Why Do My Brussels Sprout Leaves Have Ringed Brown Spots?

Circular brown spots with concentric rings on Brussels sprout leaves — often with a tan or pale grey centre and a darker brown ring or halo around the edge — are characteristic of ring spot, a fungal leaf disease caused by Mycosphaerella brassicicola. Ring spot is one of the most common leaf diseases of brassicas in cool, wet climates and can cause significant defoliation in badly affected crops, though the sprouts themselves are rarely directly destroyed.

Identifying ring spot

The spots are typically 5–20 mm in diameter, circular or slightly irregular, with the concentric ringed pattern that gives the disease its name. The centre of older spots may develop tiny black fruiting bodies — the spore-producing structures of the fungus. Multiple spots often merge on heavily infected leaves, causing large areas of dead tissue. Lower, older leaves are usually affected first, with the disease spreading upward through the canopy in wet conditions.

Conditions that favour ring spot

Ring spot spores are released during wet weather and splashed onto leaves by rain. The fungus infects leaf tissue most readily in cool, moist conditions — the typical autumn weather of northern Europe when Brussels sprouts are maturing. Plants that have been growing in the same ground for several years, or that follow brassicas without a rotation gap, are at greater risk because spores persist in old crop debris in the soil.

Managing ring spot

Remove and dispose of heavily infected leaves as soon as they are noticed. Do not compost infected material — bin or burn it to avoid recycling spores back onto the plot. After harvest, clear all crop debris thoroughly and dig or fork the soil to bury any surface debris. Copper-based fungicides can be applied as preventative or early-curative sprays in susceptible conditions, though their effectiveness is limited once infection is established. Improving plant spacing to enhance airflow through the canopy reduces the humidity that favours spore germination.

Prevention through rotation

Crop rotation is the most effective long-term preventative. Avoid growing any brassica (Brussels sprouts, cabbage, broccoli, cauliflower, kale, turnip) in the same ground for three to four years. This deprives the ring spot fungus of hosts and allows the spore reservoir in the soil to decline naturally. When starting with clean ground and clean seed, ring spot infections are typically much milder than in long-established brassica beds.