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POSTERS: Chemical control

Sensitivity of baseline and exposed Podosphaera leucotricha populations from apple to fungicides from three chemical groups
Lederson Ganan - Washington State University. Tobin Peever- Washington State University, Achour Amiri- Washington State University

Powdery mildew, caused by Podosphaera leucotricha, is among the foremost biotic challenges faced by the apple industry in the US Pacific Northwest. Site-specific fungicides, including demethylation inhibitors (DMIs), quinone outside inhibitors (QoIs) and succinate dehydrogenase inhibitors (SDHIs) have been used for decades in conventional apple orchards and some decreased efficacy has been observed recently. In 2018, sixty P. leucotricha isolates were collected from two organic (n=26) and three conventional (n=34) apple orchards in Washington State. Sensitivity to trifloxystrobin (TRI, QoI), boscalid (BOS, SDHI) and triflumizole (TRIF, DMI) was assessed on detached leaves sprayed with four doses of each fungicide and a non-treated control. Relative growth, measured after 14 days, was used to determine EC50 values and variation factors (VFs) within the tested populations. VFs for TRI, BOS and TRIF were 23, 4, and 9 in the baseline populations versus 43, 4, and 15 in the exposed populations, respectively. Overall, these results indicate some variability in sensitivity levels in P. leucotricha populations, especially for TRI both in the baseline and the exposed populations which may suggest population drifts. Further studies that include a larger population size are needed to make general conclusions about potential shifts toward reduced sensitivity.