Powdery mildew arrived this week on squash, pumpkins and other cucurbits throughout Ohio. It is a little late – we often see it by early- to mid-July. The fungus that causes cucurbit powdery mildew does not overwinter in Ohio, so the disease does not appear until spores arrive on wind currents from warmer growing areas. This fungus is an unusual plant pathogen in that it is inhibited by free water – so frequent rains may delay powdery mildew’s appearance, at least to a notable level. Signs of infection are small circular powdery growths (mycelium and spores of the pathogen) 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.
Powdery mildew is managed using powdery mildew-resistant varieties and fungicides. Development of insensitivity to overused fungicides is common in populations of the fungus that causes this disease, so it is important that a fungicide resistance management program is followed. Remember to alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. It is important to apply fungicides when the disease first appears and incidence is low. Fungicides that are effective against cucurbit powdery mildew can be found in the Midwest Vegetable Production Guide for Commercial Growers; product ratings and FRAC codes are on page 125. Our evaluations of efficacy of powdery mildew fungicides at three locations (Wooster, Columbus, South Charleston) in Ohio in 2017 indicated that Procure, Quintec, and Rally consistently provided very good control of powdery mildew on pumpkins in all three locations (see table). Approvia Top and Inspire Super were very good in two locations but fair in a third; and Merivon Xemium, Fontelis and Torino were very good in one location and fair in two. Both Bravo and Pristine performed poorly in all three locations.