Submitted to: Systematic and Applied Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/1/2005
Publication Date: 7/1/2005
Citation: Schaad, N.W., Postnikova, E., Lacy, G., Sechler, A.J., Agarkova, I., Stromberg, P., Strombuer, V., Vidaver, A. 2005. Reclassification of xanthomonas species pathogenic on citrus. Systematic and Applied Microbiology. v. 28. p.494-518. Interpretive Summary: Bacterial canker of citrus is a serious disease of citrus worldwide. The disease is currently being eradicated in Florida. Five forms of the disease have been described, cankers 'A', 'B', 'C', 'D', and 'E'. Although considerable genetic diversity exists between the five pathogens, they are currently classified as different pathogen (pathovar) forms of the same species, Xanthomonas campestris (=X. axonopodis). Based on genetic analysis (DNA-DNA relatedness assays, sequencing of the 16S-23S ribosomal intergenic spacer (ITS) regions, and amplified fragment length polymorphism (AFLP)) and phenotypic tests, we propose the five citrus pathogens be placed into three separate species. We propose the 'A', 'B, C, D', and 'E' strains be named X. smithii subsp. citri, X. fuscans subsp. aurantifolii, and X. alfalfae pv. citrumelo, respectively.
Technical Abstract: 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 between 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 (Tm-15C), 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. Taxon I and II strains share over 70% DNA relatedness to X. campestris pv. malvacearum and X. campestris pv. phaseoli var. fuscans, respectively (at Tm-15C). 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 taxon I and III strains. Taxon I and III strains utilize arabinose, maltose, and lactose and liquefy gelatin whereas taxon II strains do not. Taxon I strains hydrolyze pectate (pH 7.0 and 9.0) whereas most Taxon III strains do not. Taxon III strains utilize melezitose and 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, X. smithii subsp. citri (ex Smith) Hasse species nov., subsp. nov. nom. rev, X. fuscans subsp. aurantifolii (ex Burkholder) Gabriel et al., emend., species nov., nom. rev. and X. alfalfae pv. citrumelo (ex Riker and Jones) Gabriel et al., 1987 nov. comb. pathovar nov.