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Turfgrass Diseases: Powdery Mildew (Causal Fungus: Blumeria Graminis - Formerly Erysiphe Graminis)

This disease is almost always associated with turf in shaded areas and is most common on Kentucky bluegrass.

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Updated:

February 26, 2024

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Photo: Pete Landschoot, Penn State

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Powdery mildew is a foliar disease of lawns and parks caused by the fungus Blumeria graminis. Powdery mildew generally appears in late summer and fall and can weaken grass plants; however, the disease rarely kills turf.

Powdery mildew symptoms on Kentucky bluegass — Figure 1. Powdery mildew symptoms and signs on Kentucky bluegrass. Photo: Peter Landschoot, Penn State

Symptoms and signs

The fungal pathogen of powdery mildew, B. graminis, first appears as isolated wefts of fine, gray to white spore masses (conidia) and hyphal growth on the upper surface of grass leaves. The fungal growth eventually becomes dense and may cover the entire leaf, giving it a gray-white appearance. In severe outbreaks, entire sections of the turf stand may be dull white. Portions of older leaves that have been infected turn yellow, but plants rarely die. Occasionally, tiny dark brown or black structures called cleistothecia can be observed within the white powdery growth on leaf surfaces. These structures are the sexual stage of the fungal pathogen and contain ascospores.

Powdery mildew on Kentucky bluegrass — Figure 2. Leaves of Kentucky bluegrass affected with powdery mildew appear as though they are covered with flour or white powder. Photo: Peter Landschoot, Penn State

microscope photo of spore chains on turfgrass — Figure 3. Microscope photo of spore (conidia) chains of *B. graminis* on turfgrass leaf surface. Massive amounts of spore chains give the appearance of a white powder on leaf surfaces. Photo: Peter Landschoot, Penn State

Disease cycle

The causal fungus of powdery mildew overwinters as mycelium or spores in dead grass and in infected living grass plants. Spores of the fungus spread by wind, mowing, or foot traffic to leaves of other turfgrass plants and incite new infections. Infections are superficial and the fungus draws nutrition from leaf cells without destroying stem, crown, and root tissues.

Disease development

Conditions favorable for powdery mildew development include poor air circulation, high atmospheric humidity (but no free water on leaf surfaces), low light intensity or shade, and cool air temperatures. Kentucky bluegrass, when planted in shaded areas, is particularly susceptible to this disease.

Cultural control

Where occurrence of powdery mildew is frequent, changing landscape plantings to improve air circulation and reduce turf shading will aid in disease reduction. Prune overstory trees to allow sunlight to reach the turfgrass. Turfgrass species differ in their susceptibility to powdery mildew. Kentucky bluegrass, for example, is quite susceptible to this disease, whereas the fine fescues and tall fescue are quite resistant.