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Abstract
Clubroot is a serious soil-borne disease of crucifers caused by the obligate parasite Plasmodiophora brassicae. The genetic basis and histopathology of clubroot resistance in two Chinese cabbage (Brassica rapa ssp. pekinensis) inbred lines Bap055 and Bap246, challenged with pathotype 4 of P. brassicae, was evaluated. The Chinese cabbage cultivar “Juxin” served as a susceptible check. The resistance in Bap055 was found to be controlled by the CRa gene, while resistance in Bap246 fit a model of control by unknown recessive gene. Infection of the roots by P. brassicae was examined by inverted microscopy. Despite their resistance, primary and secondary infection were observed to occur in Bap055 and Bap246. Primary infection was detected at 2 days post-inoculation (DPI) in “Juxin,” at 4 DPI in Bap055, and at 6 DPI in Bap246. Infection occurred most quickly on “Juxin,” with 60% of the root hairs infected at 10 DPI, followed by Bap055 (31% of the root hairs infected at 12 DPI) and Bap246 (20% of the root hairs infected at 14 DPI). Secondary infection of “Juxin” was first observed at 8 DPI, while in Bap055 and Bap246, secondary infection was first observed at 10 DPI. At 14 DPI, the percentage of cortical infection in “Juxin,” Bap055 and Bap246 was 93.3, 20.0, and 11.1%, respectively. Although cortical infection was more widespread in Bap055 than in Bap246, secondary infection in both of these hosts was restricted relative to the susceptible check, and the vascular system remained intact. A large number of binucleate secondary plasmodia were observed in “Juxin” and the vascular system was disrupted at 16 DPI; in Bap055 and Bap246, only a few secondary plasmodia were visible, with no binucleate secondary plasmodia. The defense mechanisms and expression of resistance appears to differ between Chinese cabbage cultivars carrying different sources of resistance.
Highlights
Clubroot, caused by the obligate parasite Plasmodiophora brassicae Wor., is a major soil-borne disease of the Brassicaceae
Few or no galls were observed on the roots of Bap055 and Bap246 (Figure 2), with these hosts developing low levels of visible clubroot (DIs = 8.8 and 5.3, respectively; Table 1)
We found that P. brassicae follows a similar infection process in the susceptible host “Juxin.” the life cycle of P. brassicae proposed in Arabidopsis appears to represent a general model for most Brassicaceae
Summary
Clubroot, caused by the obligate parasite Plasmodiophora brassicae Wor., is a major soil-borne disease of the Brassicaceae. Transmission of P. brassicae occurs on seeds, in soil, infected plant material, irrigation water and animal manure, with the most severe clubroot outbreaks reported in the southwest, northeast and middle regions of China. Various strategies are recommended for clubroot management, including the sanitization of field implements and equipment, the application of soil amendments and chemical pesticides, and long rotations out of susceptible hosts (Hwang et al, 2014; Peng et al, 2014a; Andreote et al, 2020). The most economical and effective approach for clubroot control, is to breed varieties with genetic resistance to the disease (Diederichsen et al, 2009; Rahman et al, 2011; Peng et al, 2014b). Among the mapped resistance genes, CRa, CRbkato, and Crr have been cloned (Ueno et al, 2012; Hatakeyama et al, 2013, 2017), and all are R genes with an NBS-LRR structure (Eitas and Dang, 2010)
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