Ron Becker & Sally A. Miller
For organic growers, there are several alternative fungicides labeled for cucurbit downy mildew, including copper-based fungicides. Growers should be cautious in applying copper, as it can be phytotoxic to cucurbits and high levels in soil are toxic to earthworms and other beneficial organisms. Phytotoxicity is most common during cool, moist conditions, which are also the most favorable for downy mildew. A list of the other alternative fungicides can be found at the National Sustainable Agriculture Information Services “Downy Mildew in Cucurbits” web page. Make sure to check with OMRI and your organic certifier to determine if the suggested products are currently considered acceptable for organic production.