Genetic characterization of strains named as Xanthomonas axonopodis pv. dieffenbachiae leads to a taxonomic revision of the X-axonopodis species complex

Xanthomonas axonopodis pv. dieffenbachiae (Xad) is the causal agent of anthurium bacterial blight and listed as an A2 quarantine organism by EPPO. However, the name Xad covers a variety of strains. Here, 25 Xad strains and 88 phylogenetically related strains, including Xanthomonas type strains and representatives of other pathovars, were examined using a polyphasic taxonomic approach. Multilocus sequence analysis of seven genes showed that strains isolated from Dieffenbachia, Philodendron and Anthurium cluster into three phylogenetic groups (PG I, II and III), while the type strain of X. axonopodis clustered into a fourth group (PG IV). PG I included the type strains of X. citri subsp. citri, X. citri subsp. malvacearum, X. fuscans subsp. fuscans and X. fuscans subsp. aurantifolii. PG II included the type strains of X. euvesicatoria, X. perforans, X. alfalfae subsp. alfalfae and X. alfalfae subsp. citrumelonis. PG III included the type strains of X. phaseoli. Each PG was shown to represent a single species, based on average nucleotide identity values, DNA-DNA hybridization data and phenotypic characteristics. Therefore, strains named as Xad belong to PG I, PG II and PG III, and not to X. axonopodis (PG IV). Taxonomic proposals are made: emendations of the descriptions of X. citri, X. phaseoli and X. axonopodis, to encompass the strains of PG I, PG III and PG IV, respectively; and reclassification of X. perforans and X. alfalfae as X. euvesicatoria and emendation of the description of X. euvesicatoria to encompass all strains of PG II.