Normally symptoms are more common on older leaves, but if younger leaves are affected the symptoms become more severe. Initially small dark spots of 3-5 mm, surrounded by a yellow halo are seen on the leaf surfaces. They are green-brown or grey-black and limited by the leaf veins and eventually become 2-3 cm big. Tiny dark dots form concentric rings inside the spots. Spots may coalesce causing yellowing of the leaf. Under severe infestation it may lead to premature defoliation. Spots caused by Ring spot fungus look similar to those caused by alternaria species. The main difference is that ring spot lesions are grey and they contain black, pinhead dots in concentric rings. All above ground portions of the plant may show symptoms. Individual lesions have dark concentric rings, from black fruiting bodies, with definite edges surrounded by a yellowish zone. Under severe attacks the spots coalesce, and the entire plant may be affected and blackened. In seed stalks, the fungus causes a distortion similar to the injury caused by the herbicide 2,4-D. Dark lesions may develop on stored cabbage and may penetrate deeply.
The damage is caused by the airborne pathogen Mycosphaerella brassicicola. The spores are spread by water splashes, wind and rain. The fungus needs a period of 100% relative humidity lasting at least four days for reproduction. Temperatures of 16-20°C and poor soil drainage can support the growth of the pathogen. The disease is a problem mostly with seed production, and seeds may carry the pathogen. The fungus also overwinters on infected weed and crop hosts or in residues. Spores are spread by wind. Cool moist weather favors disease development.
To this day, we are not aware of any biological control method available against this disease. If you know of any successful method to reduce the incidence or the gravity of the symptoms, please contact us.
Always consider an integrated approach with preventive measures together with biological treatments if available. Treat your seeds pre-planting with thiram or mancozeb. During cool and moist weather apply sprays of chlorothanonil, mancozeb or copper. It is a difficult disease to control where intense vegetable production is done due to large numbers of airborne spores where environmental conditions are cool and wet, favoring spread and infection.