Onion downy mildew, caused by the oomycete Peronospora destructor, is a highly destructive foliar disease of onion. The disease does not occur every year in the Holland Marsh, Ontario, Canada, so disease forecasting can be very useful. Symptoms develop 10 – 14 days after infection and downy mildew specific fungicides must be applied prior to infection to be effective. The disease forecasting program DOWNCAST is used to time fungicide applications when there is disease risk. The IPM program operating in the Holland Marsh provides disease risk updates to local growers throughout the growing season. The risk of sporulation and infection, known as sporulation infection periods (SIPs), is based on daily air temperature, relative humidity, leaf wetness duration and temperature, plus temperature in the days following potential infection. Fungicide sprays are recommended when there are SIP’s or if disease symptoms are found on onions in the region. The presence of sporangia of P. destructor on rotorod traps is also an indication of high risk. From 2012 to 2024, the forecasting system was accurate in 11 of 13 years. Weather conditions were favorable for downy mildew in 2023 and 2024, and fungicide efficacy trials were conducted. The trials confirmed that Ridomil and Orondis Gold (mefenoxam S and oxathiapiprolin plus mandipropamid) were effective when applied at the first indication of disease risk. The DOWNCAST model alone was mostly effective and was improved with the trapping and same-day identification of sporangia. DOWNCAST continues to be a useful disease management tool for growers.
- Vegetables