Blackleg, caused by the fungal pathogen Leptosphaeria maculans (Desm.) Ces. & De Not, is an economically important disease in canola (Brassica napus L.) production. This study explored how temperature affects the Rlm gene- mediated resistance to L. maculans in canola. It also investigated the frequency of AvrLm genes and race structure of L. maculans in High Erucic Acid Rapeseed (HEAR) and canola fields in Manitoba, Canada. This study found that Rlm1, Rlm2, Rlm3 and Rlm4 in Topas isogenic lines (ILs) of B. napus are not sensitive to temperature, while Rlm3 in Quantum and Rlm4 in Falcon broke down at higher temperatures of 25/20 and 30/25 °C. The temperature had a significant effect on lesion size, where a higher temperature resulted in larger lesions in Topas ILs. Temperature also had a significant effect on L. maculans, where the optimum temperature was 25 °C for its growth and sporulation. Under different temperature treatments, different genotypes with the same Rlm genes responded differently to L. maculans. This may indicate that the genetic background is important in single Rlm gene-mediated resistance expression. This study also identified and examined 297 isolates of L. maculans from stubble collected in HEAR and canola fields in Manitoba in the 2013 and 2014 growing season. AvrLm6 and AvrLm5 were present in more than 90% of the population; AvrLm7, AvrLm4, AvrLm11, and AvrLm2 were present in more than 50% of isolates, while AvrLm3 and AvrLm9 had a very low frequency. In total, 85 races were identified, and AvrLm2-4-5-6-7-11 was identified in 14.8% of the population. AvrLm9 was not found in Manitoba in the 2014 growing season. The frequency of AvrLm genes and race structures varied over years, regions and cultivars.