RS, caused by the fungus Mycosphaerella brassicicola, occurs sporadically from season to season.
It produces dark spots scattered across the leaves with characteristic rings of dark fruiting bodies. Lesion margins are yellow, and leaf yellowing and early leaf drop occurs as RS progresses. RS can easily be confused with Alternaria, and needs microscopic examination to differentiate.
Can be important in some seasons on some crops.
RS overwinters in crop debris, on weeds or volunteer brassicas. In spring spores are produced, infecting leaves and seed pods.
Prevention and management
Use certified disease free seed to ensure no new source of infection, while good cultivation and crop rotations ensure low levels of crop debris and weeds for RS to over-winter on. Early grazing of infected plants limits the spread of RS.