Cucurbit Downy Mildew Update: August 12, 2025

Continue to be on the lookout for downy mildew on cucumber and cantaloupe. It has been confirmed in Columbia, Bucks, and Lancaster counties. The closest report on watermelon and summer squash is in South Carolina. The drier weather is currently putting the region at lower risk. However, storms are in the forecast. See the most recent Penn State Weather Outlook report. Now is a good time to make sure your cucumber and cantaloupe crops are protected with fungicides 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.

Powdery mildew (PM) is also starting to show up on cucurbit crops. Remember that fungicides that specifically target downy mildew will not manage powdery mildew. PM produces an abundance of asexual spores (the powdery growth) easily dispersed by wind. Contrary to other plant diseases, dryness is favorable for colonization, sporulation, and dispersal. Rain and free moisture on the plant surface are unfavorable. If PM is not managed well. leaves turn yellow and die prematurely, which results in fewer fruit and/or fruit of low quality because of sunscald, poor storability, and weak handles in the case of pumpkin. Use chlorothalonil in the tank mix along with a targeted fungicide such as Vivando or  DMI products such as Proline, Procure, Luna Experience, or Rhyme. Quintec and Torino also have efficacy for powdery mildew. 

 See the 2024-2025 Mid-Atlantic Commercial Vegetable Production Recommendations for a more complete list of fungicide recommendations.

Extension educators are visiting farms and scouting for disease. If you suspect downy mildew on your farm, please contact your closest Extension educator or Leah Fronk at lxf339@psu.edu. The next report will be posted on August 20, 2025.