Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Powdery mildew is a problem on cucurbits each year. All cucurbits are susceptible, however host plant resistance in many cucumber and cantaloupe cultivars has successfully managed the problem. Susceptible varieties as well as other crops like pumpkin and squash are hit hard by powdery mildew. Disease builds up during July and becomes severe in August and September. Powdery mildew is a challenge to manage, especially in hot dry conditions. Also, there is resistance in the powdery mildew pathogen to many of our fungicides such as Quadris. Therefore, fungicides must be chosen carefully.
To manage powdery mildew, select cultivars (varieties) with resistance or tolerance. Even a moderate level of resistance will improve the efficacy of a fungicide spray (and help reduce the damage if you miss a spray). Scout the field and apply the first powdery mildew spray when you see one lesion on the underside of 45 old leaves.
Always follow good resistance management guidelines. 1) Keep on a good spray schedule (a 7-day interval for powdery mildew). 2) Apply fungicides at label rate (don’t cut the rate). 3) Be sure you are getting good fungicide coverage of your plants. 4) Be aware of products that are at risk for resistance development. 5) Materials with different modes of action (FRAC codes) should always be alternated. 6) Late in the season when powdery mildew has become well established, only apply protectant fungicides such as chlorothalonil or sulfur.
Below are the fungicide programs suggested for the various crops.
Summer Squash or Cucumber: Alternate a tank mix that contains chlorothalonil and either Procure, Rally, Folicur, or Inspire Super, with a tank mix containing Pristine plus chlorothalonil.
Muskmelon: Alternate Quintec plus chlorothalonil, with a tank mix containing chlorothalonil and either Procure, Rally, Folicur, or Inspire Super.
Extensive white sporulation of powdery mildew on pumpkin leaves.
Pumpkin: Alternate Quintec plus chlorothalonil with a tank mix containing chlorothalonil and either Pristine, Procure, Rally, Folicur, or Inspire Super. An alternative and less expensive option is to alternate Micronized Wettable Sulfur with one of the above options. Sulfur may injure plants, especially at high temperatures, which is why it is only recommended for pumpkin. Certain varieties can be more sensitive.