Page 1 Stewardship of Powdery Mildew Fungicides in Perennial CropsGary Grove1, Mark Nelson1, and Chang-Lin Xiao2Washington State University1Irrigated Agriculture Research and Extension Center1,Prosser, WA and Tree Fruit Research and Extension Center, Wenatchee, WA2______________________________________________________________________Apples, apricots, cherries, grapes, hops, peaches and nectarines are plagued by thedisease powdery mildew in the Pacific Northwest. With the recent advent of strobilurinand quinoline fungicides, further development and registration of members of the DMI(sterol-inhibitor) fungicide class, and the utilization of narrow range petroleum oils,Washington growers currently have a wide variety of chemical management tools at theirdisposal (Table 1). While many of these new tools are highly effective, many of the mostvaluable fungicides carry an inherent risk of resistance problems. It has been known forsome time that resistance to DMI fungicides (Ypema et al, 1997) has developed in somecrops, but resistance to strobilurins has only recently become a commercial problem ingrapes (Wilcox et al, 2003). Resistance to quinoline fungicides has been documented oncereals in Europe (Hollomon et al, 1999). At risk are the DMI (Rally, Rubigan, Procure,Elite, Orbit, and Bayleton), strobilurin (Abound, Cabrio, Flint, Pristine, and Sovran), andto a somewhat lesser extent quinoline (Quintec) fungicide classes. Resistance developsdifferently in the former two classes. Resistance to DMI fungicides manifests itself as agradual loss of control as the disease-causing fungi becoming more tolerant to thefungicides. Compounds typically perform brilliantly when first released, and then over aperiod of years slowly deteriorate in efficacy. As a result, growers must apply thecompounds at higher rates and/or tighter spray intervals in order to maintain control.Conversely, resistance to strobilurin fungicides is “all or noneâ€?. Strobilurin compoundsmay perform brilliantly for a period of time, and then fail miserably. For example, astrobilurin fungicide may be effective during the current season but not during thesubsequent season.Agriculture cannot afford to lose these new, recently registered fungicides. Powderymildew is the most important disease of most of our perennial crops. The DMI,strobilurin, and quinoline fungicide classes comprise our first line of defense againstthese diseases. The risk of resistance to DMI and strobilurin fungicides is considered tobe high. The mechanism of resistance to quinolines is not fully understood, but theresistance risk is currently considered low to moderate (Hollomon et. al, 1999). Add tothis the fact that very few new powdery mildew fungicides are currently in thedevelopment “pipelineâ€?, it becomes readily apparent that sustained profitability of ourhop, tree fruit, and wine grape industries depends on the immediate application offungicide resistance management strategies. We cannot afford to lose the old- and newDMI, strobilurin, and quinoline fungicides. What can the grower do? Diversity (Figure1) and moderation (Table 2) in the fungicide program are keys to product preservation.Diversity and moderation mean 1) the use of multiple fungicide modes of action andmoderation means 2) use the resistance prone DMI, strobilurins, and quinoline fungicidesin a rational manner, respectively. It is important to limit the total number of applicationsof high-risk compounds per season and to limit the number of sequential applications(Table 2) of any single mode of action. ——————————————————————————– Page 2 Table 1. Fungicide choices for powdery mildew management on perennial crops inEastern Washington. Each color represents a different fungicide mode-of-action. Seefootnotes for important additional information.Crop Registration2Class1TradeNameCommonNameResistanceRiskAppleCherryGrape HopDMI(triazole)RallyMyclobutanilHigh••••••••3DMI(pyrimidine)RubiganFenarimolHigh••••••DMI(triazole)Elite orFolicur4TebuconazoleHigh••••••DMI(triazole)OrbitPropiconazoleHigh••DMI(imidazole)ProcureTriflumizoleHigh••••••StrobilurinAboundAzoxystrobinHigh••••StrobilurinFlintTrifloxystrobinHigh••••••••Strobilurin/anilidePristinePyraclostrobin/boscalidHigh••••StrobilurinCabriopyraclostribinHigh••StrobilurinSovranKresoxim-methylHigh••••QuinolineQuintecQuinoxyfenLow-Moderate••••••Horticulturalmineral oil,petroleumspray oil,narrowrangepetroleumoilJMSStylet OilNarrow rangepetroleum oilLow••••••••SulfurMicrothiolFlowablesulfurLow••••••••Carbonates ArmicarborKaligreenPotassiumbicarbonateLow••••••BiologicalSerenadeBacillussubtilisLow••••••••1 