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Assessment of boscalid, fluopyram, and fluxapyroxad sensitivity in Michigan populations of Blumeriella jaapii

Jacqueline Costa Gleason: Michigan State University

<div>Cherry leaf spot (CLS), caused by <em>Blumeriella jaapii</em> is the most important disease affecting tart cherry trees in the Great Lakes region of the United States. Early defoliation affects fruit quality and yield, and if severe, can impact the ability of trees to survive over winter. Succinate dehydrogenase inhibitor fungicides (SDHIs) have become the mainstay for control of CLS recently. Tart cherry orchards were sampled in 2016 and 2017, and more than 800 single conidium isolates were established in each year. The <em>in vitro</em> sensitivity of these isolates to boscalid, fluopyram and fluxapyroxad was examined using a minimum inhibitory concentration test (MIC). At least 50% of isolates with MICs at concentrations of 25 μg/ml or greater to boscalid, fluopyram and fluxapyroxad were present in 17, 30, and 17 of 36 orchards, respectively, in 2016 and 50% of isolates with MICs of 40 μg/ml or greater were present in 12, 14 and 1 orchard, respectively. In 2017, 50% of isolates from at least 24 and 11 of 43 screened orchards had MICs of 25 μg/ml or 40 μg/ml, respectively. We have also shown that isolates with the higher levels of reduced sensitivity observed <em>in vitro</em> are not controlled by field rates of fungicides <em>in vivo</em>. Further testing of fluopyram and boscalid is ongoing.</div>