Race typing and molecular characterization of Xanthomonas campestris pv. campestris strains occurring in Italy.

Xanthomonas campestris pv. campestris (Xcc) is the causal agent of black rot, a severe seed-borne systemic vascular disease of vegetable brassica crops. Xcc strains have been grouped into nine physiological races, with races 1 and 4 being the most widespread in Brassica oleracea crops. The characterization of Xcc races is important to identify resistance sources, establish breeding programs and to set up control strategies. To define Xccrace structure and distribution in Italy, a collection of 31 Xcc strains, isolated from six B. oleracea varieties, B. napus var. napobrassica and Crambe maritima in seven Regions, was established from a larger collection according to their geographic and host origin and PCR-based DNA fingerprints. These strains all managed to grow on the semi-selective medium FS, hydrolyzed starch, induced vascular symptoms on B. oleracea and were identified by Xcc specific primers based on the hrcC gene. The race of each strain was determined by inoculating eight differential Brassica lines belonging to five species. Xcc strains from international collections and Xcc strain races 1, 4 and 6 were included as reference strains. On the basis of compatible interaction (susceptibility) or incompatible interaction (resistance), Xcc strains isolated in Italy were classified into races 1 (35.5%), 4 (54.8%), and 6 (9.7%). Multilocus sequence analysis (MLSA) based on four housekeeping genes (dnaK, gyrB, fyuA and rpoD) showed that Xcc strains in Italy are closely related to strains isolated worldwide. The results are discussed with relation to the strain source.