We seem to be moving out of the high-moisture, root rot weather and into the hot, dry conditions that promote powdery mildew. Unlike most fungal diseases, powdery mildew is not as destructive when rains are frequent. The six common genera of powdery mildew fungi in the Midwest all prefer warm, humid days. The spores germinate on foliage when the relative humidity is 23% to 99% but not in free moisture (rain). Based on this week's weather, powdery mildew ought to be thriving soon.
Powdery mildew is a common fungal disease on many perennials, as well as on annuals, shrubs, and even trees and turf. The most common hosts in Illinois seem to be lilac, zinnia, phlox, and rose. There are many different types of powdery mildew fungi; and most are very host specific. For that reason, we will probably never see an epidemic in Illinois. Still, on one plant, the disease may spread quickly, especially in humid weather. Although this disease does not kill plants, if your zinnias, roses, or other plants are infected, that may be a major aesthetic concern. It can do a number on dogwood trees, too. Most other powdery mildews in our landscape cause symptoms in mid-July, but the one on dogwood is active all summer. We see symptoms starting much earlier on this species; watch for it now.
Symptoms of powdery mildew include a white mildew type of growth on the leaves, shoots, buds, flowers, or stems. This mildew is composed of threadlike mycelium and asexual spores of the fungus. The spores can be blown to other plant parts and cause further infection. New growth is particularly sensitive. The disease is obvious and often unsightly. Occasionally, infected foliage exhibits a purple cast rather than a white color as is true of infected apple or crabapple foliage.
To avoid problems with powdery mildew, provide conditions for adequate air flow. Plants may need to be thinned or pruned. Use recommended mature plant spacings when establishing new plants. Because the pathogen thrives in humid conditions, water the plants early in the day to promote rapid drying. Avoid syringing foliage, and try to water the soil rather than the foliage.
Resistant varieties--the easiest means of disease control--are not always available. Fungicides are available; and if sprays are begun at the first sign of mildew, control can be attained. Scout for the disease, and treat according to label directions. Often damage is minor, and treatment is not necessary; but watch plants with a history of problems, such as flowering dogwood. Consult the Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, & Garden Pest Guide for a list of registered fungicides by host and by disease. These manuals are available in your local Extension offices. Report on Plant Disease (RPD), no. 617, "Powdery Mildews of Ornamentals," is available on the Extension Web site http://www.ag.uiuc.edu/~vista/horticul.htm or in Extension offices and provides detailed information about powdery mildew.