A second gene for resistance to four pathotypes of <i>Plasmodiophora brassicae</i> identified in the <i>Brassica napus</i> cultivar ‘Mendel’

Clubroot, caused by the obligate parasite <i>Plasmodiophora brassicae</i>, poses a significant threat to the Canadian canola industry, resulting in substantial yield losses. Genetic resistance is an effective strategy for managing the disease. In this study, 137 doubled haploid (DH) lines derived from the European oilseed rape cultivar 'Mendel' were phenotyped against <i>P. brassicae</i> pathotypes 3D, 5C, and 8J, which are aggressive toward the first-generation clubroot-resistant canola cultivars, and were genotyped using genotyping-by-sequencing. Disease severity indices in the population were highly correlated among the three pathotypes, with correlation coefficients of <i>r</i> ≥ 0.82. In the DH population, 2642 high-quality Single nucleotide polymorphisms were detected by employing the <i>Brassica napus</i> 'ZS11' reference genome. A single quantitative trait locus, <i>Rcr3</i>^Mendel, was detected and mapped to a 317 kb region on chromosome A08, flanked by the markers ZS_A08_15999175 and ZS_A08_16316110. Within this region, 43 genes were identified, including a single Toll interleukin-1 receptor nucleotide-binding site-leucine-rich repeat (TNL) gene, <i>BnaA08T0102200ZS</i>. This region is homologous to the 13 415 472-15 791 728 bp region of chromosome A08 in <i>Brassica rapa</i> ECD 04, the donor of clubroot resistance in 'Mendel'. Two TNL genes, <i>BraA08g039211E</i> and <i>BraA08g039212E</i>, were identified in the ECD 04 genome. Kompetitive allele-specific PCR analysis identified eight markers that co-segregated with <i>Rcr3</i>^Mendel. Resistance to pathotype 3H was also found to co-segregate with resistance to pathotypes 5C, 3D, and 8J.