Pathotype classification of Plasmodiophora brassicae, the causal agent behind clubroot in central Europe and Sweden (2016-2020)

Abstract: Clubroot, initiated by soilborne pathogen Plasmodiophora brassicae, stands as a
significant disease affecting oilseed rape globally. Understanding the virulence characteristics
of P. brassicae populations is imperative for strategically deploying disease management
practices, particularly by selecting suitable clubroot-resistant cultivars. From 2016 to 2020,
84 isolates of P. brassicae were collected across the Czech Republic (CZ), Germany (DE),
Poland (PL), and Sweden (SW). Pathotyping was conducted utilizing 17 Brassica hosts,
incorporating the European Clubroot Differentials (ECD), Somé set, and clubroot-resistant
oilseed rape cultivar Mendel. Based on the ECD set, virulence assessments outlined the isolates into 42 distinct pathotypes, with the most prevalent being 16/31/31 (observed in DE, PL, and SW) and 16/06/12 (encountered in CZ, DE, and PL). Six pathotypes were identified according to the Somé set, with 1-4 pathotypes identified per country. Notably, P1 predominated in DE, PL, and SW, while P3 exhibited high prevalence in CZ, DE, and PL. Compared to earlier investigations; this study underscores a trend toward heightened virulence within P. brassicae populations. Furthermore, several isolates demonstrated the capability to overcome the resistance conferred by cultivar Mendel and B. rapa genotypes ECD 01 to ECD 04. Analysing all samples revealed significant negative correlations between clubroot incidence and soil pH as well as between clubroot incidence and the frequency of oilseed rape in crop rotation.