Virulence of Plasmodiophora brassicae field isolates on canola ( Brassica napus ) carrying ‘second-generation’ clubroot resistance

Clubroot, caused by Plasmodiophora brassicae, is a major disease affecting canola (oilseed rape; Brassica napus L.) production in Canada. Management relies primarily on resistant cultivars, with first-generation resistance derived from the European cultivar ‘Mendel’. Although this resistance initially provided effective disease control, the emergence of resistance-breaking isolates prompted the development of second-generation clubroot-resistant cultivars incorporating alternative resistance sources. This study evaluated the occurrence, genetic populations, and virulence of P. brassicae field isolates recovered from second-generation clubroot-resistant canola cultivars in Alberta from 2019 to 2021. Clubroot symptoms were detected in 36 fields, with disease severities ranging from 0.1% to 61%. Thirty-six field isolates were genotyped using molecular markers that distinguish two genetically distinct Canadian populations of P. brassicae: 32 belonged to population one, two to population two, and two were classified as admixtures. Greenhouse assays assessed the virulence of these isolates on seven commercial second-generation resistant cultivars (coded SG1–SG7) and three differential controls. Cultivar responses varied widely: SG7 exhibited broad resistance to nearly all isolates, whereas SG2 and SG5 were highly susceptible, including to pathotypes controlled by first-generation resistance. Pathotype 3A isolates displayed particularly high virulence across most cultivars. Overall, the effectiveness of second-generation resistance varied with both the cultivar and virulence of local P. brassicae populations. These findings underscore the importance of resistance stewardship and informed cultivar deployment to sustain effective clubroot management in Canadian canola.