Gene similarity of Plasmodiophora brassicae collections from Canada.

Clubroot, caused by Plasmodiophora brassicae Wor., is generally managed using resistant cultivars, but new, virulent pathotypes are increasing rapidly on canola (Brassica napus L.) in Canada. Information on genetic similarity among pathogen populations could inform the development of sustainable management approaches. The objective of this study was to develop whole-genome sequences of pathogen collections from across Canada, assess their genetic similarity, and compare with collections from the USA, China and Europe. In total, 52 single- spore and field collections from clubbed roots were increased on the highly susceptible ‘Mei Qing Choi’ (Brassica rapa var. chinensis). DNA was extracted from clubs and P. brassicae-induced callus cultures. Sequencing reads were mapped to the published genome of P. brassicae isolate e3 (from Europe) to remove plant DNA sequences. A phylogenetic tree prepared using R software clustered the collections into five clades. The collections from Canada were separated into four clades: one from the Prairie provinces, one of new pathotypes from Alberta, and two clades from eastern / central Canada plus British Colombia. The collections from China formed the fifth clade. There was some overlap among clades. The similarity between samples collected from a site before and after a change of pathotype was generally low. Collections from the USA grouped with eastern / central Canada. This indicated that collections across much of North America differed from the initial collections in Alberta, which in turn differed from the new pathotypes.