Why Do My Cabbage Leaves Have Circular Ring-Shaped Spots?

Circular or roughly round spots on cabbage leaves — with a dark outer ring and a lighter, sometimes pale tan or grey centre — are characteristic of ring spot disease, caused by the fungus Mycosphaerella brassicicola (also sometimes called Mycosphaerella leaf spot). This is a common brassica disease in the UK, particularly in autumn and winter on crops left standing in wet conditions. The spots are distinctive in their concentric ring pattern, which develops as the lesion matures and the outer margin continues to expand while the centre necroses.

Identifying ring spot

Mycosphaerella ring spot produces circular spots that range from a few millimetres to over 1 cm in diameter. They have a clearly defined dark border with a pale grey or tan centre. As spots mature, tiny black fruiting bodies (pycnidia) develop in the central area, visible as pin-point black dots. Spots may merge in heavy infections to affect large areas of leaf. The spots are present on both upper and lower leaf surfaces. The disease is distinct from downy mildew (which does not produce circular spots) and black rot (which produces V-shaped lesions from the leaf edge).

When it develops

Ring spot is favoured by cool, wet, overcast conditions — typical of UK autumn and early winter. Plants standing in the field from October onward in wet years often develop ring spot on older outer leaves. It is most severe on older, non-vigorous crops that have been in the ground for a long time. Freshly transplanted plants and actively growing summer crops are rarely badly affected. The disease spreads by airborne conidia (spores) and through infected plant debris that overwinters.

Management

Remove heavily spotted outer leaves and dispose of them (not on the compost heap). Harvest the crop promptly when the head is mature rather than leaving it standing for weeks once conditions are wet and cool. Strict rotation — not growing brassicas in the same ground for three to four years — reduces the soil-borne inoculum. Clear all brassica debris after harvest. Improving plant spacing and airflow reduces disease severity in susceptible conditions.