Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University

Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2unpublished). This indicates that poor control withstrobilurins under field conditions was associated withreduced sensitivity in vitro. The resistant isolates wereable to tolerate at least 100-fold higher concentration ofstrobilurins than isolates with baseline sensitivity.Two strobilurin sensitive isolates and threeresistant isolates collected in 2002 responded similarlywhen tested in another laboratory using kresoximmethyland pyraclostrobin (H. Ypema, personalcommunication). These findings and experiences inother areas of the world with strobilurin-resistant P.xanthii indicate that cross-resistance probably extendsamong multiple strobilurins (Ishii et. al. 2001).All 14 isolates obtained from non-treated plants inthe GA experiment were sensitive to trifloxystrobin(maximum concentration tolerated was 0 mg/ml for14% of the isolates, 0.5 µg/ml for 79%, and 5 µg/ml for7%). Thus applying Flint alone shifted the pathogenpopulation substantially. Large changes in thefrequency of resistance in P. xanthii populations duringa growing season have been detected with triadimefonand benomyl (McGrath 1996a).Results on resistance in the western US areinconclusive as only 1 isolate could be tested from AZand only 5 from CA because spores obtained fromthese leaves grew poorly in culture. However, the AZisolate tolerated 50µg/ml trifloxystrobin. Also, one CAisolate grew on disks treated with 50 and 100 µg/mltrifloxystrobin in one assay, but growth was reducedcompared to lower concentrations. It did not survive tobe re-tested. The maximum concentration tolerated bythe other 4 CA isolates was 0.5µg/ml trifloxystrobin.Efficacy of strobilurin fungicides changed substantiallyfrom 2001 to 2002 in the CA research field (T. Turini2002, 2003). Control of powdery mildew achieved onlower leaf surfaces with the strobilurin fungicidesCabrio (pyraclostrobin), Quadris, and Flint used alonewas 65% - 94% in 2001 whereas severity did not differsignificantly from non-treated plants in 2002. Incontrast, the DMI fungicides Rally and Procure(triflumizole) used alone provided 82% - 100% controlin 2001 and 82% - 98% control in 2002. Strobilurinfungicides used alone in AZ provided 46% - 77%control of powdery mildew on lower leaf surfaces in2002 while triflumizole provided 91% control(Matheron et.al. 2003).Strobilurin resistance did not appear to be a factorin pyraclostrobin efficacy being lower in 2000(76%)(Zitter et al 2001) than in 1999 (92%)(Drennanet al 2000) in Freeville, NY. The highest concentrationtolerated by P. xanthii isolates obtained from theFreeville experiment in 2000 was only 2µg/mlpyraclostrobin. This concentration was tolerated by80% of Freeville isolates and 83% of isolates testedfrom Riverhead, NY, in 2000.The strobilurin-resistant isolates also exhibitedreduced sensitivity to DMI fungicides. All isolates butone tolerated 100µg/ml triadimefon. The one isolateunable to grow on leaf disks treated with 100 µg/mlwas able to tolerate 50µg/ml triadimefon. Resistance toDMI fungicides is quantitative. Isolates able to tolerate100µg/ml triadimefon are resistant to triadimefon butsensitive to myclobutanil because in fungicide efficacyexperiments Bayleton was ineffective while Nova waseffective where these isolates occurred (McGrath et. al.1996). Applying the DMI fungicide Bayleton weeklyin the experiment conducted in GA also shifted thepathogen population to a high frequency (71%) ofisolates that were resistant to strobilurin fungicides andinsensitive to DMI fungicides. Using strobilurinfungicides was shown in 1999 to be effective formanaging DMI resistance (McGrath and Shishkoff2000). In 2002, however, it appears that mostindividuals in the powdery mildew fungal populationthat were insensitive to one of these chemical classeswere also insensitive to the other, consequentlyapplying either a strobilurin or a DMI fungicide shiftedthe population towards insensitivity to both. None ofthe 73 isolates tested were strobilurin-resistant andDMI-sensitive; 7% were strobilurin-sensitive and DMIinsensitive.Only 2 isolates from NY, 1 from Flinttreatedplants in GA, 4 from Bayleton-treated plants inGA, and 2 from NC were sensitive to both chemicalclasses.Although isolates were not tested from commercialproduction fields, it is prudent for growers to considerimproving their resistance management program.Using strobilurin fungicides alone, as was done in theresearch fields, exerts more selection pressure forstrobilurin resistance than using them with DMIs andcontact fungicides in a resistance managementprogram; however, the size of the population exposedto this high selection pressure in research fields isextremely small compared to that in commercial fields.Strobilurin resistance likely occurs in commercialfields, but was more easily detected in research fieldswhere plants treated with effective fungicides and nontreatedplants provided comparisons. Strobilurinresistance appears to be widespread in the US. It wasconfirmed in GA, VA, NC, and NY. It is likely in AZand CA. And efficacy of strobilurins was reduced insome mid-western states. The cucurbit powderymildew fungus produces spores wind-dispersed overlarge areas. Inoculum for powdery mildew developingon cucurbit crops is thought to be wind-dispersednorthwards through the eastern and mid-western USeach year. Occurrence of resistance in commercialfields will reduce the utility of strobilurins, includingthose not yet registered, and eliminate an importanttool for managing DMI resistance. Strobilurins andDMIs are the only systemic fungicides registered forcucurbit powdery mildew in the US. Currentrecommendations for managing fungicide resistanceinclude using a diversity of fungicides within an71