Vegetables, Berries, and Grapes: Powdery Mildew

Problem
The upper surfaces of the leaves and sometimes the stems are covered with a white powdery growth. Areas of the leaves turn brown and dry. Older leaves are affected first, progressing to younger leaves. Leaves may become cupped, showing a silvery underside. Fruit may also be covered with the white powdery growth.

Analysis
This common plant disease is caused by fungi that thrive in both humid and dry weather. The powdery patches consist of fungal strands and spores. The spores are spread by the wind to healthy plants. The fungus saps the plant nutrients, causing yellowing and sometimes death of the leaf. Fruit yield may also be reduced. A severe infection may kill the plant. Since powdery mildews attack many different kinds of plants, the fungus from a diseased plant may infect other types of plants in the garden. For a list of powdery mildews and the plants they attack, see Powdery Mildews and Some of the Plants They Infect. Under favorable conditions, powdery mildew can spread through a closely spaced planting in a matter of days or weeks.

Solution
Control powdery mildew on vegetable and berry plants with a fungicide containing chlorothalonil at the first sign of the disease. Continue spraying at intervals of 7 days as long as the disease is a problem. Make sure that your plant is listed on the label. Clean up and destroy plant debris after harvest. When available, grow varieties that are resistant to powdery mildews.