An extensive phenotypic description and an improved classification and nomenclature of the genus Xanthomonas are presented. A total of 266 strains obtained from different geographical areas, including representative strains of all species of the genus Xanthomonas and most pathovars of Xanthomonas campestris, as well as strains which might be genetically related to the genus Xanthomonas, were examined for 295 morphological, biochemical, and physiological features. Similarities among the strains were expressed numerically by using the coefficient of Sokal and Michener. Clustering was performed by using the unweighted average pair group method. The conclusions described below were reached. (i) The genus Xanthomonas comprises at least the following eight phena: X. campestris, Xanthomonas albilineans, Xanthomonas axonopodis, Xanthomonas fragariae, Xanthomonas populi, Xanthomonas maltophilia, Xanthomonas oryzae Swings et al. 1990, and X. campestris pv. graminis Egli and Schmidt 1982 [not X. campestris pv. graminis (Egli et al. 1975) ISPP List 1980]. (ii) X. populi (Rid? 1958) Rid? and Rid? 1978 is a separate species. (iii) X. maltophilia Swings et al. 1983 forms a separate species. (iv) X. campestris pv. oryzae ISPP List 1980 can no longer be regarded as pathovar of X. campestris, and its recent reclassification as a new species, X. oryzae (Swings et al., Int. J. Syst. Bacteriol. 40:309-311, 1990), is supported. (v) X. campestris pv. graminis Egli and Schmidt 1982 [not X. campestris pv. graminis (Egli et al. 1975) ISPP List 1980] seems to form a separate complex of highly related pathovars obtained from members of the Poaceae; the taxonomic implications of this are discussed. (vi) Strains of nearly all X. campestris pathovars cluster together in the X. campestris phenon. Within this species we were able to differentiate some entities on phenotypic grounds; these groups sometimes corresponded to named pathovars (e.g., X. campestris pv. manihotis, X. campestris pv. cassavae, X. campestris pv. phlei). In several other cases, pathovars were found to be heterogeneous. (vii) A number of dubious Pseudomonas species were identified as members of or as being close to Xanthomonas species. Both Pseudomonas betle and Pseudomonas hibiscicola are synonyms of X. maltophilia. We also confirmed that Pseudomonas mangiferaeindicae, Pseudomonas vitiswoodrowii, and Pseudomonas gardneri belong to X. campestris. (viii) Forty phenotypic features allow the differentiation of the eight Xanthomonas phena. (ix) A number of additional features of the genera Xanthomonas and Xylophilus are described.

The genetic diversity of 55 xanthomonad strains isolated from passion fruit plants (Passiflora spp.) and identified as Xanthomonas campestris pv. passiflorae was initially assessed by randomly amplified polymorphic DNA (RAPD) analysis. The strains showed a high level of polymorphism with almost unique fingerprints. Fifteen clusters with a similarity of approximately 70% were identified, three of which were prevalent. There was a correlation between the clusters and the geographic origin of the strains. A representative strain of each cluster, together with the pathovar reference strain, were used to verify the relationships of these strains to 18 Xanthomonas species and Pseudomonas syringae pv. passiflorae. All Xanthomonas species yielded a unique RAPD profile and no consistent relatedness to the X. campestris pv. passiflorae strains was observed. Amplification products were also analysed by repetitive (rep) primers (BOX, ERIC and REP), RFLP of the 16S-23S rDNA intergenic spacer and SDS-PAGE of whole-cell proteins. All of these approaches generated profiles characteristic for each Xanthomonas species but the taxonomic position of the X. campestris pv. passiflorae strains could not be unequivocally assigned. Finally, DNA-DNA hybridization allowed a sound taxonomic allocation of the strains to Xanthomonas axonopodis pv. passiflorae.