Alternaria fungi are the most common causes of damping-off spreading through seeds and affecting, among others, cabbage and carrot cultivations. Damping-off means reduced germination and emergence of seedlings, and it also kills plants, which are past the seedling stage. The base of seedlings affected by this disease turn dark in colour and become watery. Ultimately, the plants collapse and die. With cabbage, damping-off is caused by Alternaria brassicicola, which is a powerful pathogen and can wipe out the emerging plants totally or (when occurring in a less severe form) cause deformity among the plants. 

Alternariafungi causing damping-off in cabbage seedlings. Left, a batch of

plants damaged by damping-off and on the right, a healthy batch of plants.