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Poster: Biology & Disease Mgmt: Chemical Control


Screening for phenylamide fungicide insensitivity in Wisconsin hop downy mildew (Pseudoperonospora humuli) populations
M. MARKS (1), A. Gevens (1) (1) University of Wisconsin-Madison, U.S.A.

Pseudoperonospora humuli is the oomycete pathogen causing hop downy mildew, an important disease concern in WI. The disease is primarily managed by fungicides since varietal resistance is unavailable. Phenylamides have been shown to be systemic and highly effective in control, but due to resistance, particularly in the Pacific Northwestern (PNW) hop producing region, their use has dwindled. To date, the use of phenylamides, namely mefenoxam, has been extremely limited in WI due to the high cost and uncertainty of efficacy. In 2015, nineteen P. humuli isolates were collected from a commercial hop yard in Dane County, WI and screened for mefenoxam insensitivity using a leaf disk assay. Fourteen leaf disks were placed onto either 1% water agar (WA) or 1% WA amended with 25 μg/ml of mefenoxam (Ridomil Gold SL, Syngenta). The disks were inoculated at 3 sites with sporangial suspensions from each isolate and the incidence of sporulation at each site was tallied after 5-7 days. The percentage of total sporulating sites was calculated and isolates were considered insensitive if the incidence of sporulation on amended media was at least 50% of that on nonamended media. All isolates showed no sporulation on WA plates amended with mefenoxam, and were sensitive. Mefenoxam may be highly effective in managing hop downy mildew in this location. Additional locations will be tested to better assess the phenylamide sensitivity status and aid in grower fungicide decision-making.