Q What’s the cause of ringed spots on brassica leaves?

A Large spots of concentric rings on brassica leaves, particularly older ones, indicates ring spot, caused by a fungus (Mycosphaerella brassicicola). It causes circular brown-purple patches up to 2.5cm wide on leaves and stems. The patch is sprinkled with tiny black dots arranged in concentric rings. These are the source of the spores of the disease. Eventually, the whole leaf yellows, withers and is shed.

Ring spot disease is favoured by mild, wet conditions, and although widespread, does its worst damage in the south-west. Brussels sprout buttons are severely affected.

Caption: Ring spot tends to occur in mild, wet conditions.

Q What should I do about ring spot on brassicas?

A Burn or bin badly affected plants.

Q Can ring spot be avoided n brassicas?

A Avoid excessive manuring as lush plants are more susceptible. If plants are too lush or the site is very fertile, add 30g a sq m of sulphate of potash to promote stronger, more disease-resistant growth. Don’t grow cabbages in the same spot each year as the disease can persist. 'Roscoff'-type cauliflowers have some resistance, but are tender so can only be grown near the coast or on protected sites.

Destroy badly affected plants and rotate your crops. Seedlings should not be raised in infected soil.