Powdery mildew normally appears on pumpkins and other cucurbits in Ohio in early July. Signs of infection are small circular powdery growths on either side of the leaf. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves. Stems and leaf petioles are also susceptible, but the disease is not observed on fruit. In pumpkins, powdery mildew may also attack the “handles”, which can be further damaged by secondary pathogens. It is time to start scouting cucurbits for powdery mildew.
Powdery mildew is managed using disease-resistant varieties and fungicides. Pumpkin and squash varieties vary in resistance to powdery mildew; in general, the more susceptible the variety, the more fungicide needed. The choice of fungicide is important because insensitivity to overused fungicides is common. It is critical that a fungicide resistance management
program is followed. Alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. Fungicide applications should begin when the disease first appears and incidence is low. Fungicides that are labeled for use against cucurbit powdery mildew can be found in the Midwest Vegetable Production Guide for Commercial Growers; product ratings and FRAC codes are on page 11.
Our evaluations of efficacy of powdery mildew fungicides in Ohio in 2020 indicated that Aprovia Top, Inspire Super, Gatten, Vivando and Procure provided very good control of powdery mildew on pumpkins (see table below). Microthiol Disperss was not tested in 2020 but historically has provided good control. Quintec and Fontelis provided moderate (53-65%) control and Bravo Weather Stik, Merivon Xemium, Pristine, Torino and Rally provided poor control. The bioassay full report can be found here (pp. 15-16).
A list of products for powdery mildew management in organic cucurbits prepared by Dr. Meg McGrath of Cornell University can be found here.