The rise in virulent blackleg incidence can be partly attributed to growers increasing plantings of canola, shortening crop rotations and growing more tolerant varieties, which select for more virulent strains. Blackleg is spread by infected seed or by spores splashed about by rain or carried by the wind in the growing crop. Long distance spread of the disease is through the movement of infected seed. See picture below of infected seed When blackleg-infected seed is sown, the seedlings that emerge may be infected with lesions on the seedling leaves or stems. These infections can quickly form spores that remain infectious for only a few hours and are spread by rain to nearby healthy seedlings to start new infections.
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.