3.3.1. Blackleg Resistance

Blackleg disease, caused by *Leptosphaeria maculans,* is still the major constraint for the main canola producers including Australia, Canada, Europe, and the United States, despite blackleg resistant cultivars being adopted roughly 30 years ago and undergoing continuous improvement [27,110,111, 116–119]. The host-pathogen interaction during the disease process has been clearly understood based on pathotyping and molecular techniques (PCR, nucleotide sequencing, and gene technology) in which qualitative (major genes) and quantitative (multiple genes) resistance is triggered [120,121]. Molecular and gene technologies allied with fast development of bioinformatics has accelerated the identifications of resistance genes (*R*) in the host plants and avirulence genes (*Avr*) in *L. maculans*. At least 19 major *R* genes against *L. maculans* have been genetically mapped in *B. juncea*, *B. rapa*, and *B. napus* [119,120], with 3 genes cloned [120,122]. By contrast, nine of the 16 *Avr* genes identified in *L. maculans* have been cloned to date [123]. Reliance on a single *R* gene and little or no rotation with less susceptible crops leads to rapid evolution of *L. maculans* populations and resistance breakdown [119,120]. Using a GWAS approach, Fu et al. [116] identified 32 and 13 SNPs from Canada and Chinese accessions tightly linked with blackleg resistance which were distributed on chromosomes A03, A05, A08, A09, C01, C04, C05, and C07, in which potential SNPs located on A8 associated with resistance to 12 *L. maculans* isolates and 25 RGAs were identified within this region. Analysis of quantitative resistance (QR) to *L. maculans* in 177 double haploid lines under three experimental conditions using DArTseq markers revealed 3 SNP associations for QR on chromosomes A3, A4, and A7 [117]. New strategies for identification of genes relating to blackleg resistance is quite promising as providing an efficient way to obtain and evaluate new *R* genes in accordance with race structure of *L. maculans*, which is crucial for development of new rapeseed varieties [110,111].
