Powdery Mildew shows as a white powdery dusting on leaves, stems and buds of many flowers and shrubs in Minnesota. Common hosts include begonias, alpine current, bee balm, lilac, phlox, rose, and zinnia. The white or light gray powdery appearance over leaf and stem surfaces are caused by chains of fungal spores, which are rooted into the plant surface. The spores may rub off on your hand and may blow around in the wind. Since the mildew gets its nutrients from the plant, growth can be greatly reduced. Leaves may become puckered, deformed or have dead areas. However, only heavy infections are generally life threatening to a plant.

Both temperature and humidity are important factors in the development of powdery mildew. It is most common during periods of cool temperatures with high humidity. These conditions occur when plants are grown in crowded, damp, or shaded locations, where air circulation is poor, or when cool night temperatures follow warm days. So, in Minnesota, we see higher levels of powdery mildew in the spring and fall.

Since high humidity is an important factor, certain cultural practices can help prevent the disease, or decrease its severity. Increase air circulation and light penetration, by pruning and thinning trees and shrubs. When planting new materials, choose those which have resistance to powdery mildew, and space them far enough apart to allow good air circulation and adequate light. Remove and dispose of infected leaves in the fall to prevent overwintering and prune out highly infected branches of rose or lilac.

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