Cherry leaf spot (CLS), caused by the fungus Blumeriella jaapii, is a major disease of tart cherry (Prunus cerasus) 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, is a premix of boscalid, a succinate dehydrogenase inhibitor, and pyraclostrobin, a quinone outside inhibitor. Reduced efficacy of Pristine for CLS control observed in field trials and commercial orchards highlighted the importance of resistance monitoring. A total of 1,288 isolates from commercial orchards and 111 isolates from non-treated trees were collected in 2010 and 2011 and assayed on boscalid-amended media at concentrations of 0, 0.1, 0.5, 1, 2.5, 5, 10, and 25 g ml-1. The minimum inhibitory concentration (MIC) of boscalid was determined after incubation at 23 oC for 14 days. Isolates from non-treated trees had MIC values ranging from 0.1 to 0.5 μg ml-1 while isolates from commercial orchards ranged from 0.1 to > 25 μg ml-1 . Isolates with MIC values of > 25 μg ml-1 were considered resistant and comprised 22% and 35% of isolates in 2010 and 2011 respectively. Sequencing of the SDHB gene of resistant isolates led to the detection of the amino acid mutation H272R known to confer boscalid resistance. The occurrence of the H272R mutation in Michigan populations of B. jaapii is correlated with the reduction in sensitivity to boscalid observed in commercial orchards.
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