2.4.3. Black Rot

Black rot, caused by the bacterium *Xanthomonas campestris* pv. *campestris* (Pam.) Dowson (*Xcc*), is also one of the most destructive diseases of brassica crops in the world [98,99]. The pathogen often invades plants through hydathodes and spreads through vascular tissue, forming V-shaped lesions at the leaf margins, causing systemic infection and great loss of quality and yield [98–100]. While some resistant plant resources have been reported in *B. oleracea*, few loci/genes have been identified [101–103]. Camargo et al. identified genomic regions associated with young and adult plant resistance to black rot in linkage groups 1, 2 and 9 using a population of black rot-resistant cabbage line BI-16 and susceptible inbred broccoli line OSU Cr-7 [38]. Doullah et al. identified two genomic regions on LG 2 and LG 9 significantly associated with resistance to black rot, with a disease rating of populations from susceptible broccoli green commet P09 and resistant Reiho P01 [104]. In a later study using the same plant materials, Tonu et al. improved the previous genetic map and identified three QTLs, *XccBo(Reiho)1*, *XccBo(Reiho)2 and XccBo(Reiho)1,* for resistance to black rot, and the major QTL, *XccBo(Reiho)2*, was from parent Reiho [105]; comparison using common markers of the previous study by Camargo et al. revealed that *XccBo(Reiho)1* and *XccBo(GC)1* may be identical to the previously reported QTLs [104,105].

Iglesias-Bernabé et al. performed QTL analysis of black rot resistance (*Xcc* race 1) in the BolTBDH mapping population and identified four QTLs, including *Xcc1.1* showing overlap with the previously reported cabbage resistance locus *BRQTL-C1\_1*, *BRQTL-C1\_2* [106], *Xcc6.1* showing overlap with *BRQTL-C6*, *Xcc8.1* showing overlap with *XccBo(Reiho)2* [105] and a novel locus, *Xcc9.1* [107]; in addition, this study indicated that resistance might be related to the synthesis of secondary metabolites [107].
