Powdery mildew develops primarily on Kentucky bluegrass in the shade. Powdery mildew is caused by the fungus Erysiphe graminis, which attacks a wide range of grasses including cereals. The disease occurs most commonly on Kentucky bluegrass, bermudagrass, redtop, fine-leaved fescues, and zoysiagrasses. A number of highly specialized physiologic races of the fungus are known. Many of these races are restricted to specific species of turfgrass or to certain cultivars within a species. The races that attack cereals do not attack bluegrasses and other turfgrass.

Powdery mildew has become an increasingly important disease of ‘Baron', ‘Cheri', Fylking', ‘Kenblue', ‘Merion', ‘South Dakota Certified', ‘Windsor', and other Kentucky bluegrasses in recent years. High-nitrogen fertilizers cause a dense growth of grass that creates an ideal environment for the mildew fungus. Resistance to powdery mildew is known to exist in several cultivars of Kentucky bluegrass, bermudagrass, and in several species of bluegrass and fescues.

The disease is much more severe where air circulation is reduced and the grass is growing in shaded areas (on north and east sides of buildings, under dense trees (maples) and shrubs). It attacks chiefly in the spring, late summer, and autumn when days are mild and cloudy and nights are cool and damp. Because the fungus significantly reduces the growth of leaves, roots, and rhizomes, powdery mildew is an important cause of the deterioration of bluegrass and zoysiagrass lawns in shaded areas. A severe attack may weaken and kill the plants, especially in crowded, newly planted areas. The surviving plants are more susceptible to winterkill, drought, and attack by other disease-causing organisms.

The disease rarely is responsible for any lasting damage to turf. Shade tolerant Kentucky bluegrass varieties tend to be less susceptible to powdery mildew. Over-seeding shaded areas with these varieties will reduce powdery mildew establishment and spread. Improving air circulation by careful pruning of trees and shrubs also will help limit mildew development (and will serve to suppress some mid-summer diseases). Avoiding excess levels of nitrogen in disease-prone areas also may contribute to a reduction in mildew outbreaks.

Chemical Control - Because the disease rarely is responsible for any lasting damage to turf, there are very few instances where the cost of fungicide application would be worthwhile. Powdery mildew may be managed with preventative fungicide applications. The key is to apply these materials in a preventative mode before the disease becomes established for these materials will only protect healthy and newly developing leaves. Leaves presenting symptoms will not be affected by fungicide applications. Acti-dione RZ, Banner and Bayleton are fungicides labeled for control of powdery mildew.