In the South Carolina Grower, Clemson Plant Pathologist Tony Keinath provided vegetable growers results from a June 2025 bioassay with powdery mildew on susceptible yellow squash. All eight fungicides reduced the disease, though four worked better than the rest.
Torino, Quintec, Gatten and Vivando suppressed powdery mildew on potted bioassay plants exposed for 48 hours to spores coming from nearby infected squash plants. Bravo, Procure at the low rate, sulfur and Prolivo were less effective in suppressing the disease.
Gatten, Bravo and Procure were used at the lower labeled rate, with the 6-ounce rate of Gatten being completely effective. Procure must be used at the highest rate of 8 ounces per acre.
Bravo and sulfur did not control powdery mildew on the leaf surface.
The most effective fungicides in this bioassay were those with vapor, penetrant or curative activities. Fungicides with vapor action include an active ingredient that vaporizes after it lands on the surface of the leaf. Penetrant activity refers to fungicides that move through the leaf. Curative means the fungicides can kill powdery mildew for about 24 hours after the fungus has infected the leaf.
Prolivo has penetrant and curative activities, while Torino and Vivando have vapor, penetrant and curative activities.
Although resistance to Quintec and Torino has been found in the other parts of the country, the powdery mildew present in Charleston at the end of this year’s spring season was completely sensitive to these fungicides. Conventional growers should choose three of the four most effective fungicides and rotate them.
As a reminder, do not use Prolivo and Vivando on the same crop, since they both belong to FRAC Code 50. Prolivo must be mixed with an adjuvant for maximum efficacy.
Powdery mildew is a disease all cucurbit farmers need to be familiar with. Unlike other pathogens that need wet, rainy conditions to flourish, powdery mildew prefers a drier environment.