Blackleg, caused by the fungus Leptosphaeria maculans (Desm.) Ces. et de Not. [anamorph = Phoma lingam (Tode: Fr.) Desm.] has become the most important disease affecting canola around the world. A study was conducted to identify sources of resistance to L. maculans in a collection of B. napus plant introduction accessions. Approximately, 5% of accessions showed highly resistance (median severity <3) reaction to the L. maculans PG-4 under the greenhouse conditions and three of them performed better than commercial hybrids in the field conditions. At the same time, DNA extracted and genome-wide association study (GWAS) was done for 213 and 78 accessions for PG-4 and PG-3. The 0.1 and 0.01 percentile tails of an empirical distribution, obtained from 5,000 bootstraps, was used to determine the cut off P-value to identify significant markers. Finally, 10 and 26 significant single nucleotide polymorphism (SNP) markers associated with resistance to PG-4 and PG-3, respectively. These markers were located across 14 chromosomes (A01, A02, A03, A04, A05, A07, A08, A10, C03, C04, C05, C07, C08 and C09) out of B. napus 19 chromosomes. These markers were validated under field conditions. With exploring flanking region of each significant marker eight candidate genes were identified which involved in plant defense family such as defensing and leucine-rich repeat and serine-threonine protein kinase protein. To infer the presence of R genes in commercial canola hybrids, elite B. napus plant introduction materials, and elite canola breeding lines, they were inoculated with different L. maculans races. The results showed that, resistance gene Rlm9 was present in 18% of the genotypes evaluated; Rlm2 and Rlm3 were each present in 16% of them, while LepR1, Rlm4 and Rlm5 with present in 11, 5, and 5% each, respectively. However, we were not able to infer R gene(s) on 29% of the genotypes evaluated. Approximately 18% of genotypes were susceptible to all the races used. The hybrids with different R genes could use for hybrid rotation.