Metabolic diversity of Xanthomonas axonopodis pv. vignicola, causal agent of cowpea bacterial blight and pustule

Fifty-five strains of Xanthomonas axonopodis pv. vignicola, isolated from blight and pustule symptoms of cowpea leaves, originating from 11 countries, were characterized for their carbon-source metabolization pattern using the Biolog GN microplate system. Great variation was found between strains according to origin. Dextrin, glycogen and succinamic acid were not used by strains from Benin, Uganda or Thailand, but by all the other strains (excluding two strains from Mozambique), whereas N-acetyl-D-glucosamine and malonic acid were used by the strains from Benin, Uganda and Thailand, but generally not by the other strains. The strains from Benin, Uganda and Thailand, as well as strains from Venezuela, Brazil and Mozambique, clustered separately from the others in multivariate analysis. Nineteen substrates were used by all the strains, 47 not by any strain and 29 only by some strains. No considerable differences were found between strains isolated from blight symptoms and from pustules. Virulence of strains was not related to the metabolic pattern. The Biolog database was not representative of the diversity of X. axonopodis pv. vignicola, since all strains were identified as Xanthomonas campestris, although belonging to eight pathovars, while only eight of nine strains from Benin and both strains from Thailand were identified as X. campestris pv. vignicola. The Biolog system appeared to be useful for characterizing the diversity of X. axonopodis pv. vignicola strains. A set of representative strains based on metabolic and molecular diversity, virulence and geographic origin is suggested for screening for resistant cowpea cultivars.