Cucurbit Downy Mildew Update: August 27, 2025

With the recent rain showers, new reports of downy mildew have come in to the Penn State Plant Disease Clinic. Franklin, Juniata, and Mifflin Counties have reported cucumbers infected with downy mildew in the last 7 days. Continue to regularly scout cucumber and cantaloupe fields that are in production. The closest report on alternate squash crops has been pumpkin in Connecticut and western Ohio. The strain of the pathogen that infects pumpkin, squash, and watermelon is different from the one that infects cucumber and cantaloupe. This means that the downy mildew on your cucumbers will not infect your pumpkin or butternut squash.

The drier weather is currently putting the region at lower risk. However, if you have late plantings of cucumber and cantaloupe that are actively growing and producing fruit, be sure to protect them before the next storms arrive. Preventive applications are much more effective than applications made after the disease is detected. Since fungicides are the primary management tool for downy mildew, it is important to have a well-thought-out fungicide program that rotates among different FRAC codes for fungicide resistance management, as well as tank mixes with protectant fungicides that contain chlorothalonil (FRAC M05) and mancozeb (FRAC M03) if not already in the premix. Downy mildew targeted fungicides include, but are not limited to, Orondis Opti (FRAC 49 + M05), Ranman (FRAC 21), Elumin (FRAC 22), Zampro (FRAC 40 + 45), etc. Refer to the Cucumber chapter of the Mid-Atlantic Commercial Vegetable Production Recommendations for a more complete list. Always follow the directions for the label you have on hand. The label is the law.

This will be the last downy mildew update of the growing season.