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Selection of boscalid resistance in Blumeriella jaapii populations treated with boscalid, fluopyram, or fluxapyroxad

Cory Outwater: Michigan State University

<div>Cherry leaf spot (CLS), caused by the fungus <em>Blumeriella jaapii</em>, is a major disease of tart cherry (<em>Prunus cerasus</em>) trees, leading to early defoliation which results in poor fruit quality, reduced fruit set, and tree death. Pristine, a commonly-utilized fungicide for CLS management in Michigan from 2004-2011, is a premix of boscalid, a succinate dehydrogenase inhibitor (SDHI), and pyraclostrobin, a quinone outside inhibitor (QoI). We conducted a two year statewide survey collecting 1346 isolates from 49 orchards and found a shift in boscalid sensitivity with 22% and 35% of the population exhibiting high levels of boscalid resistance in 2010 and 2011, respectively. The presence of the H272R mutation in the <em>sdhB</em> gene known to confer boscalid resistance was identified in all ten of the highly resistant isolates sequenced. Beginning in 2013, Luna Sensation, a premix of fluopyram (SDHI) and trifloxystrobin (QoI), and Merivon, a premix of fluxapyroxad (SDHI) and pyraclostrobin, were widely adopted to replace Pristine for CLS management. To assess the potential of these new SDHIs for the selection of boscalid resistance, 24 isolates from four-tree replicates that were either non-treated or treated with Pristine, Merivon or Luna Sensation were collected at three time points during 2012. At the end of the sampling period Pristine, Merivon and Luna Sensation appeared to be selecting for boscalid resistance with 54%, 79% and 33% of isolates recovered with MIC values <strong>></strong>25 mg/ml.</div>