Identification of quantitative trait loci for resistance to Xanthomonas campestris pv. campestris in Brassica rapa

Resistance to six known races of black rot in crucifers caused by Xanthomonas campestris pv. campestris (Pammel) Dowson is absent or very rare in Brassica oleracea (C genome). However, race specific and broad-spectrum resistance (to type strains of all six races) does appear to occur frequently in other brassica genomes including B. rapa (A genome). Here, we report the genetics of broad spectrum resistance in the B. rapa Chinese cabbage accession B162, using QTL analysis of resistance to races 1 and 4 of the pathogen. A B. rapa linkage map comprising ten linkage groups (A01-A10) with a total map distance of 664 cM was produced, based on 223 AFLP bands and 23 microsatellites from a F-2 population of 114 plants derived from a cross between the B. rapa susceptible inbred line R-o-18 and B162. Interaction phenotypes of 125 F-2 plants were assessed using two criteria: the percentage of inoculation sites in which symptoms developed, and the severity of symptoms per plant. Resistance to both races was correlated and a cluster of highly significant QTL that explained 24-64% of the phenotypic variance was located on A06. Two additional QTLs for resistance to race 4 were found on A02 and A09. Markers closely linked to these QTL could assist in the transference of the resistance into different B. rapa cultivars or into B. oleracea.