Ralstonia solanacearum isolated from Rosa spp. in the Netherlands is closely related to isolates from other sources in the Netherlands

In 2018, during an annual survey in the Netherlands, Ralstonia solanacearum phylotype II (phy II) was found in asymptomatic greenhouse rose plants at three geographic locations. These findings were remarkable, since previous findings of Ralstonia sp. in rose always concerned Ralstonia pseudosolanacearum phylotype I (phy I). Since R. solanacearum phy II had never been found in rose before, there was no information available on the virulence of R. solanacearum phy II on rose. Therefore, in this study R. solanacearum phy II isolates PD 7421 and PD 7394, isolated in 2018 from asymptomatic ornamental rose (Rosa spp.), were assessed for their virulence in two rose cultivars (Armando and Red Naomi) at two temperatures. Disease severity was assessed and re-isolations were performed from symptomatic plants, in order to confirm the presence of R. solanacearum. Plants showing no symptoms at the end of the experiment were also included for the re-isolations in order to evaluate the presence of R. solanacearum phy II in a latent state. Roses inoculated with Ralstonia pseudosolanacearum phy I isolate PD 7123, an isolate previously shown to result in high disease severity in ornamental rose, showed severe symptoms at both temperatures. In contrast, no typical symptoms were acquired for R. solanacearum phy II isolates PD 7421 and PD 7394 on the two rose cultivars included in this study, irrespective of the temperature. However, latent infections upon inoculation of these isolates on rose did occur. R. solanacearum phy II is known as a major potato pathogen, causing brown rot in potato. Whole genome multilocus sequence typing analysis demonstrated that the R. solanacearum phy II isolates from rose were closely related to R. solanacearum phy II isolates previously found in seed potato and surface water in the Netherlands. Because of this close genetic relatedness, the virulence of PD 7421 and PD 7394 was also assessed in potato plants, where both isolates caused severe disease symptoms on both the above ground plant and the daughter tubers. This implies that rose can act as a reservoir for R. solanacearum phy II and, in this way, asymptomatic rose plants could potentially be involved in spreading the disease.