Pods with fungal lesions usually produce infected seed, which is often shrivelled and may not germinate. Invasion of the crown is the most damaging phase of the disease. This usually results from cotyledon and leaf infection at the seedling stage. Badly affected plants are ringbarked at ground level by cankers and lodge . Less severely affected plants remain standing, but pods fail to fill because their sap flow is restricted, and seed is pinched.
Susceptible varieties can be a complete loss.
Cause Blackleg is caused by the fungus Leptosphaeria maculans . The fungus survives in undecomposed infected crop residues for several years, so sowing into old canola paddocks can result in heavy losses. Stems colonised by the fungus constitute a disease risk for next season's crops planted within five kilometres, particularly those planted downwind. Ascospores are released from perithecia after rain in autumn and winter and are widely dispersed by wind to infect crops. Conidia are produced in the lesions and washed down the stem to infect the base of the stem.