Evaluation of clubroot resistance in Chinese Cabbage and its inheritance in the European turnip line ‘IT033820’, a new genetic resource

Abstract

Clubroot caused by the protist Plasmodiophora brassicae is one of the most destructive diseases of Brassica crops. Developing Chinese cabbage cultivars with durable clubroot resistance (CR) is an important goal of breeding programs, which will require new genetic resources to be identified and introduced. In this study, we evaluated resistance to P. brassicae race 4 using 26 Chinese cabbage (B. rapa ssp. pekinensis ) cultivars compared to the clubroot-susceptible Chinese cabbage inbred line ‘BP079’ and the clubroot-resistant European turnip (B. rapa ssp. rapifera) inbred line ‘IT033820’. No symptoms of clubroot disease were found in ‘IT033820’ infected with P. brassicae race 4, whereas the Chinese cabbage cultivars exhibited disease symptoms to various degrees. The Chinese cabbage cultivars that were reported to be clubroot-susceptible were susceptible to P. brassicae race 4; however, seven of the 20 cultivars reported to be clubroot-resistant were susceptible to this race of P. brassicae to varying degrees. Resting spores of P. brassicae were abundant within the infected root tissues of ‘BP079’, as revealed by light microscopy and scanning electron microscopy (SEM), but they were not detected in root tissues of ‘IT033820’. Although resting spores were not detected by light microscopy in root tissues of the clubroot-resistant Chinese cabbage cultivar ‘Kigokoro 75’, a few spores were observed by SEM. The F1 hybrids from a cross between ‘IT033820’ and ‘BP079’ showed no disease symptoms, and all BC1P1 progenies from a cross between the F1 hybrid and ‘IT033820’ exhibited a resistance phenotype. In the BC1P2 population from a cross between the F1 hybrid and ‘BP079’, this trait segregated at a ratio of 3(R):1(S) (χ2 = 1.333, p = 0.248) at a 5% significance level. Inoculated BC1P2 plants were either highly resistant or highly susceptible to the pathogen, indicating that the CR to race 4 of P. brassicae carried by ‘IT033820’ is dominant. In the F2 population, this trait segregated at a ratio of 15(R):1(S) (χ2 = 0.152, p = 0.696) at a 5% significance level, suggesting that CR in ‘IT033820’ is mainly controlled by two dominant genes. Therefore, ‘IT033820’ represents a promising genetic resource for developing durable CR breeding lines in Chinese cabbage.