Genomic and phenotypic characterization of Xanthomonas cynarae sp nov., a new species that causes bacterial bract spot of artichoke (Cynara scolymus L.)

A bacterial disease of artichoke (Cynara scolymus L.) was first observed in 1954 in Brittany and the Loire Valley, France. This disease causes water-s oaked spots on bracts and depreciates marketability of the harvest. Ten str ains of the pathogen causing bacterial spot of artichoke, previously identi fied as a member of the genus Xanthomonas, were characterized and compared with type and pathotype strains of the 20 Xanthomonas species using a polyp hasic study including both phenotypic and genomic methods. The ten strains presented general morphological, biochemical and physiological traits and G +C content characteristic of the genus Xanthomonas. Sequencing of the 16S r RNA gene confirmed that this bacterium belongs to the genus Xanthomonas, an d more precisely to the Xanthomonas campestris core. DNA-DNA hybridization results showed that the strains that cause bacterial spot of artichoke were 92-100 % related to the proposed type strain CFBP 4188(T) and constituted a discrete DNA homology group that was distinct from the 20 previously desc ribed Xanthomonas species. The results of numerical analysis were in accord ance with DNA-DNA hybridization data. Strains causing the bacterial bract s pot of artichoke exhibited consistent determinative biochemical characteris tics, which distinguished them from the 20 other Xanthomonas species previo usly described. Furthermore, pathogenicity tests allowed specific identific ation of this new phytopathogenic bacterium. Thus, it is concluded that thi s bacterium is a new species belonging to the genus Xanthomonas, for which the name Xanthomonas cynarae is proposed. The type strain, CFBP 4188T, has been deposited in the Collection Francaise des Bacteries Phytopathogenes (C FBP).