Improvement of the taxonomy of the genus Xanthomonas and especially of Xanthomonas campestris, which is subdivided into more than 125 pathovars, is discussed. Recent contributions to the taxonomy of Xanthomonas are reviewed, and on the basis of these data and unpublished data from several laboratories, the usefulness of different phenotypic, chemotaxonomic, and genotypic techniques is discussed. The heterogeneity of several X. campestris pathovars has been demonstrated by sodium dodecyl sulfate electrophoresis of whole-cell proteins and fatty acid fingerprinting. The host selectivity of the pathovars is not correlated with their relationships as revealed by DNA-DNA hybridization experiments. In order to reveal the phylogenetic relationships among X. campestris pathovars and their relationships to other Xanthomonas species, it will be necessary to perform extensive DNA-DNA homology studies as an essential part of a polyphasic approach. At present, six DNA homology groups within X. campestris have been delineated. A systematic approach to improve the taxonomy of the genus Xanthomonas is proposed.

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Two pseudomonad strains that produce a yellow cellular pigment, in addition to a diffusible fluorescent pigment on Kings medium B, were isolated from cankers on walnut trees. Biochemical properties, such as a positive oxidase reaction, a negative arginine dihydrolase reaction, and the production of a fluorescent pigment, in addition to the results of an extensive nutritional characterization study and DNA-DNA hybridization experiments, indicated that these strains belong to a new Pseudomonas rRNA group I species. This conclusion was supported by the results of a determination of the sequence of the PCR-amplified 16s rRNA gene and a comparison with the 16s rRNA genes of other bacterial species. The genomic DNAs of the strains had a base composition of 63 mol% G+C. The name Pseudomonusfluvescens sp. nov. is proposed. Strain B62 (= NCPPB 3063) is the type strain of the species.During a study of Xanthomonas campestris pv. juglandis causing cankers on walnut trees, it was observed that some of the cankers were covered with a bright yellow bacterial mass oozing from the lesions. It was assumed that this material consisted primarily of X. campestris pv. juglandis cells, since similar phenomena are not uncommon with cankers caused by other bacterial plant pathogens. However, isolation of bacteria from the lesion revealed that the largest proportion of the bacteria were fluorescent pseudomonads that produce a yellow cellular pigment. X. campestris pv. juglandis was also present, although in smaller numbers. In this paper we describe the results of a taxonomic study of this canker-inhabiting pseudomonad and ascribe this organism to a new species, for which the name Pseudomonas flavescens is proposed. MATERIALS AND METHODSBacterial strains. Two pseudomonad strains were isolated 6 weeks apart from walnut trees in Orinda, Calif. Strain B62T (T = type strain) was isolated from canker tissue, and strain B62-D5 was isolated from bacterial ooze on a canker. Other strains used for comparative purposes are listed in Table 1. YDCP medium (17) was used for isolation and maintenance of strains.Nutritional and biochemical characteristics. Denitrification, poly-P-hydroxybutyrate accumulation, the oxidase reaction, the presence of arginine dihydrolase, and hydrolysis of gelatin, starch, and Tween 80 were examined by using methods described by Palleroni and Doudoroff (23). Rutin glycosidase activity was tested by using the basal medium of Ayers et al. (2) (1 g of ammonium sulfate per liter, 0.2 g of potassium chloride per liter, 0.2 g of magnesium sulfate per liter, 15 g of agar per liter; pH 7.2) supplemented with rutin and glucose as described elsewhere (5). P-Glucosidase activity, pectolytic activity, ice nucleation activity, the ability to cause soft rot of potatoes, the production of levan, and hypersensitivity were examined by the methods described previously (9). Carbon source utilization tests were conducted by adding carbon sources (O.l%, wt/vol) to the mineral base media of Ayers et al. (2) and Palleroni and Doudoroff (23). Mos...