EPPO Global Database

Taxonomic revision of Ralstonia solanacearum

Since 1975, Ralstonia solanacearum (Smith) Yabuuchi et al. has been included in the EPPO A2 list of pests recommended for regulation as quarantine pests, and this bacterium is also a regulated pest in many EPPO member countries. R. solanacearum has a large range of economically important hosts, such as: Solanum tuberosum (potato), S. lycopersicum (tomato), S. melongena (aubergine), Musa spp. (banana), Nicotiana tabacum (tobacco), and many ornamental plants. Despite being classified as a single species, its large host range, pathogenic specialization, cultural and physiological properties, as well as its phylogeny supported the fact that R. solanacearum would probably be better described by a species complex. Before the use of molecular tools, R. solanacearum was subdivided into races (based on host range) and biovars (based on biochemical properties). However, it was later shown that these race/biovar subdivisions did not correspond to phylogenetic relationships among R. solanacearum strains, and therefore could not be retained to separate them into taxonomically distinct groups. In the 2000s, the concept of ‘species complex’ became more widely accepted and phylogenetic studies identified 4 phylotypes (based on ITS region sequence analysis) within this complex: Phylotype I (strains of Asian origin), Phylotype II (strains of South American origin), Phylotype III (strains which have evolved in African highlands), and Phylotype IV (strains of Indonesia).

More recently, the taxonomy of the R. solanacearum species complex was revised using a combination of genomic and proteomic methods, and it was proposed to separate this complex into 3 distinct species: Ralstonia pseudosolanacearum, Ralstonia solanacearum and Ralstonia syzygii. This new species delimitation was found to be congruent with the previous phylotype classification: R. pseudosolanacearum (Phylotypes I and III), R. solanacearum (Phylotype II) and R. syzygii (Phylotype IV). In addition, it was suggested that R. syzyggi was composed of 3 subspecies. In summary, the following taxonomic revision of the
R. solanacearum complex was proposed:

Ralstonia pseudosolanacearum sp. nov.
Ralstonia solanacearum (with a revised description)
Ralstonia syzygii (with a revised description)

Ralstonia syzygii subsp. syzygii subsp. nov.
Ralstonia syzygii subsp. indonesiensis subsp. nov.
Ralstonia syzygii subsp. celebesensis subsp. nov.

Table summarizing the different taxonomic changes for Ralstonia solanacearum.

Smith, 1914	Pseudomonas solanacearum (originally described as ‘Bacterium solanacearum’ by Smith in 1896)
Yabuuchi et al., 1992	Burkholderia solanacearum
Yabuuchi et al., 1995	Ralstonia solanacearum
Fegan & Prior, 2006	Ralstonia solanacearum species complex
Prior & Fegan, 2005	Phylotype I	Phylotype II	Phylotype III	Phylotype IV
Safni et al. 2014	R. pseudosolanacearum	R. solanacearum	R. pseudosolanacearum	R. syzygii

Consequences for EPPO:

As a result of these taxonomic changes, the EPPO A1 and A2 lists of pests recommended for regulation as quarantine pests (see also EPPO RS 2017/158) have been modified as follows: R. solanacearum remains on the A2 List (but now corresponds to the newly revised description), R. pseudosolanacearum is added to the A2 List, and R. syzygii is added to the EPPO A1 List.

In the EPPO Global Database, several significant changes will have to be made to reflect these taxonomic changes. This illustrates the challenges that taxonomic revisions may represent in the management of information systems (e.g. treatment of past and new information), and more generally in plant health (e.g. policy, diagnostics, management).

1) The code that was previously attributed to R. solanacearum (RALSSO) is now attributed to the species complex (i.e. R. solanacearum sensu lato). New codes have been created for R. pseudosolanacearum (RALSPS), R. solanacearum (RALSSL), R. syzygii (RALSSY), R. syzygii subsp. celebesensis (RALSSC), R. syzygii subsp. indonesiensis (RALSSI), and R. syzygii subsp. syzygii (RALSSS).

2) As it is now clear that the concepts of races/biovars should no longer be used to separate strains into taxonomic groups, the codes for R. solanacearum race 1 (PSDMS1), R. solanacearum race 2 (PSDMS2), R. solanacearum race 3 (PSDMS3) will be deactivated in the future. The information (e.g. lists of hosts, geographical distributions) currently attached to these codes will gradually be transferred to the species complex (RALSSO). Finally, new geographical distributions and new lists of host plants will have to be constructed for the newly proposed species and subspecies: R. pseudosolanacearum (RALSPS), R. solanacearum (RALSSL), R. syzygii subsp. celebesensis (RALSSC), R. syzygii subsp. indonesiensis (RALSSI), and R. syzygii subsp. syzygii (RALSSS).

Sources

Prior P, Ailloud F, Dalsing BL, Remenant B, Sanchez B, Allen C (2016) Genomic and proteomic evidence supporting the division of the plant pathogen Ralstonia solanacearum into three species. BMC Genomics 17:90. DOI 10.1186/s12864-016-2413-z

Safni I, Cleenwerck I, de Vos P, Fegan M, Sly L, Kappler U (2014) 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. International Journal of Systematic and Evolutionary Microbiology 64, 3087-3103.