Abstract
Many clubroot resistant (CR) cultivars of Chinese cabbage (Brassica rapa ssp. pekinensis) have been bred so far, but their usage is limited because the capacity for resistance breaks down with time. This degradation is caused by a pathogenic variation in the causal fungus Plasmodiophora brassicae. We attempted to accumulate 3 CR genes, CRa, CRk, and CRc, through marker-assisted selection. Five doubled haploid CR lines with an individual CR locus were used as breeding materials. The CR lines were crossed with each other. A subsequent selection for resistance was performed using sequence characterized amplified region markers in segregating generations. As a result, 4 homozygous lines for 3 resistance genes and the F1 hybrids between them were developed. CR pyramiding lines were inoculated with 6 field isolates of P. brassicae. The homozygous lines for 3 CR genes, whether selfed or crossed, exhibited exceedingly high resistance against all of the isolates. Morphological characters of F1 hybrids were comparable to those of a control cultivar, but the degree of heterosis was less than expected, which is probably because of genetic similarity of the parents. The results of this study prove that clubroot resistance can be reinforced through the accumulation of varied resistance genes in B. rapa.
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