Brassica napus is a globally important oilseed crop. Leptosphaeria maculans is the causative agent of blackleg disease in Brassica napus. Two types of resistance mechanisms are known namely qualitative (vertical or R - mediated) and quantitative (horizontal) resistance. R-mediated resistance is well-studied and used extensively to manage L. maculans infection in B. napus. However, the genetic basis of quantitative resistance (QR) in this pathosystem is not fully understood but is apparently due to several multiple minor effect or a few major effect loci. Unlike minor effect loci that are difficult to dissect, major effect loci could be Mendelized to study the causative gene(s) controlling the QR trait in B. napus. Nevertheless, no QR genes have been cloned in this pathosystem, hence, the underlying mechanism is unknown. The work presented in this thesis focuses on the analysis of the major effect BLMR2 locus, which was originally derived from the highly resistant B. napus cultivar Surpass 400. In chapter three, the non-race specificity and intermediate resistance phenotype of the BLMR2 locus to multiple L. maculans isolates at seedling and adult plant stages was described. Chapter four focuses on the narrowing down of the BLMR2 region originally mapped on B. napus chromosome N10 using fine-mapping approaches. The alignment of the genetic and the physical map, based on the reference genomes of B. napus and Arabidopsis thaliana, facilitated the identification of the candidate disease resistance related cytochrome P450 gene. In chapter five, RNA sequencing (RNA-Seq) was employed to understand the mechanism underlying QR in B. napus and to confirm the differential expression of the identified candidate gene. By analyzing the global changes in gene expression, we indicated that the mechanism behind the R and QR strategies in B. napus – L. maculans pathosystem is comparable compared to the susceptible Westar cultivar. The findings in this thesis benefit the canola industry and the broader scientific community involved in canola, blackleg and resistance breeding by providing valuable information about a major effect horizontal resistance locus; the associated markers may be used in marker-assisted selection (MAS) to improve the canola crop.