Characterisation of Xanthomonas campestris pv. campestris isolates from South Africa using genomic DNA fingerprinting and pathogenicity tests
Black rot caused by Xanthomonas campestris pv. campestris (X. c pv. campestris) is a major disease constraint to cabbage production. The control of black rot is difficult and resistant cultivars could play an important role in reducing the losses due to the disease. Information on the distribution and diversity of X. c pv. campestris is critical before any meaningful disease resistance screening can be done. However, little is known about the diversity and international significance of South African X. c pv. campestris strains. To assess the genetic diversity and international significance of X. c pv. campestris strains in South Africa, strains of the pathogen were obtained from cabbage growing districts in Gauteng, Mpumalanga and North West Provinces of South Africa in 2010. International strains were obtained from international culture collections. Isolates from South Africa were purified and race typed using differential sets of Brassica spp according to Nickerson–Zwaan protocols. Four races, race 1(14%), race 3 (7%), race 4 (68%) and race 6 (10%) of the pathogen were identified. Repetitive DNA polymerase chain reaction–based fingerprinting using Eric– and Box–primers were used to assess the genetic diversity. Polyacrylamide gel electrophoresis allowed clear and reproducible differentiation of the PCR products. Of the amplified loci for South African isolates 5 loci were present in at least 90 % of the isolates for Eric–profiles and 6 in at least 80% of the isolates for Box–profiles. Of these prominent loci, none had corresponding high presence in international isolates. While no loci had a presence greater than 51% and 61% for Eric– and Box– profiles in international isolates, respectively, several loci among South African isolates were unique to isolates from specific geographic origin. Generated fingerprints of X. c pv. campestris were similar for the South African isolates and distinguishable from those of X. c pv. armoraciae and X. c pv. raphani reference strains. However, when international X. c pv. campestris were considered, no profile pattern was observed to be unique to international X. c pv. campestris isolates as was the case with South African isolates. Eric– and Box–PCR profiles of international isolates varied widely with some isolates having profile patterns similar to those of reference strains. Cluster analysis divided X. c pv. campestris into two major groups, the South African group and the international isolates group. The South African group could be divided into subgroups, which clustered according to the geographical origin of the isolates. The same was observed for international isolates, which generally clustered isolates according to country of origin. However, isolates from different countries also clustered together. A few X. c pv. campestris strains of international origin clustered with the South African isolates group. Furthermore, a few South African isolates were clustered in the international isolate group. Although X. c pv. campestris distribution may be unique to its geographical origin, our findings, based on the present data set, suggest wide spread of the pathogen both at national and international level. The existence of different races, genetic variability and international distribution of the pathogen should be considered when resistant crucifer cultivars are bred to control black rot of crucifers.