable.ThereporteddistributionofP. triticina (Fig.2)isstronglyin luencedbylanduseintermsofhostavaila‐bility (wheat cropping) and irrigation. The reportedrange also includes a large area of ephemeral habitat,where,duetothemobilityofitsspores,P. triticina infectscropsduringclementseasons.Themapofknownoccur‐rences therefore includes areaswhere it cannot persistyearroundbecausetheclimateistoocold(e.g.,Canada),orwhere it is toodry(e.g.,Egypt). TheremaybeareaswhereP. triticina isbeingexcludedbycompetitionwithother foliar pathogens such as Pyrenophora tritici‐repentis(Al‐Naimietal.2005).Thereporteddistributionmayalsobeaffectedbymis‐orunder‐reportingofP. trit‐icina occurrences. Consideringhow thesevarious factorsaffectthereporteddistributionofP. triticina,itisneces‐sarytoemployacomplicated logicwhencomparingdif‐ferent scenarios and sets ofmodelled state variables inCLIMEX to the knowndistribution. The Ecoclimatic In‐dex (EI)depicts the relative climatic suitability of areasfor year‐round persistence of the pathogen (i.e., estab‐lishment);theAnnualGrowthIndex(GIA) indicatesrela‐tive climatic suitability for growth (i.e., infection/outbreak).

TheCLIMEXparameters (Table 1)were ittedbasedonthebiologyof leafrustpathogenandadjustedaccordingtoitsknowndistribution,usinganaturalrainfallscenar‐io. Subsequently, an irrigation scenario (2.5 mm day‐1applied as top‐up) was run and the results comparedwithxericareaswherecroppingisconductedunderirri‐gated conditions. A composite climate suitability mapwas created by (1) combining the natural rainfall andirrigationscenarioresultsusingthedatafromSiebertetal. (2005), (2) by including both the EI (annual persis‐tence)andGIA(seasonalgrowth)values,and(3)byonlyconsidering the modelled range that falls within thewheat‐growingregionsoftheworld. Thegeneralmeth‐odology used to it themodel, alongwith an accessibleguidetointerpretationofCLIMEXmodelsisprovidedbyBeddowetal.(2010).

To estimate the climatic niche for leaf rust, distributiondata from Australia, Brazil, Pakistan and the UnitedStateswere used to it theparameters. In theU.S., leafrustcansurvivewinterinthesouthernstatesandisdis‐seminated by wind‐blown urediniospores spreadingthroughoutthewheatgrowingareasoftheGreatPlains.TwoCLIMEXindices, theEcoclimaticIndex(EI)andtheAnnualGrowthIndex(GIA),needtobeconsideredinor‐dertodifferentiatetheregionswherewheatleafrustcanpersist year round (EI>0) and regionswhere infectionsonlyoccurduringfavourableseasons(GIA>0).

TheSoilMoisture Indexparameterswere initially ittedtotheobserveddistributionofP. triticina intheUnitedStates. The lower soil moisture threshold (SM0) wassubsequentlysetto0.18tore lectdistributiondataintheSindhinsoutheastPakistanandtobetter ittheavailabledata on the distribution of the disease in southwesternAustralia. The lower (SM1) and upper (SM2) optimumsoilmoisture parameterswere set as 0.7 and 1 respec‐tively,where1is ieldcapacity.Theupperthresholdforgrowth(SM3)wassetat1.5,re lectingtheconditionstoowetforthegrowthofwheatandleafrust.

The Temperature Index parameters were informed bythe environmental conditions required for wheat leafrust(Table2). Accordingtothecontrolledenvironmentstudieson leafrust, theoptimumtemperature forsporegermination, appresorium formation and penetration is15°C,withtheoptimaltemperaturerange15‐20°Cforsubstomatal vesicle development (Clifford 1981). Ever‐smeyeretal.(1980)showedthattheoptimumtempera‐

Table1.CLIMEXParameterValuesforPuccinatriticina

Parameter Description Value

Moisture

SMO lowersoilmoisturethreshold 0.18

SM1 loweroptimumsoilmoisture 0.7

SM2 upperoptimumsoilmoisture 1.0

SM3 uppersoilmoisturethreshold 1.5

Temperature

DV0 lowertemperaturethreshold 10°C

DV1 loweroptimumtemperature 15°C

DV2 upperoptimumtemperature 25°C

DV3 uppertemperaturethreshold 32°C

ColdStress

DTCS coldstressdegree‐daysthreshold 10°C

DHCS coldstressdegree‐daysaccumulationrate ‐0.0005week‐1

HeatStress

TTHS heatstresstemperaturethreshold 33°C

THHS temperaturethresholdstressaccumulationrate 0.01week‐1

DryStress

SMDS soilmoisturedrystressthreshold 0.18

HDS drystressaccumulationrate ‐0.02week‐1

WetStress

SMWS soilmoisturewetstressthreshold 1.5

HWS wetstressaccumulationrate 0.015week‐1

IrrigationScenario

2.5mmday‐1astop‐upthroughouttheyear

PDD 150°Cdays*

ThresholdHeatSum

numberofdegree‐daysaboveDV0neededtocompleteonegenerationthreshold

*TheAnnualThresholdHeatSum(PDD)wascalculatedusingtheminimumsurvivaltemperatureof10°C,theoptimaltemperature25°Candaperiodof10daysforonegeneration:(25°C‐10°C)×10days=150degreedays.

