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.

Bacterial leaf blight of aroids is caused by a heterogeneous group of xanthomonads listed as Xanthomonas axonopodis pv. dieffenbachiae (Xad) on the EPPO A2 quarantine list. Recently, Xad strains were shown not to belong to X. axonopodis but to the species X. citri, X. phaseoli and X. euvesicatoria. Here, to verify the pathovar designation, 11 representative strains were tested for pathogenicity on six aroid genera. They had overlapping host ranges and only the strain isolated from Syngonium showed host specificity. The X. citri strains, isolated from various hosts, showed dissimilarity in virulence to the tested aroid genera. The X. phaseoli strains, isolated from Anthurium and Syngonium, were generally more virulent and, additionally, induced systemic infections. The X. euvesicatoria strains, isolated from Philodendron, were scored as not pathogenic on the tested aroids. Four representative strains were genome sequenced and showed a variable virulence-associated gene content. Pathogenicity to aroids was correlated with the presence of three specific T3 effector genes and with a T6SS gene sequence. Together, the phylogenetic and pathogenic differentiation among Xad strains justifies the installation of three pathovar epithets for the pathogens on aroids: X. phaseoli pv. dieffenbachiae comb. nov. for the strains isolated from Anthurium; X. phaseoli pv. syngonii comb. nov. for the strain isolated from Syngonium; and X. citri pv. aracearum comb. nov. for the strains isolated from Aglaonema, Xanthosoma and Dieffenbachia. It is proposed that phytosanitary regulations for xanthomonads on aroids are restricted to these three pathovars.

Xanthomonas axonopodis pv. dieffenbachiae, the causal agent of bacterial blight of Araceae (aroids), is a regulated pest in several countries and is included in the EPPO A2 List. Reference strains of Xanthomonas axonopodis pv. dieffenbachiae have recently been reclassified into the species Xanthomonas phaseoli, Xanthomonas citri and Xanthomonas euvesicatoria on the basis of different features, including multilocus sequence analysis, average nucleotide identity and homology in DNA–DNA hybridization analyses. Based on pathogenicity tests, Constantin et al. (2017) proposed naming the pathogens on aroids as X. phaseoli pv. dieffenbachiae, X. phaseoli pv. syngonii and X. citri pv. aracearum. Recommendations are made on how to deal with these changes for the group of pathogenic bacteria for Araceae. The name Xanthomonas axonopodis pv. dieffenbachiae on the EPPO List should be adjusted to the names proposed in the taxonomic study by Constantin et al. (2016). The current EPPO Diagnostic Standard is directed at strains pathogenic on Anthurium. They mainly belong to X. phaseoli pv. dieffenbachiae, but some also to X. citri pv. aracearum that are not detected by the EPPO Diagnostic Standard. Xanthomonas phaseoli pv. syngonii strains are also aggressive, but with a host range restricted to Syngonium. The pathogenicity specific to aroids of the bacterial isolates reclassified as Xanthomonas euvesicatoria was not confirmed and no pathovar epithet has been retained for these strains.