**6. Identification of clubroot resistance in Brassica species and their relatives**

Brassica species are the major sources that are used to identify clubroot resistance. In the *Brassica* genus, three diploid species are the natural progenitors of three amphidiploid species, which is the famous triangle of U, explaining the evolutionary relationship of Brassica species. The close evolutionary relationship of Brassica species suggests that it is relatively easy to transfer clubroot resistance from species to species through interspecific hybridization and gene introgression. Extensive searching for the clubroot resistance has been performed in Brassica species, especially *B. rapa*, *B. oleracea*, and *B. napus*, and the European turnips in *B. rapa* are found to contain dominant resistance and those clubroot resistance sources have been widely used *in B. rapa* and *B. napus* breeding.

In *B. rapa*, there are various types of vegetables such as Chinese cabbage, Shanghai Pak-choy, and turnip. Clubroot disease causes heavy yield losses in Chinese cabbage production in Eastern Asian countries, especially in Japan, South Korea, and China. Fortunately, European turnip contains dominant clubroot resistance which is commonly used in Chinese cabbage hybrid cultivar development through crosses of Chinese cabbage and resistant European turnips. The clubroot resistance in European turnips has been extensively tested and geneti‐ cally analyzed under field conditions or using artificial inoculation under controlled environ‐ mental conditions [57]. Before 1960, breeders in the Netherlands developed various clubrootresistant turnip cultivars which were used to control the most serious disease in fodder turnip production, and also those clubroot-resistant turnip cultivars were used to differentiate pathogen and study clubroot infection under different field conditions [57–59]. For example, the European Clubroot Differential (ECD) set has been selected [59] and are currently used by other researchers.

*B. oleracea* vegetables such as cabbage, broccoli, and cauliflower are tested to identify clubroot resistance. As the clubroot resistance in *B. oleracea* was analyzed, the results in genetic analyses showed that susceptibility was dominant over resistance, and recessive genes were inferred to explain the inheritance of clubroot resistance in diallel analysis [60]. In another diallel analysis of F1 kale populations, it was also found that additive effects are inferred based on the assessment of broad sense inheritability [61]. In addition, there are several other investigations on the clubroot resistance in *B. oleracea*; and in most cases, recessive inheritance of clubroot resistance was identified. For example, 71 accessions of cabbage, broccoli, and curly kale were tested and most of them showed some levels of resistance to clubroot, while all the F1 populations of these resistant and susceptible *B. oleracea* accessions were susceptible [62, 63]. Further analysis indicated that multiple loci are involved in the clubroot resistance in *B. oleracea*, but it was not easy to determine how many loci control clubroot resistance in the analysis of F1, F2, and backcross populations of *B. oleracea* [64]. Moreover, 44 landraces of Portuguese coles (*B. oleracea*) were tested to identify clubroot resistance and three accessions showed high levels of clubroot resistance [65].

In *B. napus*, rutabaga cultivars are identified to contain dominant clubroot resistance. In one report, the clubroot resistance in rutabaga was suggested to be controlled by one dominant resistance gene [66]. To investigate the diversity of clubroot pathogen (*P. brassicae*), the Williams differential set was suggested; [67] and in this set, there are two rutabaga accessions that show clubroot resistance in several reports [66, 68, 69]. Vigier et al. [70] tested 31 cultivars and breeding lines of spring canola under controlled environmental conditions and found that several Swedish accessions showed clubroot resistance, but the resistance was not recovered in the subsequent progenies. In another report, the clubroot resistance from rutabaga was transferred into cabbage through interspecific hybridization and results indicated that all the F1 hybrids were resistant to clubroot disease [71].

Radish (*Raphanus sativus*) is a Brassica relative and there are several reports that focus on the identification and transfer of clubroot resistance to Brassica species. Rowe [72] tested 68 radish cultivars and breeding lines collected from several countries and found that all Japanese and most Dutch radish cultivars were completely resistant to clubroot. Akaba et al. [73] used *B. napus*–radish chromosome additional lines to analyze clubroot resistance and found that one chromosome additional line, the c-type, showed a high level of clubroot resistance. More recently, quantitative resistance loci (QTL) mapping for clubroot resistance in radish has been performed and one major gene on one linkage group was found to control the high level of clubroot resistance in radish [74].

As discussed earlier, European turnips contain dominant clubroot resistance genes which makes gene mapping easier than in *B. oleracea* varieties. To control clubroot disease, Chinese cabbage hybrid cultivars were developed by introducing clubroot resistance from European turnips into Chinese cabbage in Japan [75]; and currently, clubroot-resistant Chinese cabbage cultivars containing turnip clubroot resistance genes are being used in Japan, South Korea, and China.
