They are also rain-splashed to nearby plants, causing secondary infections. On seedlings, the fungus can grow down through the cotyledon and leaf petioles and directly penetrate the stem base, causing cankers - see Figure 8 . Typical severe blackleg crown canker Prolonged moist weather favours rapid spread and development of the disease. If seed infection is the only source of inoculum the disease incidence in the crop will normally be relatively low. The high incidence of infected residues has made seed infection of little importance in disease epidemics occurring in Western Australia.
Susceptible varieties can be a complete loss.
All canola residues of any age are a source of inoculum and risk to subsequent crops. The document Managing blackleg in 2001 provides a full description of ways to manage blackleg in each rainfall and disease risk zone, supplementing the brief description below. Only sow varieties with very high levels of resistance, and practice crop rotation. All varieties recommended for sowing in Western Australia have good adult plant resistance to blackleg. However, since all varieties currently grown are susceptible at the seedling stage, there is still a need to maintain good management and hygiene practices to keep blackleg disease to a minimum, even when using resistant varieties with good adult plant resistance.