Evaluation of a core collection of Brassica rapa vegetables for resistance to Xanthomonas campestris pv. Campestris

The identification of juvenile resistance to isolates Xcc512 (race 1) and Xcc524 (race 4) of Xanthomonas campestris pv. campestris (Xcc) was performed in a screening of Brassica rapa vegetable core collection with 210 accessions representing the genetic and geographic diversity of the specie. Twentyfour plants per accession were screened against the isolates Xcc524 (race 4) and Xcc512 (race 1) at the third thrue-leaf. The conventional rating criteria of the mean Disease Index (DI) and the percentage of resistant seedlings (%R) were compared and adopted as the criteria to rank the accessions for their interest as sources of resistance. A great variety of reactions was found between and within accessions of the B. rapa core collection, ranging from complete resistance to full susceptibility. Sources of resistance to isolate Xcc524 were found among the broccoleto, choy-sum, and Chinese cabbage gene pools. The turnip cultivar type group was the only one where the number of resistant accessions was significantly lower than expected. One hundred and twenty one accessions presented at least 20% of resistant plants to this isolate. Thirty-six accessions presented more than 70% of resistant plants to this isolate and so, they should be considered as potential and useful sources for direct use in breeding programs of Xcc resistance. In contrast, from the 210 accessions tested with isolate Xcc512 (race 1), it was found that 195 (92%) were completely susceptible with all the plants rated in the 7 to 9 classes and only one accession, the Chinese cabbage “Chang Puh Early” (B-31), presented 25% of resistant plants. This resistance is rare. Besides, other accessions that were screened such as Chinese cabbage “Chang Puh Medium Early” (B-32), and broccoletos “Cima di rapa tardivo di Marzo” (K-9011) and “Tardivo” (HRI-5213), are presented respectively 19, 16, and 16%. All these four accessions are new sources of resistance and they can be exploited in breeding programmes for blackrot resistance with race 1 isolates. Since they exhibit resistance to the two major worldwide Xcc races (1 and 4), they can be used to transfer this resistance through interspecific hybridization to other B. rapa morphotypes and to Brassica oleracea, where there are no cultivars with complete resistance to these Xcc races.