Alternaria brassicicola is a fungus which belongs to the family Pleosporaceae. The species has branched hyphae which are between 1.5 and 7.5 micrometre wide. In the beginning the hyphae are transparent but later their colour turns to brown or olive brown. The conidium are between the 5 and 8 micrometre wide and can be up to 70 micrometre long.