Identification and characterization of Ralstonia spp. causing bacterial wilt disease of vegetables in Mali

Ralstonia solanacearum species complex is the most destructive and economically important bacterial pathogen of many plant species around the world. It is a particular constraint to the production of the solanaceous vegetables such as tomato, chilli pepper and African eggplant in West Africa. Its broad host range, ability to survive in the soil for long periods and ability to sustain latent infection, make it difficult to control. Identification, characterization and mapping the distribution of the causal Ralstonia spp. strains are necessary to help design effective control strategies. Wilted tomato, African eggplant and pepper plant samples were collected from fields in different regions of Mali. The causal bacterial strains were isolated from the samples and all were identified as Ralstonia pseudosolanacearum. Multiplex PCR using four phylotype specific primer pairs identified the presence of phylotype I strains distributed across all the vegetable production areas whereas phylotype III strains were limited to areas of Sikasso, Koulikoro and Segou to the south and east of Bamako city. Phylogenetic analysis of part of the conserved endoglucanase virulence gene sequences revealed four sequevars with most phylotype I strains identified as sequevars 46 and 31 with only a few as sequevar 14 and one sequevar 18. The phylotype III strains were all in the sequevar 23–48 group. Pathogenicity testing of a selected subset of the strains isolated in Mali showed them all to be pathogenic to the susceptible tomato cultivar Roma. To our knowledge, this is the first study on the molecular diversity within the Ralstonia spp. complex in Mali. This information is important for developing wilt management strategies including breeding for resistance and specifying quarantine restrictions.