Scheduling Fungicide Applications for Cucurbit Downy Mildew Control on Pickling Cucumber in Michigan Using Disease Forecasters

Cucumber production is important to Michigan's economy and was valued at more than $45 million in 2019. Cucurbit downy mildew (CDM), caused by Pseudoperonospora cubensis, is an annual threat to Michigan's cucumber production, and fungicides must be applied frequently to prevent major yield losses. Our objective was to evaluate the disease forecasting models TOM-CAST, BLITE-CAST, and DM-CAST for their application in scheduling fungicide applications for CDM control. Field trials were conducted in 2021 and 2022 to evaluate each disease forecaster at different spray thresholds compared with 7- and 10-day programs and an untreated control. In 2021, all treatments received applications of cyazofamid alternated with oxathiapiprolin/chlorothalonil alternated with ametoctradin/dimethomorph plus chlorothalonil. The 2022 fungicide program was similar to that of 2021 except that cyazofamid was tank-mixed with chlorothalonil. Treatment plots were visually assessed for the foliar area (%) with CDM symptoms, and the relative area under the disease progress curve was determined at the end of each season. The results indicate that using DM-CAST or BLITE-CAST to schedule fungicide applications limited CDM and was similar to the 7-day program. The 7-day program received 7 (2021) and 6 (2022) applications, whereas DM-CAST and BLITE-CAST required 4 to 8 or 5 to 6 applications, respectively, depending on the threshold and year. This is the first study to evaluate these disease forecasters for scheduling fungicide application intervals for CDM.