Two Alternaria fungi, A. brassicae and A. brassicicola, cause these leaf spots on most brassica species.
A. brassicae causes light brown circular spots with large yellow margins on leaves in autumn and early winter, mainly in the South Island. A. brassicicola causes dark circular spots with narrower yellow margins on leaves in summer, mainly in the North Island.
ALS reduces leaf efficiency, decreasing crop yield and feed quality.
ALS spreads via wind-borne spores produced by lesions on crop residue or weeds. Wet, humid weather favours spore production. Infected seed can also introduce infection into a crop.
Prevention and management
Sowing clean seed minimises infection, along with good cultivation to work in all brassica crop residue. Grazing early forage that is becoming infected effectively contains the spread of the disease.