Identification of Race‐Specific, Clade‐Specific and Broad‐Spectrum Resistance to <scp> <i>Xanthomonas campestris</i> </scp> pv. <i>campestris</i> in <i>Brassica</i> Diversity Fixed Foundation Sets

ABSTRACT Black rot, caused by Xanthomonas campestris pv. campestris (Xcc), is the most damaging bacterial disease of vegetable brassicas ( Brassica oleracea ) worldwide. The prevalence of several genetically diverse Xcc races makes breeding for varietal resistance challenging. In this study, we have screened diversity fixed foundation sets (DFFSs) of B. oleracea and Brassica napus for resistance against isolates belonging to prevalent Xcc races 1, 4, 5 and 6. The DFFSs are designed to capture the genetic diversity available within the respective Brassica species gene‐pools in smaller subsets of lines. Our findings revealed that resistances to race 1 and 4 were largely absent in B. oleracea but more prevalent in B. napus . Notably, resistance to race 4 was particularly common in B. napus (63% of lines showed resistance, 13% of lines showed partial resistance). Conversely, resistance to races 5 and 6 was more common in B. oleracea than in B. napus . In B. oleracea , there was no significant association between disease index and morphotype but, among the B. napus morphotypes, swede was the most susceptible to races 1, 5 and 6 but not to race 4. Ten B. oleracea and 67 B. napus lines showed resistance to more than one isolate. Further testing of a subset of these lines demonstrated that resistances were effective against additional diverse Xcc isolates in a race‐specific, clade‐specific or broad‐spectrum manner. The resistant lines identified in this study offer a valuable resource for breeding programmes aimed to achieve durable and sustainable control of Xcc.