Cherry leaf spot (CLS), caused by the fungus Blumeriella jaapii, is a major disease of tart cherry (Prunus cerasus L.) trees, leading to early defoliation that results in uneven ripening and poor fruit quality in the current season, reduced fruit set in the following season, and increased potential for winter injury and tree death. Pristine (BASF Corporation, Research Triangle Park, NC), a commonly used 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 was observed in field trials and commercial orchards and highlights the importance of fungicide resistance monitoring. A total of 1,189 isolates from 31 commercial orchards in Michigan, I 1 1 isolates from nontreated trees (four locations in Michigan and two locations in Ohio), and 133 isolates from a research orchard were collected during 2010, 2011, and 2012 and assayed on boscalid-amended media at concentrations ranging from 0 to 25 mu g ml(-1). Because of the very slow growth rate of B. jaapii in culture, we determined the minimum inhibitory concentration (MIC) of boscalid as opposed to the effective concentration that inhibits mycelial growth to 50% of the control. Isolates from nontreated trees had MIC values ranging from 0.1 to 0.5 mu g ml(-1); the MIC of isolates from commercial orchards ranged from 0.1 to >25 mu g ml(-1), and isolates from the research orchard ranged from 2.5 to >25 mu g ml(-1). Isolates with MIC values >= 25 mu g ml(-1) were considered boscalid resistant and comprised 0% of the nontreated isolates, 30.4% of the commercial isolates, and 42.1% of the research orchard isolates. Sequencing of the sdhB gene of resistant isolates led to the detection of the amino acid mutation H260R, which is known to confer boscalid resistance in other phytopathogenic fungi. Our results indicate that the occurrence of the H260R mutation in Michigan populations of B. jaapii is correlated with the reduction in sensitivity to boscalid observed in commercial orchards.