Reclassification of Xanthomonas campestris pv. citri (ex Hasse 1915) Dye 1978 forms A, B/C/D, and E as X. smithii subsp citri (ex Hasse) sp nov nom. rev. comb. nov., X. fuscans subsp aurantifolii (ex Gabriel 1989) sp nov nom. rev. comb. nov., and X. alfalfae subsp citrumelo (ex Riker and Jones) Gabriel et al., 1989 sp nov nom. rev. comb. nov.;: X. campestris pv malvacearum (ex Smith 1901) Dye 1978 as X. smithii subsp smithii nov comb. nov nom. nov.;: X. campestris pv. alfalfae (ex Riker and Jones, 1935) Dye 1978 as X. alfalfae subsp alfalfae (ex Riker et al., 1935) sp nov nom. rev.;: and "var. fuscans" of X. campestris pv. phaseoli (ex Smith, 1987) Dye 1978 as X. fuscans subsp fuscans sp nov.

Bacterial canker of citrus is a serious disease of citrus worldwide. Five forms of the disease have been described, cankers "A", "B", "C", "D", and "E". Although considerable genetic diversity has been described among the causal agents of the five forms of citrus canker and supports multiple taxons, the causal agents currently are classified as pathovars citri ("A"), aurantifolii ("B/C/D") and citrumelo ("E") of a single species, Xanthomonas campestris pv. citri (or X. axonopodis pv. citri). To determine the taxonomic relatedness among strains of X campestris pv. citri, we conducted DNA-DNA relatedness assays, sequenced the 16S-23S intergenic spacer (ITS) regions, and performed amplified fragment length polymorphism (AFLP) analysis, using 44 strains representative of the five recognized forms of citrus canker. Under stringent DNA reassociation conditions (T-m,-15 degrees C, three distinct genotypes of citrus pathogens were revealed: taxon I included all "A" strains; taxon II contained all "B",,"C", and "D" strains; and taxon III contained all "E" strains. The three citrus taxa showed less than 50% (mean) DNA-DNA relatedness to each other and less than 30% (mean) to X campestris pv. campestris and X axonopodis pv. axonopodis. Taxa I and II strains share over 70% DNA relatedness to X. campestris pv. malvacearum and X. campestris pv. phaseoli var. fuscans, respectively (at T-m- 15 degrees C). Taxon III strains share 70% relatedness to X campestris pv. alfalfae. Previous and present phenotypic data support these DNA reassociation data. Taxon II strains grow more slowly on agar media than taxa I and III strains. Taxa I and III strains utilize maltose, and liquefy gelatin whereas taxon II strains do not. Taxon I strains hydrolyze pectate (pH 7.0) whereas Taxon II strains do not. Taxon III strains utilize raffinose whereas Taxon I strains do not. Each taxon can be differentiated by serology and pathogenicity. We propose taxa I, II, and III citrus strains be named, respectively, Xanthomonas smithii subsp. citri (ex Hasse, 1915) sp. nov. nom. rev. comb. nov., Xanthomonasfuscans subsp. aurantifolii (ex Gabriel et al., 1989) sp. nov. nom. rev. comb. nov., and Xanthomonas alfatfae subsp. citrumelo (ex Riker and Jones) Gabriel et al., 1989 nov. rev. comb. nov. Furthermore, based on the analysis of 40 strains of 19 other xanthomonads, we propose to reclassify X. campestris pv. malvacearum (ex Smith, 1901) Dye 1978 as X. smithii subsp. smithii sp. nov. comb. nov. nom. nov.; X. campestris pv. atfalfae (ex Riker and Jones) Dye 1978 as X. alfalfae subsp. alfalfae (ex Riker et al., 1935) sp. nov. nov. rev.; and "var. fuscans" (ex Burkholder 1930) of X. campestris pv. phaseoli (ex Smith, 1897) as X fuscans subsp. fuscans sp. nov. (c) 2005 Elsevier GmbH. All rights reserved.