**11. Management of clubroot resistance for effective utilization**

are available and the resistance in these sources are dominant, which makes it easier to transfer clubroot resistance through interspecific and intraspecific hybridization. Rutabaga cultivars have been identified as clubroot-resistant sources [66, 67, 69]. However, the genetics of clubroot resistance in rutabaga is complicated so it is difficult to develop molecular markers that are

In Chinese cabbage, the dominant clubroot resistance from European turnips has been successfully used to develop clubroot-resistant Chinese cabbage. Since the gene mapping has been performed extensively in Chinese cabbage, molecular markers closely linked to clubroot resistance loci that are used in gene mapping can be easily selected to transfer clubroot resistance genes in the development of Chinese cabbage cultivars through molecular markerassisted selection. Since canola, the amphidiploid *B. napus*, has a very complex genome, most of the molecular markers developed in *B. rapa* may not be polymorphic and cannot be directly used in canola. Additional efforts are required to develop molecular markers in canola when the mapped clubroot resistance loci in Chinese cabbage are transferred into canola. Currently, most clubroot resistance genes in European turnips have not been intensively investigated and mapping and cloning of these clubroot resistance genes in European turnips will allow using

*B. rapa* (Chinese cabbage) BC1 A03 *Rcr1* fine mapped [103] *B. rapa* (Chinese cabbage) F2 A03 *CRb* fine mapped [104] *B. rapa* (G004 line) F2 A08 *Crr1a* fine mapped [86] *B. rapa* (Chinese cabbage) A03 *CRa* fine mapped [93] *B. oleracea* (Anju) DH O2, O5, pb-Bo(Anju)1, pb-Bo(GC)1 [24]

> N13, N15, N16 and N19

*B. rapa* (Shinki) F2 A03 *CRb* [81]

*B. rapa* (Chinese cabbage) F2, BC1 A03 CR gene fined mapped [95] *B. rapa* (turnip line) F2 A03, A08 Two major QTL (Pb-Br3, Pb-Br8 and [105] *B. rapa* (European turnip) F2:3 A03 *Crr3* [85, 87] *B. rapa* (G004) F2 A06 *Crr4* [88, 106] *B. rapa* (Chinese cabbage) F2 A03 and A02 *CRk and CRc* [107]

**Table 1.** Clubroot resistance QTL/gene mapped/fine mapped in different Brassica species by different research studies

*B. oleracea* (kale) F2:3 LG1, 2, 5 Nine QTL (Pb-Bo1 to Pb-Bo9) with

LGs

DH - Two QTL (pb-3 and pb-4) [83]

**Populations LG QTL/genes Reference**

Nineteen QTL identified on different

phenotypic variance 20-88%

[91]

[84]

closely linked to the dominant clubroot resistance genes.

these genes effectively and efficiently in canola breeding.

*B. napus* (synthetic line) DH N02, N03, N08,

**Brassica species (R sources)**

14 Plants for the Future

*Brassica oleracea* (Bindsachsener) The Brassica genomes (A, B, and C genome) are crucially important to provide novel genetic inheritance for economically important traits that can be used for the overall improve‐ ment of crop production and quality. For example, single genome of diploid *B. rapa* (Agenome) holds more than 230 R-gene sequences in 16 gene families [108], among which over 8 genetic loci have been identified in different research studies, which have function‐ al properties for clubroot disease resistance. There are possibilities of the existence of more R-genes specific to the clubroot disease resistance and their allelic variations in different genetic pools or wild relative species. Effective utilization of resistance loci and their allelic variations may enhance the durability of resistance against clubroot disease in different Brassica species.

As a long history of clubroot disease revealed relatively high evolutionary patterns of the pathogen, *P. brassicae*. In various cultivating geographical regions of Brassica crops, persistence of *P. brassicae* pathotypes with high levels of pathogenicity poses challenges to breed durable clubroot-resistant cultivars. The breakdown of clubroot-resistant cultivars has become a serious problem in Chinese cabbage and leafy cabbage in China, Korea, and Japan [109, 110]. Effective management of resistance genetic resources in breeding novel culti‐ vars could enhance the performance of resistance loci in different Brassica species for sustainable, more durable, and cost-effective control of the clubroot disease.
