‘New’ pathotypes of Plasmodiophora brassicae in Canada are not new.

Clubroot, caused by Plasmodiophora brassicae Wor., is an important disease of brassica crops. Clubroot is generally managed using resistant cultivars, but new pathotypes are increasing rapidly on canola (Brassica napus L.) in Canada. The genetic sequences of P. brassicae collections from across Canada were assessed to quantify the genetic variation among populations. Many of the collections were also pathotyped (Williams’ system + others) to determine how closely related the new pathotypes were to the pathotypes that had previously been predominant on canola.. In total, 43 single-spore and field collections of P. brassicae from Canada, the USA, and China were sequenced and clustered into five clades. The collections from Canada separated into four clades: two primarily from canola on the Prairies and two primarily from other regions of Canada, but 3 of 4 clades contained multiple pathotypes from various hosts. The ‘new’ pathotypes from the Prairies generally clustered together, separately from the pathotypes that had previously been predominant on canola. At two sites in central Canada where a rapid breakdown in resistance had been documented, SNPs in about half of the genes differed between collections before and after the change. Population genomic analysis indicated that the ‘new’ pathotypes were likely not the result of point mutation, but rather presented genotypes that had been maintained in the pathogen population at low frequency through balancing selection, and became dominant at the site in response to selection from repeated exposure to pathotype-specific resistance.