<i>Ralstonia solanacearum</i> Species Complex in Australia

The <i>Ralstonia solanacearum</i> 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 <i>R. solanacearum</i> (phylotype II), <i>R. pseudosolanacearum</i> (phylotype I and III), and <i>R. syzygii</i> (phylotype IV). Knowing which <i>Ralstonia</i> species and subspecies are established in Australia is important to Australia's biosecurity and market access. The goal of this study was to analyze Australia's <i>Ralstonia</i> culture collections and to assign the isolates to the modern taxonomic groups. The results shed light on the identity, distribution, and pathogenicity of the <i>Ralstonia</i> strains in Australia. <i>Ralstonia solanacearum</i>, <i>R. pseudosolanacearum</i> phylotype I, and <i>R. syzygii</i> phylotype IV-11 are present in Australia but have limited geographic ranges. We identified two aberrant RSSC strains that have genetic similarity to <i>R. syzygii</i> 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 <i>Ralstonia</i> lineages remain undetected in Australia, providing evidence that they are absent. These include <i>R. pseudosolanacearum</i> phylotype III and the phylotype I mulberry infecting strains; <i>R. solanacearum</i> strains IIC and the Moko causing strains; and <i>R. syzygii</i> subsp. <i>celebesensis</i>, and <i>R. syzygii</i> subsp. <i>syzygii</i>. This study fulfilled Koch's postulates for the Australian strains, <i>R</i>. <i>solanacearum</i> wilted potato plants, and <i>R. pseudosolanacearum</i> wilted blueberry plants, the hosts from which they were initially isolated. The data supports the hypothesis that Australia has native and introduced strains of <i>Ralstonia</i>.