Canola (B. napus L.), one of the most valuable oilseed crops in the world, has been reported with significant crop losses up to 100% due to blackleg disease caused by Leptosphaeria maculans. Genetic resistance is a primary method to control blackleg, and the highest levels of resistance can be introduced from Brassica species containing the B-genome through interspecific hybridization. With successful introgression of resistance, the BC3 recombinant lines, derived between resistant B. juncea UM lines and susceptible B. napus L. cv. Westar, showed high levels of resistance to two L. maculans isolates 03-15-03 and PG4-1M at the seedling stage. In the analysis of blackleg resistance in synthetic hexaploid Brassica lines using susceptible B. juncea UM3086, the resistance against L. maculans isolate 03-15-03 in F2, BC1, BC2 and BC2F2 populations indicated that two genes are inherited in backcross populations and each gene can confer the same level of blackleg resistance. The high number of resistant phenotypes and compatibility of interspecific crosses in hexaploids crossed with tetraploid Brassica imply that synthetic lines are a feasible tool for developing blackleg resistance.