Michigan is the nation's leading producer of cucumbers (Cucumis sativus) for pickling, valued at $45.4 million in 2019. Cucurbit downy mildew (CDM) caused by oomycete Pseudoperonospora cubensis is an annual threat to pickling cucumber production in the state. Prior to 2004, CDM on cucumber in the U.S. was controlled via host resistance that was integrated into commercial cultivars in the 1950s. In 2004, a new population of P. cubensis appeared that was highly virulent on previously resistant cucumber cultivars resulting in a CDM epidemic throughout the southeast and Atlantic coast; Michigan reported a CDM outbreak in 2005. Currently, prophylactic fungicides are relied on for disease control, but the pathogen has become resistant to key products. Our goal was to improve CDM disease management for Michigan's pickling cucumber growers by 1) evaluating 13 currently labelled fungicides for efficacy under field conditions and 2) evaluating the TOM-CAST, BLITE-CAST, and DM-CAST forecasters at different thresholds for their application in scheduling fungicide applications. Trials were conducted in 2021 and 2022 at the Michigan State University Plant Pathology Research Farm in Lansing, Michigan. Disease severity was assessed by visually estimating the foliar area showing symptoms (%) and calculating the area under the disease progress curve (AUDPC). Oxathiapiprolin + chlorothalonil was the most effective fungicide whereas fluopicolide, dimethomorph, and pyraclostrobin were ineffective. Chlorothalonil and mancozeb provided a moderate level of control but are not recommended as stand-alone treatments. When fungicide applications were scheduled using BLITE-CAST or DM-CAST models, CDM levels were similar to those observed with the 7-day program but did not consistently reduce total fungicide applications. These results provide Michigan growers with information to implement effective and sustainable CDM management strategies.
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