Moisture helps in the infection process. In a single season, one infected seedling could spread the infection to many surrounding plants. Thus, a canola crop planted from diseased seed could develop scattered areas of infection all over the field. See picture below of an infected seedling. Another kind of spore is present in infected stubble when the disease has been around for some time.
Ascospores are very common in heavily infested regions.
Infected canola stubble discharges thousands of these spores into the air from May until October. The spores are wind-borne and can travel several kilometers in the air before landing. If they land on canola or volunteer canola plants in nearby fields, the spores can begin new infections and continue the disease cycle. Infected stubble will produce ascospores for three to five years, with peak production normally occurring in the second year after infection. Ascospore production usually drops to low levels in subsequent years and stops after five years unless soil conditions are extremely dry.