Phenotypic and genetic diversity of Xanthomonads associated with Anacardiaceae

We present here a description of phenotypic and genetic diversity of Xanthomonads associated with Brazilian pepper in Reunion Island, ambarella in the French West Indies, and mango in 14 countries. Biochemical and physiological profiles, carbohydrate utilization fingerprints and sensitivity to antibiotics and heavy metal salts have been determined to assess phenotypic diversity. Relationships among strains were also examined by isozyme analysis of esterase, phosphoglucomutase and superoxide dismutase and by RFLP analysis using a cluster of hrp genes as a probe. Grouping of strains was consistent for the 3 approaches used. These techniques allowed to differentiate: Apigmented strains isolated from mango in Brazil, Apigmented strains isolated from mango in all other countries and from Brazilian pepper in Reunion Island, Apigmented strains isolated from ambarella in the French West Indies, Yellow-pigmented strains from mango, with a high heterogeneity among strains. These results indicate that there is a need for a comprehensive taxonomic evaluation of xanthomonads associated with anacardiaceae using a polyphasis approach. Our data suggest that i) apigmented strains isolated from mango (except Brazil) and Brazilian pepper form a homogenous group representative of the pathovar magiferaeindicae and ii) other strains (i.e. yellow-pigmented strains from mango and apigemented strains from mango in Brazil and from ambarella in the French West Indies) are probably members of pathovars other than mangiferaeindicae. The incidence of these strains on mango production remains mostly unknown. The genetic diversity of strains representative of pv. mangiferaeindicae was further analyzed the RFLP analysis using and avirulence gene and an insertion sequence (IS) as probes. Four main lineages were identified, that are in correlation with geographic origin of strains and their host of origin (mango or Brazilian pepper). As analysis of the pathogenicity of strains indicated that they are also diverse for their pathogenicity : strains were more aggressive when inoculated on the host species from which they originated. The population structure of xanthomonads associated with mango, characterized by their phenotypic, genetic and pathogenic diversity, should be taken in consideration when evaluating germplasm in order to obtain efficient and durable resistance against a range of bacterial pathogens as comprehensive as possible.