Why Is There White Powder on My Cabbage Leaves?

A white or off-white powdery coating on the upper surface of cabbage leaves — spreading in patches that can eventually cover much of the leaf — is powdery mildew, caused by Erysiphe cruciferarum (the powdery mildew specific to brassicas). Unlike downy mildew, which appears as grey fuzz on the leaf underside in cool, wet conditions, brassica powdery mildew grows on the upper leaf surface and thrives in warm, dry conditions with moderate air humidity. It is typically a late-season disease, most common from August onward in dry years.

Powdery mildew versus downy mildew

The two diseases are often confused but are clearly distinguishable. Powdery mildew: white powdery growth on the upper leaf surface, dry weather, warm conditions, no corresponding grey growth underneath. Downy mildew: yellow patches on the upper surface with corresponding grey-purple fuzzy growth on the leaf underside, cool wet weather, particularly in spring and autumn. They are caused by completely different organisms with different biology and respond to different management approaches.

Why powdery mildew appears

Erysiphe cruciferarum spores germinate and spread in warm, relatively dry conditions — unlike most fungal diseases, it does not need free moisture on the leaf surface. High air humidity combined with warm temperatures and dry soil is the classic trigger. Plants that are drought-stressed (inconsistent watering, competitive from weeds) are significantly more susceptible. Crowded plants with poor air movement allow higher local air humidity around the leaves, favouring infection.

Management

Water plants consistently to avoid drought stress. Improve plant spacing for better air circulation. Remove and dispose of heavily mildewed leaves — this reduces the spore source and slows spread. Potassium bicarbonate solution (one teaspoon per litre of water) applied as a foliar spray creates an alkaline surface that kills spores — safe and effective for organic growing. The disease is primarily cosmetic on outer cabbage leaves and rarely threatens the head quality.