Why Does My Broccoli Have White Powder on the Leaves?

A white or grey floury coating on the surface of broccoli or cauliflower leaves is powdery mildew — a fungal disease that is common in warm, dry summers and early autumn. It is not usually fatal to established brassicas, but it weakens plants, reduces yield quality and indicates growing conditions that favour disease over the plant. On cauliflower, severe mildew near harvest time can spread to the curd itself.

What causes powdery mildew on brassicas

Powdery mildew is caused by several fungal species and thrives specifically in warm days combined with cool nights and low soil moisture — conditions where the air is humid but the soil is relatively dry. This is why it peaks in late summer and early autumn. Plants crowded together with poor air movement between them are consistently worst affected. Unlike downy mildew, powdery mildew sits on the leaf surface and does not require a wet leaf to spread.

Is the crop still safe to eat?

Broccoli and cauliflower affected by powdery mildew are safe to eat. The florets and heads are unaffected if the mildew is confined to the outer leaves. Remove and bin heavily affected leaves rather than composting them. If the mildew has spread to the curd or head, harvest immediately, cut away any affected parts and use the rest — it will not keep well.

Slowing the spread

Remove affected leaves as soon as they appear. Improve air circulation by ensuring plants are not touching each other. Water at soil level rather than overhead, and water in the morning so the foliage dries before evening. A bicarbonate of soda spray (one teaspoon per litre with a drop of washing-up liquid) can slow spread by altering leaf surface pH. Potassium bicarbonate-based products are more effective and food-safe.

Prevention next season

The most effective prevention is correct spacing — at least 60cm between plants for broccoli, 75cm for cauliflower. Avoid late-summer nitrogen applications that promote the soft, susceptible growth mildew colonises most readily. Clear all brassica debris at the end of the season and rotate crops to reduce the pathogen load in the soil.