Most Plasmodiophora brassicae Populations in Single Canola Root Galls from Alberta Fields are Mixtures of Multiple Strains.

Clubroot, caused by Plasmodiophora brassicae, is an important disease on canola in Alberta, Canada. The pathogen is grouped into pathotypes according to their virulence reaction on differential hosts. Multiple pathotypes or strains are known exist in one field, one plant, or even one gall. This study was conducted with the objective of testing the prevalence of the coexistence of multiple strains in a single gall. In all, 79 canola clubroot galls were collected from 22 fields across Northern Alberta in 2018. Genomic DNA extracted from these single galls was analyzed using RNase H-dependent PCR (rhPCR). The rhPCR primers were designed to amplify a partial sequence of a dimorphic gene, with one primer pair specific to one sequence and the other primer pair specific to the alternative sequence. The amplification of both sequences from DNA obtained from a single gall would indicate that it contains two different P. brassicae strains. The rhPCR analyses indicated that the P. brassicae populations in 50 of the 79 galls consisted of more than one strain. This result emphasizes the need for cautious interpretation of results when a single-gall population is subject to pathotyping or being used as inoculum in plant pathology research. It also confirms that the maintenance of pathotype diversity within single root galls is a common occurrence which has implications for the durability, and stewardship, of single-gene host resistance.