Powdery mildew, caused by Erysiphe necator, is the most common and destructive disease of grapes (Vitis spp.) worldwide. In Michigan, it is primarily controlled with fungicides, including strobilurins (quinone outside inhibitors [QoIs]). Within the United States, resistance to this class of fungicides has been reported in E. necator populations in some east coast states. Among 12 E. necator isolates collected from five Michigan vineyards in 2008, one carried the G143A single-nucleotide mutation responsible for QoI resistance. This isolate was confirmed to be resistant in a conidium germination assay on water agar amended with trifloxystrobin at 0.001, 0.01, 0.1, 1, 10, or 100 μg/ml and salicylhydroxamic acid (100 mg/liter). The mutant isolate was able to germinate on media amended with 100 μg/ml trifloxystrobin, whereas a representative wild-type isolate did not germinate at concentrations higher than 0.1 μg/ml. In 2009, 172 isolates were collected from a total of 21 vineyards (juice and wine grapes): three vineyards with no fungicide application history (baseline sites), six research vineyards, and 12 commercial vineyards. QoI resistance was defined as the effective concentration that inhibited 50% of conidial germination (EC50) > 1 μg/ml. Isolates from baseline sites had EC50 values mostly below 0.01 μg/ml, while isolates that were highly resistant to trifloxystrobin (EC50 > 100 μg/ml) occurred in five research and three commercial wine grape vineyards at frequencies of 40 to 100% and 25 to 75% of the isolates, respectively. The G143A mutation was detected in every isolate with an EC50 > 1 μg/ml. These results suggest that fungicide resistance may play a role in suboptimal control of powdery mildew observed in some Michigan vineyards and emphasizes the need for continued fungicide resistance management.
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