Cherry leaf spot (CLS), caused by the fungus Blumeriella jaapii (Rehm) v. Arx. is a devastating disease of tart cherry (Prunus cerasus 'Montmorency') in the Great Lakes region of the U.S. Michigan is the largest producer of tart cherries in the United States, and control of CLS is critical for the maintenance of high-yielding, healthy trees. Succinate dehydrogenase inhibitor (SDHI) fungicides are widely used for CLS management, but fungicide resistance has been reported for the SDHI fungicide boscalid, and other alternative SDHI fungicides have been in use commercially since 2013. In 2016, 36 tart cherry orchards in Michigan were sampled, and 883 single conidium isolates were established. In 2017, 42 orchards were sampled, and 898 isolates were established. These isolates were tested in vitro for sensitivity to the SDHI fungicides fluopyram, fluxapyroxad, and boscalid, and minimal inhibitory concentrations (MICs) were determined. Experiments using fungicide-treated potted trees inoculated with B. jaapii isolates with known MICs were done to establish the relevance of MIC to fungicide efficacy in the field. Results suggest that populations of B. jaapii have shifted towards resistance to fluopyram and fluxapyroxad. Cross-resistance between boscalid, fluopyram and fluxapyroxad is confirmed by the number of tested isolates that are resistant to the three of them (n=418), which represents 26% of the isolates tested. Management of disease must be consistent, and it is the most important form of preventing against the development of resistance to the SDHIs.
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