The aim of the research conducted in 2007-2009 was the evaluation of the pathogenicity of eight Plasmodiophora brassicae Wor. isolates collected from clubroot-infested white cabbage growing in different areas of Poland. Breeding materials from white cabbage resistant and susceptible to P. brassicae were used for this purpose. Cabbage seeds were infected by submerging them into a spore suspension. The screening of plant clubroot resistance using a nine-degree scale based on root symptoms was carried out at the eight-week-old seedling stage and at harvest time on a field infected by P. brassicae spores - called the ‘death field’. Differences in the pathogenicity of the collected isolates were determined. Isolates from Szczecin, Ostromęczyn, Krojczyn and Maszkienice were defined as the most virulent. Plants of the ‘Kilaxy’ F1 white cabbage cultivar confirmed resistance to all isolates, while plants of the susceptible HTM line from the ‘Hitoma’ F1 white cabbage cultivar were characterised by the highest infestation level. The evaluation of adult plants in the field with high P. brassicae spore contamination confirmed seedling test results. Different susceptibility was observed between sub-lines from the ‘Oregon 123’ and ‘Badger Shipper’ cultivars with resistant genes. ‘Oregon 123’ sub-lines were less susceptible to applied isolates than ‘Badger Shipper’ sub-lines at both the seedling stage and during the field test. A double-stage plant screening to pathogen reaction caused the elimination of the most susceptible plants at the seedling stage. Plants with a certain resistance level were planted in the ‘death field’.
