Text adapted from Turfgrass Pest Management: A Guide to Major Turfgrass Pests in Oklahoma. Oklahoma Cooperative Extension Service.

CAUSE: Powdery mildew is caused by fungus Erysiphe araminis, which is an obligate parasite. This means the fungus can only complete its life cycle in association with a living host. When the host plant dies, the fungus normally dies as well.

HOSTS: Kentucky bluegrass.

SYMPTOMS: A white to gray powdery fungal growth develops on infected leaves (Figure 1). Heavily infected leaves turn yellow and die slowly, resulting in weakened plants that may be killed by environmental stress.

DISEASE CYCLE: The fungus overwinters on living, infected plants. Spores are produced in spring and become airborne. Spores that land on a susceptible plant germinate quickly and infect the host (Figure 2). New spores can be produced within one week on newly infected leaves. The disease can, therefore, spread very rapidly. The spores live for only a few days, but they can infect susceptible grass leaves over a wide temperature range, and fungal growth and disease development does not require a film of free moisture on the leaf surface.

FACTORS FAVORING DISEASE DEVELOPMENT: Powdery mildew is generally observed during the spring and autumn, because spore production and infection of leaves are favored during cool, humid, cloudy conditions. The disease is usually more severe in shaded areas with poor air circulation, and especially on turfgrass that has been fertilized heavily with nitrogen.

CONTROL: The best strategy for controlling powdery mildew is to include shade-tolerant grass cultivars in the seed mixture when the turf is established. Additional measures include selectively pruning or placement of ornamental shrubs and trees to allow good penetration of sunlight to the turfgrass canopy. This will also help reduce the humidity over the turfgrass by increasing the air circulation. In areas where the disease is particularly severe, fungicides or alternative shade-adapted ground covers may be required.

Maintain mowing height above two inches in mildew prone areas to enhance rooting and provide greater leaf surface for energy production. Water as needed to avoid drought, and fertilize to promote vigorous, but not lush, growth.

Figure 2. -- "Schematic of how fungal spores germinate and infect leaves of turfgrass", Courtesy of C.S.S.A.