Ralstonia solanacearum species complex in Australia

Abstract

The Ralstonia solanacearum species complex (RSSC) causes vascular wilt of many crops and is considered one of the most destructive plant pathogenic bacteria worldwide. The species complex was recently resolved into a stable taxonomy of three species aligning with the previously determined phylotypes, namely R. solanacearum (phylotype II), R. pseudosolanacearum (phylotype I and III), and R. syzygii (phylotype IV). Knowing which Ralstonia species and subspecies are established in Australia is important to Australia’s biosecurity and market access. The goal of this study was to analyse Australia’s Ralstonia culture collections and to assign the isolates to the modern taxonomic groups. The results shed light on the identity, distribution, and pathogenicity of the Ralstonia strains in Australia. Ralstonia solanacearum, R. pseudosolanacearum phylotype I, and R. syzygii phylotype IV-11 are present in Australia but have limited geographic ranges. We identified two aberrant RSSC strains that have genetic similarity to R. syzygii based on sequevar analysis, but do not yield a phylotype IV multiplex PCR band, similar to the known aberrant strain ACH732. The aberrant strains may represent a novel species. Three new sequevars were determined, 72, 73 and 74. Several Ralstonia lineages remain undetected in Australia, providing evidence that they are absent. These include R. pseudosolanacearum phylotype III and the phylotype I mulberry infecting strains; R. solanacearum strains IIC and the Moko causing strains; and R. syzygii subsp. celebesensis, and R. syzygii subsp. syzygii. This study fulfilled Koch’s postulates for the Australian strains, R. solanacearum wilted potato plants, and R. pseudosolanacearum wilted blueberry plants, the hosts from which they were initially isolated. The data supports the hypothesis that Australia has native and introduced strains of Ralstonia.