Powdery mildew is the name associated with diseases seen on many hosts. Symptoms include a powdery white growth on the surface of leaves, stems, and other plant parts. There are many powdery mildew fungi. Most that attack landscape plants thrive in hot, humid conditions. Recently, however, you may have seen a powdery mildew that has been enjoying your turfgrass; and we have experienced typical Illinois spring weather. This powdery mildew is not typical of most powdery mildews in the landscape.
Powdery mildew of turf is caused by a fungus called Erysiphe graminis. It is most common on Kentucky bluegrass, bermudagrass, redtop, fine-leaved fescues, and zoysiagrass. Look for the disease in shady areas of turf where air circulation is minimal. The fungus thrives during cool (55 to 70 degrees F), humid, cloudy weather. The disease can spread rapidly as well. Under ideal conditions (for the fungus), the conidia (spores) may even germinate and produce infection within 2 hours of landing on a leaf.
Although we donít usually see powdery mildew as a major problem on turf, a severe attack may weaken and kill plants, especially in crowded, newly planted areas. The disease is most common in spring, late summer, and fall in Illinois. Mild, cloudy days followed by cool, damp nights favor development of powdery mildew on turf.
There are several nonchemical recommendations to help control this disease. Although it helps to have a strong stand of turf, excess nitrogen fertilization actually increases powdery mildew problems, so try to fertilize based on soil tests or at least avoid high-nitrogen fertilization. Cultural practices to increase the penetration of light, movement of air, and drying of turf also help. This may mean pruning surrounding plants, spacing new plants, and watering early in the day. It also helps to mow frequently and keep the turf at the recommended height. For bluegrass this is 1-1/2 to 2-1/2 inches. There is resistance reported to powdery mildew in some cultivars of Kentucky bluegrass, bermudagrass, and several species of fescues, but I could not find specific recommendations. At the very least, look for resistance information in your garden center and use shade-tolerant cultivars in shady sites. Some information on turfgrass cultivars can be found on the University of Illinois Extension turf Web site, http://www.turf.uiuc.edu/.
You will find fungicides that are effective against powdery mildew of turf listed in the Home, Yard, and Garden Pest Guide and the Commercial Landscape and Turfgrass Pest Management Handbook. Fungicides can be applied when the disease is first evident and will help keep the disease in check. Still, chemicals act only as a temporary stop to disease development, which is why we concentrate on nonchemical control measures. More information can be found in Report on Plant Disease, no. 406, available on the Extension VISTA Web site, http://www.ag.uiuc.edu/%7Evista/pubs.html.