Bridgehead invasion of a monomorphic plant pathogenic bacterium: X anthomonas citri pv. citri , an emerging citrus pathogen in M ali and B urkina F aso

Summary Molecular epidemiology studies further our understanding of migrations of phytopathogenic bacteria, the major determining factor in their emergence. Asiatic citrus canker, caused by X anthomonas citri pv.  citri , was recently reported in M ali and B urkina F aso, a region remote from other contaminated areas. To identify the origin and pathways of these emergences, we used two sets of markers, minisatellites and microsatellites, for investigating different evolutionary scales. Minisatellite typing suggested the introduction of two groups of strains in M ali ( DAPC  1 and DAPC  2), consistent with microsatellite typing. DAPC  2 was restricted to Bamako district, whereas DAPC  1 strains were found much more invasive. The latter strains formed a major clonal complex based on microsatellite data with the primary and secondary founders detected in commercial citrus nurseries and orchards. This suggests that human activities played a major role in the spread of DAPC  1 strains via the movement of contaminated propagative material, further supported by the frequent lack of differentiation between populations from geographically distant nurseries and orchards. A pproximate B ayesian C omputation analyses supported the hypothesis that strains from B urkina F aso resulted from a bridgehead invasion from M ali. Multi‐locus variable number of tandem repeat analysis and A pproximate B ayesian C omputation are useful for understanding invasion routes and pathways of monomorphic bacterial pathogens.