Why Do My Brussels Sprout Leaves Have a White Powdery Coating?

A white or greyish powdery coating on the upper surface of Brussels sprout leaves is the characteristic sign of powdery mildew — a fungal disease that is quite different in its biology and management from downy mildew despite the similar name. Powdery mildew on brassicas tends to appear in late summer or early autumn, particularly after a period of warm, dry weather followed by cooler nights. It is rarely fatal to established plants but weakens them and, in heavy infections, can affect the quality of the sprouts.

Identifying powdery mildew

Powdery mildew appears as white or pale grey, flour-like patches on the upper surface of the leaves, and occasionally on the stems and sprouts. Unlike downy mildew, the powdery growth is on the top of the leaf, not underneath. In severe infections the whole upper leaf surface may be covered, and the leaves may start to yellow and die back. The disease spreads by airborne spores which land on dry leaf surfaces.

Why powdery mildew affects stressed plants most

Plants that are water-stressed are significantly more vulnerable to powdery mildew. The connection seems counterintuitive — a disease that spreads in dry conditions, worse on dry plants — but this is consistent across many host species. Keeping Brussels sprout plants consistently watered, particularly during dry spells in late summer when mildew pressure is highest, is one of the most effective preventative measures. Equally, overcrowded plants with poor airflow are more susceptible than well-spaced ones.

Treatment and control

Remove the most heavily affected leaves and dispose of them. For chemical treatment, a dilute solution of sodium bicarbonate (baking soda) sprayed onto the foliage can slow the spread of powdery mildew. More effective are proprietary sulphur-based fungicide sprays, which can be applied preventatively in susceptible conditions. Potassium bicarbonate products are also available. None of these cures existing infection but they slow further spread. Improving watering and airflow are the most sustainable long-term measures.

Does powdery mildew affect the sprouts?

Powdery mildew primarily affects the leaves, and the sprouts themselves are usually unaffected unless the infection is very severe. Even heavily infected plants can still produce a worthwhile crop if the main stems and the developing sprouts remain healthy. Sprouts that show mildew symptoms directly are rare but can be trimmed of affected outer leaves. The inner sprout is usually fine.