Powdery mildewIDENTIFICATION – In mid- to late summer, you may notice a gray-white film on a plant, and the plant may begin to lose leaves. The culprit is powdery mildew. You’re most likely to see this problem on roses, phlox, zinnias, bee balm and lilacs (in the illustration).
This condition won’t kill the plant, but it’s not very attractive. Symptoms look the same, but specific fungi cause powdery mildew on different plants. In other words, the fungus that causes these symptoms on roses won’t cause the same disease on zinnias.
FAVORITE CONDITIONS – Humid weather, with warm days and cool nights, encourages powdery mildew.
CONTROL – As with many plant diseases, the easiest way to control powdery mildew is to plant resistant cultivars. But there are a few things you can do to help your existing plants. The spores survive the winter in leaves and plant debris, so clean up around infected plants to get rid of as many spores as possible. Make sure your plants have enough sun and good air circulation, as this will cut down on powdery mildew.
Fungicides are very effective for preventing this fungus. Read the label to make sure the fungicide controls powdery mildew, not downy mildew, a different disease. You’ll probably need to apply it more than once, starting before you see symptoms. You can’t “cure” powdery mildew — prevention is key. Spraying horticultural oil on plants until the leaves are covered, top and bottom, cuts down on the problem. Test this on a few leaves first, to make sure the leaves don’t burn. Wait for a cloudy day to spray anything on your plants — the combination of sun and sprays can damage foliage.