Development of spring-type Brassica napus single-gene lines for differentiating Plasmodiophora brassicae races

Clubroot, caused by Plasmodiophora brassicae, is a major threat to Brassica crops, including canola (Brassica napus). We developed six spring-type B. napus single-gene lines (SGLs), each carrying a distinct clubroot resistance (CR) gene (Rcr1, Rcr3, Rcr5, Rcr8, Rcr9, or Rcr10), to classify pathogen races and support breeding strategies. CR genes from B. rapa ('Siloga', 'Purple Top White Globe', T19, ECD01) and B. napus ('Mendel') were introgressed into the susceptible line DH16516 through interspecific or intraspecific crosses, followed by backcrossing, marker-assisted selection, and microspore culture to produce doubled-haploid lines. The presence of Rcr1 was confirmed by gene cloning, while genome-wide marker analysis reduced the residual donor genome in the Rcr3 line to 32.8 %. The resulting SGLs exhibited spring-type growth and were morphologically comparable to DH16516 under greenhouse and field conditions. Using these SGLs, 35 P. brassicae field strains from Western Canada were differentiated into 24 races, with avirulence allele frequencies ranging from 22.9 % to 57.1 %. None of the six CR genes conferred universal resistance, highlighting the need for gene stacking in cultivar development. Compared with existing differential sets, these spring-type SGLs provide more precise race profiling and better represent Canadian canola backgrounds. This resource establishes a robust platform for monitoring pathogen diversity and guiding the strategic deployment of CR genes for durable clubroot resistance.