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 weather this past week, powdery mildew ought to be thriving soon. We discussed powdery mildew briefly in issue no. 10 of this newsletter in the article on dogwood powdery mildew. Powdery mildew is a common fungal disease problem on many perennials as well as annuals, shrubs, and even trees and turf. The most common hosts in Illinois seem to be lilac, zinnia, phlox, and rose; but certainly, other species are affected. 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 of this disease in Illinois. Still, on one plant, the disease may spread very quickly, especially in humid weather. Despite the fact that this disease does not kill plants, if your zinnias, roses, or other plants are infected, that may be a major aesthetic concern to you.|
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 very 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 in the planting. This may mean that plants need to be thinned or pruned to allow better air movement. 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 are the easiest means of disease control. If you have had problems with powdery mildew in the past, look through garden catalogues for varieties with resistance to this disease pathogen. Ask for similar information at nurseries. The Internet is also a good reference for such information. Try searching under the host name and the disease name. For example, the Chicago Botanic Garden has a list of 10 Monarda cultivars resistant to powdery mildew.
Fungicides can be used to control powdery mildew. Scout for the appearance of the disease, and then treat the plants according to label directions. Often damage is minor, and this is not necessary; but watch plants that have had a history of problems. Consult the 2001 Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide for a list of registered fungicides by host and by disease. These manuals are available in your local Extension offices or by calling (800)345-6087. Report on Plant Disease no. 617, “Powdery Mildews of Ornamentals” is available at http://www.ag.uiuc.edu/~vista/horticul.htm or in Extension offices and provides detailed information about powdery mildew.