Fungicide Sensitivity of Cucurbit Powdery Mildew in Charleston, SC

From Clemson Plant Pathologist Tony Keinath

A fungicide sensitivity bioassay was done between June 1 and June 10, 2023, to find out which fungicides can be used against the current outbreak of powdery mildew on squash, watermelon, and other cucurbits.

Methods: Yellow summer squash plants ‘Early Prolific’ with three true leaves were treated with one of 10 fungicides, water, or not treated. Four plants were treated with the highest field rate of each fungicide (for four replications). The following day potted plants were placed outdoors among summer squash and zucchini plants at Coastal REC that had obvious symptoms and signs of powdery mildew. Plants were left outside for 24 hours, then moved to a greenhouse. The third oldest leaf on each plant was examined under a dissecting microscope at 20X magnification so individual colonies of powdery mildew on the top of the leaf could be counted.

Results: The number of colonies of powdery mildew ranged from 0 to 248 per leaf.

All leaves treated with Gatten, Vivando, Quintec, or Procure had no powdery mildew. These four fungicides are among the best to manage powdery mildew on cucurbits. Applications should begin either when the first colonies are seen on the crop or when powdery mildew is known to be active in the area.

Leaves treated with Torino, Torac, and Prolivo had low levels of powdery mildew. Resistance to Torino has been reported in the Northeast, but the isolate present in Charleston is still considered sensitive. Torac is typically used as an insecticide, but this pesticide also is recommended to manage powdery mildew on tomato in the 2023 Southeast Vegetable Crop Handbook. Growers who apply Torac to cucurbits may be able to skip a week of fungicides for powdery mildew early in the season. Although Prolivo was effective both with and without an adjuvant in this test, the company strongly recommends adding an adjuvant if this fungicide is used.

As seen in 2017, Endura was still ineffective due to resistance to the boscalid active ingredient.

The organic products 97% paraffinic oil (JMS Stylet Oil was used) and 70% neem oil (Triple Action Neem Oil was used) did not control powdery mildew at all compared to the non-treated control. In a previous test, paraffinic oil also provided inconsistent control of cucurbit powdery mildew in the greenhouse. To manage powdery mildew on organic cucurbits, Organocide (92% fish oil + 8% sesame oil) or sulfur are better organic-approved biopesticides than other oils.