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Integrated Approach for Detection of Nonculturable Cells of Ralstonia solanacearum in Asymptomatic Pelargonium spp. Cuttings

Ralstonia solanacearum (biovar 2, race 3) is a soil and water-borne pathogen that causes serious diseases in several solanaceous hosts. It can also infect geranium plants, posing an important threat to their culture when latently infected cuttings are imported from countries where the pathogen is endemic. R. solanacearum can be present in very low numbers in asymptomatic geranium cuttings, and/or in a particular stressed physiological state that escapes direct isolation on the solid media usually employed. Consequently, an integrated protocol has been developed to analyze asymptomatic geranium cuttings routinely. The first screening tests include isolation and co-operational-polymerase chain reaction (Co-PCR), based on the simultaneous and co-operational action of three primers from 16S rRNA of R. solanacearum. This method was selected as the most sensitive one, able to detect only 1 cell/ml including nonculturable cells. When isolation is negative but Co-PCR is positive, the bioassay in tomato plants is proposed, since stressed bacterial cells or those present in low numbers that do not grow on solid media can be recovered from inoculated tomato plants and retain pathogenicity. This methodology has been demonstrated to be useful and has allowed us to assess the relevance of the physiological status of bacterial cells and its implications in detection. It also reveals the risk of introducing R. solanacearum through asymptomatic geranium material when relying only on bacterial isolation.