[Alternaria leaf blight, leaf spot, fruit rot,]
A. brassicae overwinters in and on the outside of seeds and on the crop residues of agricultural crops and weeds. The spores on the seed can cause early loss of the entire plant. Following primary infection, numerous new spores are formed and the fungus can disperse in splashes and via the wind. The spores cause blotches on the leaves, stems, flower stems and siliquas. The new seed is subsequently infected. The fungus can survive for many years on seed stored in dry, cool places.
A. brassicae causes dark spots on all parts of the plant above ground. The lesions are dark brown with a yellow halo. Concentric rings can be seen in the lesions, the oldest part of which sometimes falls out. Affected leaves can turn completely yellow and drop. In Brussels sprouts, blotches can appear on the sprouts.
The optimal temperature for germination is 21-28 °C. A leaf wetness period of three hours is required for successful infection at 20-25 °C (Mridha and Wheeler, 1993). Optimum conditions for sporulation are a relative humidity of over 91% and temperatures of between 24 and 28 °C (Humpeherson-Jones and Phelps, 1989).
- wide rotations;
- changing sowing dates to avoid conditions that are favourable to the fungus;
- removing or fully processing crop residues.