Effect of canola (Brassica napus) cultivar rotation on Plasmodiophora brassicae pathotype composition

Abstract

In Canada, clubroot (Plasmodiophora brassicae) disease is managed mainly by planting clubroot resistant (CR) canola (Brassica napus). New pathotypes of P. brassicae have emerged recently, however, which are virulent on most CR canola cultivars. To understand the impact of cultivar rotation on pathotype abundance, greenhouse experiments were conducted in which different canola cultivar rotations were grown in a soil mix containing equal amounts of pathotypes 5X and 3, which are virulent and avirulent, respectively, on CR canola. The rotation treatments included: T1, the same susceptible cultivar planted over four cycles; T2, the same CR cultivar planted over four cycles; and T3, different CR cultivars planted in each cycle. Clubroot severity increased from cycles one to four in all treatments, with the exception of one CR cultivar in T3 that may carry a different source of resistance. Pathogen populations were recovered with a susceptible bait crop and pathotyped on the differentials of Williams plus a CR host (B. napus ‘Mendel’). The percentage of galls classified as pathotype 5X in T1 declined from 50% to 6.7% over the course of the experiment, while galls classified as pathotype 5X increased from 50% to 66.7% in both T2 and T3. Quantitative PCR analysis of the soil with pathotype 5X-specific primers generally confirmed an increase in 5X DNA. The results suggest that continuous planting of CR canola favours a rapid proliferation of virulent pathotypes of P. brassicae, as indicated by the increases in pathotype 5X observed in this study.