Phenotypic and phylogenetic analysis of <i>Verticillium longisporum</i> strains from European and Canadian oilseed rape fields

Abstract Verticillium longisporum is a vascular fungal pathogen of oilseed rape (OSR) that causes Verticillium stem striping. The pathogen segregates into three lineages originating from at least three hybridization events between four different Verticillium genomes, leading to the hybrid lineages A1/D1, A1/D2 and A1/D3. Although A1/D1 is considered the most relevant lineage in OSR, extensive studies addressing the geographical spread and phylogenetic relationships of Verticillium lineages are lacking. In the present study, the genetic diversity and relatedness of V . longisporum isolates from European and Canadian OSR fields were analysed. Based on genotyping by sequencing, genetic subgroups were identified and assigned to geographic origins and pathogenic traits. To assess the agronomic importance of different lineages, representative isolates were tested for pathogenicity and aggressiveness on OSR under greenhouse conditions. This study confirms that A1/D1 is the prevalent and most aggressive lineage of V . longisporum in European and Canadian OSR fields. Genetic clusters within the A1/D1 lineage were weakly correlated with geographic origin, but not with aggressiveness on winter OSR. Isolates from the United Kingdom, Latvia and Canada occurred only in a single and distinct subclade, indicating a more recent introduction, whereas isolates from locations where the disease has been known for a long time (France, Germany, Poland, Sweden) were less uniform and could be assigned to several subclades. Our results indicate that Verticillium stem striping in the United Kingdom, Latvia and Canada may have derived from separate and different single introductions of the pathogen in the recent past.