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ture for the latent period is 15 ‐ 26 °C. According toRoelfs(1989),atemperaturerangeof15‐25°Cisopti‐mal for all stages of leaf rust infection, including sporegermination,penetration,growthandsporulation.Thus,the lower (DV1) and upper (DV2) optimal temperaturelimitsweresetto15°Cand25°Crespectively.Thisopti‐maltemperaturerange isgreaterthanusually foundformanyorganisms,butmaybeafeaturethatiscommontosome plant pathogens (e.g., Kriticos et al. 2013),wherepathogen growth rates are driven by photosynthate,whichmayberelativelyabundantacrossawiderangeoftemperatures.Thelowertemperaturelimitfordevelop‐ment (DV0)was set to10 °Cand theupper limit (DV3)wasset to32 °C, according to theexperimentsbyEver‐smeyer et al. (1980) and Tollenaar (1985), where at10°Cleafrustculturesfailtoreachthesporulationstageandabove32°Ctherewasnosporegermination.

A degree‐day cold stress mechanism was itted to thecold limits of theover‐wintering regions for leaf rust intheUnitedStatesidenti iedbyRoelfs(1989).Thethresh‐oldof10degreedaysaboveDV0(10°C)andstressaccu‐mulation rate of ‐0.0005 week‐1 matched the reportedlimitsclosely. Under thismechanism, thespeciesneedstoexperienceaminimumof10degree‐daysabove10°Ceachweek,otherwisecoldstressaccumulates.Thiscoldstress mechanism is related to the balance of anabolicand catabolic processes, re lecting the need for the or‐ganismtogrowataminimumrateinordertooffsetbasalrespiration processes. In the case of ield infections byfungal phytopathogens, this probably re lects the needfor the plant to generate suf icient photosynthate to beco‐optedbythefungus.

Thethresholdforheatstresswassetat33°C,justabovethe maximum temperature for spore germination(32.2°C)(Eversmeyeretal.1980;Tollenaar1985).Therateofheatstressaccumulationwassetat0.01week‐1tomatch the known distribution of leaf rust in theUnitedStates.

BasedonthedistributionofleafrustintheUnitedStates,thethresholdforDryStresswassetat0.18andtherateof stress accumulation was set at ‐0.02 week‐1. WetStresswassettomakepartsofthePaci icnorth‐westoftheUnitedStatesunsuitableforP. triticina,withathresh‐old soilmoisture level (SMWS)of1.5 andan accumula‐tionrate(HWS)of0.015week‐1.Theheatsumnecessaryto complete a generationwas used to limit the area inwhichP. triticina cancompleteasinglegeneration.Theittedvalueof150degreedaysabovethebasetempera‐turefordevelopment(10°C)accordedwiththeapproxi‐matenorthernmostrecordnotedbyRoelfsetal. (1992)inCanada.

Figure5reportsthemodelledGrowthIndex(GI)forPuc‐cinia triticina, which shows areas suitable for seasonaldevelopment of the disease within the global extent ofwheatproduction reportedbyYouet al. (2015).HighervaluesofGI indicateareaswithclimatesmorefavorableto development of the disease. These potential occur‐renceestimatesweremodelledusingacompositeofnat‐ural rainfall and irrigation based on the irrigated areasreported by Siebert et al. (2005). Overall, the CLIMEXmodel for wheat leaf rust occurrence accords with theknown distribution of this disease and provides a toolsuitable for assessing climate suitability and potentialyield losses caused by wheat leaf rust. In the UnitedStates, theCLIMEXcomposite riskmap (Fig.5)matchestherecentreportsfromU.S. ieldandexperimentalplotsobservations(Fig.3),where leaf rustoccurs throughoutsouth‐easternstates,SouthGreatPlainsandNorthGreatPlains (Kolmer et al. 2007). It also agreeswith the re‐portsforwheatleafrustoccurrenceinotherwheatgrow‐ing regions, including the eastern prairies of Canada,Mexico,ArgentinaandUruguayinSouthAmerica,south‐ernChina,Russia,SouthAfricaandAustralia. Thecom‐posite risk model correctly identi ies the xeric parts ofEgypt, Pakistan and Indiawherewheat is grown underirrigation,andwhereP.triticinahasbeenrecorded.

Table2.TemperatureandmoisturerequirementsforP.triticina(fromRoelfsetal.1992)

Light FreeWater

Minimum Optimum Maximum

Germination 2 20 30 Low Essential

Germling 5 15‐20 30 Low Essential

Appressorium 15‐20 None Essential

Penetration 10 20 30 NoEffect Essential

Growth 2 25 35 High None

Sporulation 10 25 35 High None

Temperature°C

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Figure5. Modelledglobalclimatesuitability(GI) forPucciniatriticinaasacompositeofnaturalrainfallandirrigationbasedontheirrigationareasidenti iedinSpatialProductionAllocationModel(SPAM2005).

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ACKNOWLEDGEMENTS

HarvestChoice would like to acknowledge Philip Pardey for his significant help in preparing this brief. This brief was prepared with sup-port from the CGIAR Research Program on Wheat led by CIMMYT (Interna onal Maize and Wheat Improvement Center) and ICARDA (Interna onal Center for Agricultural Research in the Dry Areas), and the Bill and Melinda Gates Founda on by way of the Har-vestChoice project with addi onal support from the Commonwealth Scien fic and Industrial Research Organisa on (CSIRO) and The Interna onal Science and Technology Prac ce and Policy Center (InSTePP), University of Minnesota.About HarvestChoice 

HarvestChoice generates knowledge products to help guide strategic investments to improve the well-being of poor people in sub-Saharan Africa through more produc ve and profitable farming. Learn more at www.harvestchoice.org. 

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