Fungicide Sensitivity of Cucurbit Powdery Mildew in Charleston, SC

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Clemson Plant Pathologist Tony Keinath believes Gatten, Vivando, Quintec and Procure fungicides provide the best management options for cucurbit producers managing powdery mildew.

Photo courtesy of South Carolina Grower: Small colonies of powdery mildew (orange circles and others not marked) on a leaf treated with water before exposure to powdery mildew spores produced on naturally infected plants. (The yellow spots are thrips damage, which was not part of the experiment.)

According to the The South Carolina Grower, a fungicide sensitivity bioassay was conducted between June 1 and June 10 to determine which fungicides can be used against the current outbreak of powdery mildew on squash, watermelon and other cucurbits.

All leaves treated with Gatten, Vivando, Quintec or Procure showed no powdery mildew. 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, South Carolina, is still considered sensitive. Torac is normally used as an insecticide, but this pesticide is also 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.

Source: The South Carolina Grower