Black spot of cabbage [Alternaria brassicicola]
All cruciferous plants, oilseed rape, brassica vegetables,and cruciferous weed.
Infection in the fall comes from infected plant residues from previous crops, or microsclerotia in soil or in seeds. Primary infection comes from conidiospores (asexual reproductive spore) which are spread with the wind as well as infected harvest residues. The newly formed conidiospores on the leaves may initiate an epidemic in the following spring. For this to occur, temperatures of between 17 and 25°C, and a period of rain, or humidity of 95-100% is required for at least 3 consecutive days. The fungus infects the plant through the stoma or penetrates the wax layer directly.
Pattern of damage
Small (0.5–1 mm), brown, round spots can be found on the cotyledons. Leaves have angular, black necroses which frequently have yellow edges. On older leaves, larger round, sharply delineated spots are also formed (2–15 mm),. The infected sites alternately show light and dark, annular zones. Infected sites on main and secondary shoots, as well as branches of the inflorescences and fruit, are longitudinal (1–5 mm), sharply delineated black or light-gray spots with black outlines (Picture 1). This typical damage can be seen onthe pods of oilseed rape (Picture 2). They have many round, small (0.5-3 mm), black or light-gray (with black edge) necroses. In very humid weather, a dark-gray fungal lawn may develop.
New crops should be sown with the largest possible distance to recently harvested potential sources of infections. Control of cruciferous weeds
Good control is achieved with oilseed rape fungicides. Treatment of inflorescence against white mold provides added control against Alternaria in the ripening phase