Polyphasic taxonomic revision of the Ralstonia solanacearum species complex: proposal to emend the descriptions of Ralstonia solanacearum and Ralstonia syzygii and reclassify current R. syzygii strains as Ralstonia syzygii subsp. syzygii subsp. nov., R. solanacearum phylotype IV strains as Ralstonia syzygii subsp. indonesiensis subsp. nov., banana blood disease bacterium strains as Ralstonia syzygii subsp. celebesensis subsp. nov. and R. solanacearum phylotype I and III strains as Ralstonia pseudosolanacearum sp. nov.

Abstract

The Ralstonia solanacearum species complex has long been recognized as a group of phenotypically diverse strains that can be subdivided into four phylotypes. Using a polyphasic taxonomic approach on an extensive set of strains, this study provides evidence for a taxonomic and nomenclatural revision of members of this complex. Data obtained from phylogenetic analysis of 16S-23S rRNA ITS gene sequences, 16S–23S rRNA intergenic spacer (ITS) region sequences and partial endoglucanase (egl) gene sequences and DNA–DNA hybridizations demonstrate that the R. solanacearum species complex comprises three genospecies. One of these includes the type strain of Ralstonia solanacearum and consists of strains of R. solanacearum phylotype II only. The second genospecies includes the type strain of Ralstonia syzygii and contains only phylotype IV strains. This genospecies is subdivided into three distinct groups, namely R. syzygii , the causal agent of Sumatra disease on clove trees in Indonesia, R. solanacearum phylotype IV strains isolated from different host plants mostly from Indonesia, and strains of the blood disease bacterium (BDB), the causal agent of the banana blood disease, a bacterial wilt disease in Indonesia that affects bananas and plantains. The last genospecies is composed of R. solanacearum strains that belong to phylotypes I and III. As these genospecies are also supported by phenotypic data that allow the differentiation of the three genospecies, the following taxonomic proposals are made: emendation of the descriptions of Ralstonia solanacearum and Ralstonia syzygii and descriptions of Ralstonia syzygii subsp. nov. (type strain R 001T = LMG 10661T = DSM 7385T) for the current R. syzygii strains, Ralstonia syzygii subsp. indonesiensis subsp. nov. (type strain UQRS 464T = LMG 27703T = DSM 27478T) for the current R. solanacearum phylotype IV strains, Ralstonia syzygii subsp. celebesensis subsp. nov. (type strain UQRS 627T = LMG 27706T = DSM 27477T) for the BDB strains and Ralstonia pseudosolanacearum sp. nov. (type strain UQRS 461T = LMG 9673T = NCPPB 1029T) for the strains of R. solanacearum phylotypes I and III.