Keeping Ahead of Powdery Mildew - Garden Pest Tip

by Arzeena Hamir

Powdery mildew is a common disease on many types of plants. In fact, it is one of the oldest plant diseases on record - Theophrastis wrote of powdery mildew on roses in 300 B.C. Although different species of fungi cause the disease on different plants (Erysiphe infects vegetable crops and flowers; Podosphaera species infects apples and stone fruits; Sphaerotheca species infects berries, roses, some vegetable crops, and stone fruits; and Uncinula necator infects grapes), the infections are all characterized by a powdery white to gray fungal growth on leaves, stems and heads.

Contary to popular belief, powdery mildew generally does not require free water to establish and grow. Infection can actually occur on dry leaves. Warm temperatures and shady conditions encourage the fungus to grow and spread. However, the spores and mycelium are sensitive to extreme heat and direct sunlight.

Symptoms
Powdery mildew usually shows up on leaf and stem surfaces and does not directly affect most vegetable fruits. However, it can affect the flavor of melons and squash and reduce their yield. Woody species such as grapes, fruit trees, roses, crape myrtle, and sycamore are more seriously affected; new growth is often distorted. The young fruit of apples and grapes can also develop rough skin due to powdery mildew.