Identification and characterization of resistance for Plasmodiophora brassicae race 4 in cabbage (Brassica oleracea var. capitata)

Cabbage is one of the most important cruciferous vegetables worldwide including China. Clubroot, caused by Plasmodiophora brassicae, occurs on most cruciferous crops and causes great yield losses. Utilization of resistant cultivar is the principal way to combat clubroot. To provide resistant sources for breeding, 102 cabbage genotypes were evaluated for seedling resistance to P. brassicae race 4, which is the predominant race in China. Results of repetitive identifications showed that Xiangan336 was highly resistant to clubroot, and four resistant genotypes including Verheul, Bindsachsener, Zhouyebai and 2358 were identified as resistant. Marker analysis suggested that Xiangan336 carried the clubroot resistance gene CRb. To clarify the infection characteristic of P. brassicae and deduce the resistance mechanism of cabbage, the infection process in Xiangan336 and Jingfeng No.1 (highly susceptible) were observed and compared. The significantly lower level of primary infection in Xiangan336 than that in Jingfeng No.1 at 4 dai (days after inoculation) indicated the early defense against P. brassicae in resistant genotype. However, the accumulated secondary plasmodia in Xiangan336 were more abundant than in Jingfeng No.1 from 7 to 21 dai and then declined significantly after 28 dai, suggesting that the clubroot-specific resistance mechanism may restrict the proliferation of P. brassicae after 28 dai. Histological observation verified that the development of P. brassicae from secondary plasmodia to resting spores was restricted within cortex cells in Xiangan336. The identified resistant genotypes will be of great value for resistance breeding and the analysis of infection process will further improve our understanding of resistance mechanism.