classes in italics are at moderate to high risk for resistance development2compound registered for use on crops indicated3 Section 18 registration4tebuconazole product for use on hops ——————————————————————————– Page 3 Figure 1. Fungicide resistance management is facilitated by use of multiple fungicidemodes-of-action. Each color represents a different mode-of-action. The spray programshould be as diverse and “colorfulâ€? as possible.In general, DMI resistance can be managed by limiting the number of applications to nomore than 3 per season and no more than 2 sequential applications, tank mixing withother modes of action (oils, sulfur, carbonates), and by application at maximum labeledrates. Resistance to strobilurins can be managed by limiting the number of strobilurinapplications per season (no more than 3 per season and no more than 2 sequentialapplications), use in single or block application in alternation with fungicides from adifferent group and use early in the season before powdery mildew is well established.Do not alternate strobilurin products as cross-resistance has been documented in thefungicide class. Good spray coverage is essential.Table 2. Sample fungicide programs for management of powdery mildew of grapevines.Treatment sequence positions the highly efficacious and resistance-prone compoundsduring the peak period of berry susceptibility. These program iteratiosn represent goodproduct stewardship because they utilize 5 modes of action, use no more than 2 totalapplications of a single mode of action per season, and do not include the sequential useof any resistance-prone fungicide class. There are numerous other iterations ofacceptable programs.Program Flatleaf6-12â€?ShootsPrebloomBloomBloom + 14 Bloom + 28CoverAOilOilStrobilurin QuinolineDMIStrobilurinSulfurBOilOilQuinolineDMIQuinolineStrobilurinSulfur ——————————————————————————– Page 4 The most important concept to appreciate with regard to fungicide resistancemanagement is that only a very small percentage of individuals in a pathogen populationhave the potential for resistance to any given fungicide mode of action. The objective ofresistance management are to reduce the populations of pathogens exposed to a givenmode of action as well as reduce the duration and frequency of that exposure, therebyreducing the opportunity for those few individuals with resistance potential to becomepredominant in the population.The bottom line for the grower or consultant:• utilize cultural practices to reduce pathogen populations whenever possible. Forexample, removal of overwintering infected terminals by dormant pruning helps reducethe primary inoculum level of apple powdery mildew. Delaying the initial irrigation ofcherries early in the spring may help to delay the onset of powdery mildew epidemics.Limiting grapevine vigor reduces powdery mildew pressure in vineyards.• use (either as alternations or tank mixes) as many fungicide modes of action (classes)as possible in the disease management program.• use fungicides protectively before powdery mildew has become a problem.• do not use resistance-prone compounds to attempt to get a powdery mildew epidemicunder control. Narrow range petroleum oils and carbonates are the best eradicativefungicides.• limit the number of applications of resistance-prone fungicide classes to no more than 3per growing season.• adjust spray volume per acre based on the size and volume of the crop to attainexcellent spray coverage.• apply fungicides at rates specified on the fungicide label (do not reduce rates).• avoid making more than 2 consecutive applications of DMI, quinoline, or strobilurinfungicides• include low resistance risk compounds (sulfurs, carbonates, and petroleum spray oils) inthe spray program as much and whenever possible.• apply fungicides during good conditions for sprayingALWAYS READ THE PESTICIDE LABEL AND FOLLOW RESISTANCEMANAGEMENT GUIDELINES! ——————————————————————————– Page 5 ReferencesHollomon, D.W., Wheeler, I., Dixon, K., Longhurst, C., and Skylakakis, G. 1999.Defining the resistance of the new powdery mildew fungicide quinoxyfen. PesticideScience 51(3): 347-351.Wilcox, W.F., Burr, J.A., and Riegel, D.G. 2003. Practical resistance to Qol fungicidesin New York populations of Uncinula necator associated with quanitative shifts inpathogen sensitivities. Phytopathology 93:S90.Ypema, H.L., Ypema, M., and Gubler, W.D. 1997. Sensitivity of Uncinulanecator to benomyl, triadimefon, myclobutanil, and fenarimol in California. PlantDis. 81:293-297.