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

Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2through an experiment conducted with muskmelon in1999 to evaluate resistance management strategies forDMI fungicides. A preliminary report has beenpublished (McGrath and Shishkoff 2000).Fungicide sensitivity was determined for isolatesobtained in 2000 from a fungicide efficacy experimentconducted in research fields in Freeville, NY, wherecontrol obtained with pyraclostrobin formulated asBAS 500 was not as effective as expected based onprevious results (Zitter et al 2001). Isolates were alsocollected from the efficacy experiment conducted inRiverhead, NY, for comparison. A leaf-disk bioassaywith 0, 0,2, 2, 20, and 40µg/ml pyraclostrobin was usedto determine strobilurin sensitivity (McGrath et al1996). Assays were repeated at least once for eachisolate. Fungicide sensitivity was determined forisolates obtained from commercial fields in CA andfungicide efficacy experiments conducted in researchfields in AZ, CA, GA and NC, as well as NY, wherecontrol obtained with strobilurin fungicides was noteffective in 2002, contrasting with previous years.Leaves were collected on 22 July, 8 Oct, and 17 Octafter the last of four, five, and six applications of astrobilurin fungicide (Flint or Quadris) made inexperiments conducted by J. David Moore in Chula,GA, M. T. McGrath in Riverhead, NY, and Gerald J.Holmes in Clayton, NC, respectively. Leaves were alsocollected from non-treated (control) plants and plantsthat had been treated weekly with triadimefonformulated as Bayleton® in GA. Isolates were obtainedfrom the leaves. Strobilurin sensitivity was determinedusing 0, 0.5, 5, 50, and 100µg/mltrifloxystrobin. Sensitivity totriazole fungicides was alsodetermined using 5, 50, and100µg/ml triadimefon.2002). Reduced control in 2002 was evident for thetreatment with Quadris alternated with Nova and Bravoas well as when Quadris was used alone (Table 1).Reduced efficacy was also observed in KY, NJ, IL, MI,and VA in 2002 (W. Nesmith, S. A. Johnston, M.Babadoost, M Hausbeck, and C. Waldenmaier,personal communications).Isolates of Podosphaera xanthii resistant tostrobilurin fungicides were obtained from the GA, NC,and NY research fields. Four of nine NY isolates, 19 of21 GA isolates, and 13 of 15 NC isolates from plantstreated weekly with Flint or Quadris were able to growwell on leaf disks treated with 100 µg/mltrifloxystrobin in the bioassay. Strobilurin resistancewas also detected in the research fields in VA (Olaya,personal communication). Strobilurin sensitivityappeared to be qualitative as reported in other areas ofthe world (Ishii et. al. 2001). The maximumconcentration tolerated by most of the 73 isolates fromGA, NC, and NY in 2002 (89%) was either 0.5 or 100µg/ml trifloxystrobin. Azoxystrobin baseline sensitivitydistribution had been investigated in North America. Inone study with P. xanthii isolates collected in 1998 and1999 from several locations in North America, thegeometric mean of the baseline was 0.258 µg/ml andthe individual values ranged from 0.107 to 0.465 µg/ml(Olaya et. al 2000). In another study, 0.5-1 µg/ml wasthe maximum concentration tolerated by 60% of 72isolates collected from 1990 to 1996 in six states; 6%were able to grow, but only slightly, on leaf diskstreated with 5 µg/ml (Shishkoff and McGrath,RESULTS and DISCUSSIONStrobilurins used alone on aweekly schedule (use pattern notlabeled) did not effectivelycontrol cucurbit powdery mildewin 2002 in several fungicideefficacy experiments conductedin research fields. Isolates ofPodosphaera xanthii were collected for testingfrom fields in CA, GA, NC, and NY. Powderymildew had been controlled well by Flint orQuadris applied alone and Quadris applied inalternation with Nova and Bravo in fungicideefficacy experiments conducted yearly from1998 to 2001 in NY (Tables 1 and 2). Flintused alone effectively controlled cucurbitpowdery mildew in efficacy experimentsconducted in other states before 2002. Degreeof control in 2001 was 100% in GA (Langston et al2002), 80% in MI (Hausbeck et al 2002), 91% in VA(Alexander et al 2002), and 90% in DE (Everts et al,70