Reports on Plant Diseases

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 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.

Disease Cycle

Powdery mildew fungi live chiefly on the outer surface of the host plant. They obtain food and water by means of small, branched, rootlike organs (haustoria) that penetrate the grass leaf or sheath and enter the surface layer or epidermal cells (Figure 2). The mildew appears powdery because it produces tremendous numbers of microscopic chains of spores (conidia). During cool (55 to 70 F; 13 to 21 C), humid cloudy weather, spores are continuously produced for 7 to 14 days until the host tissue dies. The conidia are easily carried by air currents to other grass plants in the same or neighboring turf areas, where they produce new infections and start the cycle once again. The conidia are capable of germinating and producing infections within 2 hours after landing on a leaf. New conidia are produced within a week after infection occurs.

As the fungus matures, the grayish white powdery growth forms dense mycelial mats. Occasionally, speck-sized, black, fungus-fruiting bodies (cleistothecia) develop in the mycelial mats during autumn. These bodies are especially evident on dead grass leaves. Sexual or overwintering spores (ascospores) are sometimes produced in the cleistothecia. Cleistothecia, however, are not common in turfgrasses. The powdery mildew fungus survives the winter as cleistothecia on dead plant tissues and as mycelial mats on living grass plants. The ascospores and/or conidia are released in early spring and produce the initial infections.