Conventional and Biopesticide Fungicides for Cucurbit Downy Mildew Control on Cucumber in Michigan

Michigan ranks first in the nation in cucumber production for processing (pickling), contributing over $45 million to the state's economy in 2019. Cucurbit downy mildew, caused by the oomycete Pseudoperonospora cubensis, is the most important disease of cucumber and an annual threat to yields. Fungicides are the most important means to control P. cubensis. Our goal was to monitor currently labelled conventional fungicides and biopesticides for efficacy over the course of an entire cropping period under high pathogen pressure. Field trials were conducted at the Michigan State University Plant Pathology Research Farm in 2021 and 2022. Fungicides were applied weekly, and disease severity was evaluated by visually assessing the percentage of foliar area showing downy mildew symptoms. At the end of the season, the relative area under the disease progress curve (rAUDPC) was calculated using the disease severity data. Based on rAUDPC results, oxathiapiprolin premixed with chlorothalonil was the most effective fungicide at controlling P. cubensis and was significantly better than all other conventional fungicides in both years tested. In both years, all conventional treatments had a significantly lower rAUDPC than the untreated control except pyraclostrobin, dimethomorph, and fluopicolide. No biopesticide fungicide reduced the rAUDPC compared with the untreated control in either year evaluated.