PHENOTYPIC AND GENETIC DIVERSITY OF XANTHOMONADS ASSOCIATED WITH ANACARDIACEAE

Abstract

Bacterial black spot (BBS), associated with Xanthomonas pv. mangiferaeindicae, is a serious threat to mango production in many tropical and subtropical areas. Chemical control is unsatisfactory and integrated management strategies, including the use of rigorous prophylactic measures, adapted cultural practices and weakly susceptible cultivars, are used. Breeding prograrns have been developed in several countries and a thorough evaluation of the susceptibility to BBS of new cultivars is therefore important. 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 from 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 with the 3 methods used. These techniques allowed to clearly 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 from ambarella in the French West Indies, - Yellow-pigmented strains from mangd, with a high heterogeneity among strains. These results indicate that i) there is a need of a comprehensive taxonomic evaluation of xanthomonds associated with Anacardiaceae using a polyphasic approach, ii) apigmented strains isolated from mango (except Brazil) and Brazilian pepper form a homogenous group representative of pathovar mangiferaeindicae, and ii) other strains (e.g. yellow-pigmented strains from mango and apigmented 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 by RFLP analysis using an avirulence gene and an insertion sequence (IS) as probes. Four main lineages characterizing either geographic origin of strains or host of isolation (mango vs. Brazilian pepper) were found. An analysis of the pathogenicity of strains from both host species indicated that host specialization occurs (i.e. strains were more aggressive on the host species from which they originated). The occurrence of several xanthomonads associated with mango and the diversity existing within pathovar mangiferaeindicae should be considered for germplasm evaluation. (Resume d'auteur)