Why Are My Brussels Sprout Leaves Turning Black at the Edges?

Brown-yellow or blackening lesions at the leaf margins of Brussels sprout plants, often with a distinctive V-shape pointing inward from the leaf edge, are the classic signs of black rot — a bacterial disease caused by Xanthomonas campestris. Inside the lesion, the leaf veins typically turn black or very dark brown. This vascular discolouration is the key diagnostic feature that distinguishes black rot from other leaf margin problems. Black rot can also affect the stems and sprouts in severe cases.

How black rot spreads

The black rot bacterium enters the plant through the natural leaf openings (hydathodes) at the leaf margins or through wounds. It then travels through the plant's vascular system, turning the veins black as it colonises them. Wet weather and rain splash spread the bacteria from plant to plant. Contaminated soil, infected seed, and infected crop debris in the soil are the main sources of infection. The disease spreads fastest in warm, wet conditions.

What to do with infected plants

Remove and destroy infected plants — do not compost them as the bacteria can survive in compost and infect subsequent crops. If the infection is partial, remove infected leaves and cut away any visibly discoloured stem tissue, though this is only appropriate for mild cases. Sanitise cutting tools with garden disinfectant or methylated spirits between plants to avoid spreading the bacteria mechanically.

Prevention through crop rotation

Black rot bacteria can persist in soil and plant debris for several seasons. Do not grow brassicas (cabbages, Brussels sprouts, broccoli, kale, cauliflower) in the same ground for at least three years after an outbreak of black rot. This rotation breaks the disease cycle and allows the bacterial population in the soil to decline to harmless levels. When buying plants or seeds, source from reputable suppliers with certified disease-free stock.

Distinguishing black rot from other problems

The V-shaped lesion with blackened veins is fairly distinctive. Tip burn — a physiological problem caused by calcium deficiency or inconsistent watering — also causes leaf margin browning but without the blackened veins. Caterpillar damage causes irregular holes and notches rather than edge lesions. If you are uncertain, cut a cross-section of an affected stem — blackened vascular tissue running through the stem is a strong confirmation of black rot.