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

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 <i>P. brassicae</i> 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 <i>P. brassicae</i>: 32 belonged to population one, two to population two, and two were classified as admixtures.