3.3.2. Clubroot Resistance

Clubroot, caused by the soil-borne biotrophic pathogen *Plasmodiopora brassicae*, has long been an important canola disease and has recently been increasing in Europe, China, India, Nepal, and Australia [124–129]. The yield losses by this disease can be around 60–90% when susceptible canola/rapeseed cultivars are planted in *P. brassicae* infested field [129]. Chemical treatment or sanitation practices for eradication of the pathogen from infested cultivating soil is impractical and impossible [113], which suggests utilization of resistant cultivars is the most effective and sustainable solution [125]. Clubroot resistance (CR) genes and gene loci contributing to the resistance in Brassica oilseed crops have been identified and mapped. Clubroot resistance has been discovered to be controlled by dominant loci in *B. rapa*, with two resistant genes being cloned, [130,131], 13 CR loci mapped on chromosomes A01, A02, A03, A06, A08 and the most recently A05 with CrrA5 [125,132,133]. At least 20 QTL in the C genome of *B. oleracea* [28,134] and more than 30 QTL in the A C genome of *B. napus* conferring CR have also been identified and mapped [134]. The CR gene (*Rcr6*) on chromosome B3 of *B. nigra* (BB, n = 8), was the first CR gene identified and mapped in the B-genome of Brassica species [132]. Utilization of modern genomic approaches e.g., GBS (genotyping by sequencing), GWAS, and RNA sequence analysis has facilitated identification of a number of CR candidate genes corresponding to specific race/pathotype of *P. brassicae* in *B. oleracea*, *B. rapa*, *B. napus*, which were summarized by Zhou et al. [135]. These valuable CR resources need to be further explored and precisely identified by molecular markers for breeding CR and dealing with resistance overcome by novel strains of *P. brassicae* [28,134,136]. By mapping RNA sequences of *B. napus* genotypes with different response to *P. brassicae* pathotype 5X to the *B. napus* reference genome (AST\_PRJEB5043\_v1), Galindo-González et al. [70] found differential expression in these genotypes whereby the standard defense-related proteins e.g., chitinases and thaumatins, and salicylic acid-mediated responses were up regulated in the resistant genotype coupled with mostly inhibited jasmonic acid-mediated responses. The study also identified major proteins relating to *P. brassicae* resistance and susceptibility e.g., calmodulin binding protein 60g (CBP60g), systemic-acquired resistant deficient 1 (SARD1), and xyloglucan endotransglucosylase/hydrolase (XTH).
