P-291 - Effectiveness of an infection risk model in tailoring fungicide application against cucurbit downy mildew during the growing season

Cucurbit downy mildew (CDM), caused by Pseudoperonospora cubensis, significantly impacts cucurbits globally. Cucurbits are susceptible to CDM and growers currently depend on calendar-based fungicide applications for disease control. A 24-h infection model has been developed to assess disease risk to facilitate fungicide application. However, the effectiveness of the model in guiding fungicide application during the season has not been determined. Field experiments were conducted in North Carolina (summer) and South Carolina (fall) in 2023 and 2024. Ranman (cyazofamid), Bravo (chlorothalonil) and Orondis Opti (oxathiapiprolin + chlorothalonil), were applied to cucumber and squash singly or in alternation, either prophylactically or based on the model, alongside an untreated control. In both locations, model-timed applications had comparable levels of disease as to calendar-based applications and significantly (P < 0.0001) lower levels of final disease severity and rates of disease increase compared to the untreated control. The decrease in severity resulted in marginal increase in yield in fungicide treated plots compared to the untreated control. Model- or calendar-based application of alternating Ranman with Orondis Opti was the most effective program. Applying fungicides based on disease risk may be more effective than calendar-based application in controlling CDM during the season especially when disease pressure is not extremely high. Benefits from making risk-based applications will be greater when conditions favorable for CDM occur intermittently during the season.