Molecular evaluation of Plasmodiophora brassicae collections for the presence of divergent genetic pathogen populations before and after the release of clubroot resistant canola

Plasmodiophora brassicae Woronin, the cause of clubroot of crucifers, is now established across most of Alberta, Canada, and has become a major constraint in canola (Brassica napus L.) production. Disease control is achieved mainly by planting clubroot-resistant (CR) canola varieties. The first CR canola in Canada was released in 2009. In 2013, severe clubroot was detected on CR canola in several fields in Westlock County, Alberta, and found to be caused by a 'new' strain of P. brassicae termed '5X'. These strains have been shown to be genetically distinct from other P. brassicae strains in Alberta ('population one') and belong to a second population ('population two') of the pathogen. In this study, multiple molecular markers, specific for 'population two', were used to determine the occurrence of members of this population over time throughout central Alberta. Two hundred and twenty-four clubroot galls collected from 2005 to 2016 from non-CR canola varieties were examined. The presence of 'population two' DNA was detected at low levels in at least 47 of the galls, and it was the predominant population of P. brassicae in another 10 of the samples. These samples were found in Flagstaff County starting in 2008, Westlock County starting in 2009, the County of Vermillion River in 2011 and Red Deer County in 2014. Although relatively uncommon, members of this population were widespread, occurring at locations 168 km apart prior to the release of CR canola.

Automated summary

A new strain of Plasmodiophora brassicae, termed '5X', has been discovered in Canada, belonging to a second population of the pathogen. Molecular markers specific for this population have shown its presence in central Alberta, with low levels detected in some galls from 2005 to 2016, and predominant in others. Despite being relatively uncommon, this new population was found to be widespread across different counties in Alberta prior to the release of clubroot-resistant canola varieties.