We are finding powdery mildew on squash and in high tunnel tomatoes. Powdery mildew is a common summertime problem of squash, pumpkins and other cucurbits, lately appearing in 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. 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 on page 108 in the Midwest Vegetable Production Guide for Commercial Growers; product ratings are on page 109.
We don’t often see powdery mildew of tomatoes in open fields in Ohio, but it can occur and be quite damaging in high tunnels and greenhouses. As with cucurbits, tomato powdery mildew must be managed proactively – by regular scouting and application of fungicides as soon as the disease appears. Signs are white powdery growth (fungus hyphae and spores) on leaves, and symptoms include leaf yellowing and necrosis. The fungicides sulfur, Inspire Super, Rally and Switch can be used on tomatoes in greenhouses and high tunnels for powdery mildew management. The Switch label prohibits use on small-fruited tomato varieties in the greenhouse/high tunnel.
