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Abstract: The planting of clubroot resistant (CR) canola (Brassica napus) is the most effective method to manage clubroot, a soilborne disease caused by Plasmodiophora brassicae. In recent years, many P. brassicae isolates capable of overcoming resistance have been detected, often in mixtures with avirulent isolates. To improve understanding of the effect of low concentrations of virulent isolates on host resistance, three CR canola cultivars (‘45H29’, ‘L135C’ and ‘L241C’) were inoculated with pairs of isolates representing virulent/avirulent pathotypes (2*/2, 3*/3 and 5*/5) of P. brassicae, collected after or before the introduction of CR canola, respectively. Clubroot severity was significantly higher in all nine experimental treatments (low virulent + high avirulent) than in the negative control NC1 (high avirulent), and higher in seven of nine experimental treatments than in the negative control NC2 (low virulent). Disease severity was positively correlated with P. brassicae biomass in planta, as determined by quantitative PCR analysis 28 - 35 days after inoculation (dai). These results suggest that low concentrations of virulent isolates compromised the clubroot resistance in canola, facilitating infection by avirulent isolates. In a second study, the expression of 205 P. brassicae genes encoding putative secreted proteins was compared following inoculation of the canola ‘45H29’ with pathotypes 5I (avirulent) and 5X (virulent) of the pathogen. Sixteen of these genes were differentially expressed at 14 dai, and additional monitoring at multiple timepoints (7, 14 and 21 dai) indicated that, collectively, 12 of the 16 genes were upregulated in pathotype 5X. The relative expression of the same 16 genes was also compared in the interaction between the canola ‘Westar’ and pathotype 5I (virulent on ‘Westar’), with 12 of the genes showing similar expression patterns as in the ‘45H29’/5X interaction. Given their common expression patterns in two compatible interactions, it is possible that these genes play a role in clubroot pathogenesis. In a third and final study, clubroot resistance was introgressed from the European Clubroot Differential (ECD) 02 into the B. rapa accessions CR 2599 and CR 1505 (‘Emma’), and into the B. napus accession CR 1054 (‘Westar’). The distorted segregation ratios suggest that the two resistance genes are on different chromosomes and that two genes interact in an epistatic manner to confer resistance. Genotyping was conducted with 144 PCR-based markers in two of the three F2 populations. Linkage and QTL analysis with the polymorphic markers identified two QTLs on chromosome A03 associated with resistance to P. brassicae pathotypes 5X and 5G in Popl#1, whereas only the second QTL on chromosome A03 was associated with resistance to these pathotypes in Popl#2. The two QTLs clustered in genomic regions on the A03 chromosome of B. rapa where the CRa/CRbKato gene(s) are mapped. In addition, the Crr1 gene on the A08 chromosome of B. rapa was detected in the two populations. Therefore, the phenotypic and molecular data confirm the existence two CR genes in ECD 02.