Infection of canola pods by Leptosphaeria maculans and subsequent seed contamination

Abstract

Blackleg disease caused by the pathogen Leptosphaeria maculans is the most devastating disease of canola (Brassica napus, oilseed rape). This disease occurs in all canola-growing regions of the world except China. It has been suggested that L. maculans contaminated seed and admixture (associated plant debris) could lead to a blackleg epidemic in China and as a consequence, restrictions on importations into China were placed on canola seed from Australia and Canada in 2009. We show that canola pods infected by L. maculans can lead to seed contamination, and resultant seedling infection, which then leads to cankering in adult plants. The fungus can sexually reproduce over summer on stubble derived from these plants. Airborne sexual spores are then released in the following year– thus completing the life cycle of the fungus from a contaminated seed and providing a potential source for an epidemic, particularly in countries such as China where canola cultivars do not have high levels of resistance to L. maculans. Furthermore, sexual fruiting bodies can also be produced on admixture. The presence of blackleg lesions on canola pods correlated with seed contamination by the blackleg fungus. Viability of L. maculans is reduced on contaminated seed over a twelve month period. Surveying blackleg disease in field trials in Australia showed that the presence and degree of stem cankers did not correlate with the level of pod infection. This suggests that pod lesions are likely to arise as a result of new infection events, rather than the pathogen moving from pre-existing infections (stem cankers) onto the pods. Furthermore, pod infections are likely to be a result of seasonal conditions rather than specific to regions where canola is cropped at a high intensity.