Identification of Plasmodiophora brassicae Wor. isolate suppressing clubroot resistance in ‘Kilaxy’ F₁ white cabbage

The purpose of this study was the evaluation of the pathogenicity of Plasmodiophora brassicae Wor. field isolates gathered from clubroot-infected white cabbage Brassica oleracea var. capitata L. f. alba plants grown in different areas of Poland. The virulence of seven isolates of the pathogen was tested. Breeding materials from white cabbage resistant and susceptible to P. brassicae were used as standard objects for isolate virulence screening. Cabbage seeds were infected by submerging them into a resting-spore suspension. A screening of plant resistance using a nine-degree scale based on root symptoms was carried out at the eight-week-old plantlets stage and at harvest time on afield infected by P. brassicae spores. Isolates from Pobiednik and Grębałów were defined as more virulent. The P isolate from Pobiednik also infested plants of the clubroot resistant ‘Kilaxy’ F₁ cultivar, which indicated the suppression of clubroot resistance. Its reaction to the other six isolates confirmed the resistance of this cultivar. The susceptible HTM line was characterised by the highest infestation level. A difference in the susceptibility level was noticed between lines derived from those with resistant genes, i.e. Oregon123 and ‘Badger Shipper’. The Oregon123 line was less susceptible to the applied isolates. The effect of the isolates used for seed inoculation on the reaction of the dormant plants to the pathogen on the clubroot field was observed.