2.4.2. Clubroot

Clubroot, caused by the soil-borne pathogen *Plasmodiophora brassicae*, is one of the most devastating diseases of Brassica crops, including broccoli [86–88]. Plants infected by the pathogen form galls on roots, which prevent plant uptake of nutrients and water and become stunted and wilt under warm weather [89]. *B. olearcea* lacks germplasm highly resistant to clubroot, although it has been identified and studied for mining resistance loci/genes in its close relatives, such as turnip, radish and rapeseed [90–93]. The resistance gene *CRa* has been introduced from *B. rapa* to *B. olearcea* by distant hybridization and MAS [94]; in this process *CRa*-specific markers SC2930-Q-FW/SC2930-RV were applied for detection of *CRa* gene in the F1 and each backcross plants, enabled successful introgression of the *CRa* gene into the cabbage inbred lines. In recent years, commercial broccoli varieties with the *CRa* resistance gene, bred by the Syngenta Corporation, are available on the market of China, but the MAS process is not available.

While highly clubroot-resistant germplasms are lacking, some moderate clubroot resistance has been identified in *B. oleracea* [95–97]. There are two studies on genetic mapping for resistance loci related to broccoli, although both of them used broccoli as susceptible parents. These studies are useful for the rapid introduction of clubroot resistance from other subspecies/related species to broccoli with MAS [95,96]. Rocherieux et al. generated F2:3 segregation populations by crossing clubroot-resistant kale and clubroot-susceptible broccoli and constructed a restriction fragment length polymorphism (RFLP) based genetic map. The populations were infected by five isolates and two to five QTLs were identified depending on the isolates; one of these QTLs, *Pb-Bo1*, showed broad-spectrum resistance detected in all isolates [95]. Using populations of crossing resistant double-haploid line (Anju) with a susceptible double-haploid line (GC), Nagaoka et al. identified five CR-QTLs, *pb-Bo(Anju)1*, *PbBo(Anju)2*, *PbBo(Anju)3* and *PbBo(Anju)4* derived from Anju and *pb-Bo(GC)1* from the susceptible parent GC; this study also provided specific primer sequences linked to CR loci and a comparison with known *B. rapa* CR genes [96].
