Characterisation of Xanthomonas campestris pv. campestris isolates from South Africa using genomic DNA fingerprinting and pathogenicity tests

The genetic diversity of Xanthomonas campestris pv. campestris isolates from South Africa was evaluated using 28 isolates obtained from the Johannesburg Fresh Produce Market. Samples were collected from cabbage supplies from farms in Gauteng, Mpumalanga and North West Provinces. Strains were isolated from small sections of infected cabbage leaf samples and cultured on Yeast Dextrose Agar. Isolates identity was confirmed by ELISA and Pathogenicity test. Pathogenicity tests were performed by inoculating leaves of known susceptible cabbage seedlings. Infection symptoms induced could be categorized into three groups, ranging from typical to non-typical black rot symptoms. Four differential Brassica cultivars with known avirulence genes were used for race typing done by spray inoculation. Four races, namely 1, 3, 4 and 6, were identified. Of the 28 isolates, four were identified as race 1, two as race 3, 19 as race 4 and three as race 6. Repetitive DNA polymerase chain reaction-based fingerprinting using Eric- and Box-primers was used to assess the genetic diversity. Generated fingerprints of X. c pv. campestris were relatively similar. Cluster analysis could not strictly group isolates by their geographical origin, suggesting limited diversity of Xanthomonas campestris pv. campestris strains within cabbage producing regions in South Africa.