Epidemiology of clubroot disease and pathogenic variation among isolates of Plasmodiophora brassicae from oilseed rape growing in Europe

Clubroot, caused by Plasmodiophora brassicae, is one of the most destructive diseases in oilseed rape (OSR) cultivation and has become increasingly important in central Europe. The disease has been monitored by collaborators through field surveys in the Czech Republic, Denmark, France, England and Scotland, Germany, Poland and Sweden. Infected plants and soil samples were collected randomly from clubroot-infested fields in different countries. Location, soil type, soil pH, plant genotype and rotation regime were recorded for each field. The presence of P. brassicae-resting spores in the soil was assessed by standard bioassays. Pathotype classification of the P. brassicae-populations was conducted on two or three known differential sets. Additionally, the
degree of virulence of the collected isolates was analysed on the clubroot-resistant OSR cv. Mendel. Clubroot monitoring revealed that the disease occurred in different regions in all stated countries with different intensities. It
was a particularly major issue when OSR was grown in a 2- or 3-year rotation, which could lead to a rapid increase in clubroot severity. A slight significant negative correlation was found between soil pH and the disease incidence in infested fields. Variation in pathotype distribution was observed in different countries. In the Czech Republic and Poland, there were nine pathotypes
according to the evaluation system of Williams, four pathotypes based on Somé et al. and 15 with the European Clubroot Differential (ECD) set. In Germany,
five pathotypes were found according to Somé et al. and 20 according to the ECD set. In France, six pathotypes were classified according to the set of Somé et al. Although the population of P. brassicae appears to be very diverse in the UK, three pathotypes were found to be dominant when tested on the ECD set. In Sweden, pathotypes were evaluated earlier according to Williams and four pathotypes were found. In 2010, a range of isolates originating from different Brassica species were evaluated on the ECD set and different virulence patterns were observed. From all populations tested for virulence on cv. ‘Mendel’, several isolates were found to be moderately or highly virulent. These virulent populations were not restricted to a small geographic area in different countries.