Disease

Powdery mildew

Erysiphales (Podosphaera, Golovinomyces, Erysiphe and others)

A fungus that grows on the leaf surface rather than inside it. Unlike most leaf diseases it needs no free water: warm days, cool nights, crowded canopies and shade favor it.

Signs

• White to gray powdery spots on either leaf surface, often the upper, that merge to cover the leaf.
• Badly infected leaves yellow and may drop.
• Often starts on lower leaves of vegetables and young leaves of trees and shrubs; worst in shade.

What favors it

Warm days, cool nights, moderate to high humidity without wet leaves, shade, and crowded canopies.

Prevention

• Grow resistant varieties.
• Space and prune for airflow and light into the canopy.
• Avoid heavy nitrogen, which grows soft, susceptible leaves.

Often confused with

Sources

• Powdery mildew in the flower gardenUniversity of Minnesota Extension
• Powdery MildewMontana State University Extension
• Powdery Mildew on VegetablesUniversity of Maryland Extension
• Powdery Mildew on VegetablesUC Statewide IPM Program

Why →