Evaluation of A Weather-based Decision Support System in Scheduling Fungicide Applications to Control Cucurbit Downy Mildew

Cucurbit downy mildew (CDM), caused by Pseudoperonospora cubensis, is one of the most damaging diseases of cucurbits worldwide. Disease control primarily relies on calendar-based application of fungicides, which does not explicitly consider the risk of disease as dictated by prevailing weather. To optimize fungicide timing based on forecasted weather, a decision support system (DSS) was developed to estimate the daily risk of infection risk of cucurbits by P. cubensis and derive optimal decision thresholds for fungicide application. However, the effectiveness of this DSS in tailoring fungicide applications to control CDM has not been established. Field experiments were conducted in North Carolina (summer) and South Carolina (fall) in 2023 and 2024. Cyazofamid, chlorothalonil, and oxathiapiprolin + chlorothalonil premix, were applied to cucumber and squash plots singly or in alternation based on a calendar (weekly application) or using the DSS strategy, alongside a non-treated control. At both locations, DSS-timed applications had comparable levels of disease as the calendar-based applications, resulting in significantly (P < 0.0001) lower levels of final disease severity and rates of disease increase, compared to the non-treated control. The decrease in severity resulted in a significant (P < 0.0001) increase in yield in fungicide treated plots compared to the non-treated control except in 2024. Although both the DSS- and calendar-based strategy resulted in comparable levels of disease control, the DSS-based strategy resulted in fewer or the same number of applications as the calendar-based strategy. For comparable fungicides treatments, fungicide use efficiency was higher for the DSS- than for the calendar-based strategy in >50% of the disease cases examined. These results show that applying fungicides based on the DSS is likely to be more effective than calendar-based strategy in controlling CDM especially when disease pressure is not high. The DSS is expected to help cucurbit growers maximize the efficiency of fungicide use in disease control by allowing more informed decisions. Greater benefit from using the DSS will be realized when conditions favorable for CDM occur intermittently during the season.