Spatial-temporal and quantitative analysis of growth and EPSI production by Ralstonia solanacearum in resistant and susceptible tomato cultivars

One susceptible and two resistant cultivars of tomato were tested for diffe rences in infection by Ralstonia solanacearum and for the subsequent multip lication, colonization, and production of the wilt-inducing Virulence facto r, exopolysaccharide I (EPS I). Bacterial ingress into the taproot was fast est in the susceptible cv. Marion, followed by the resistant cvs. L285 (fiv efold slower) and Hawaii 7996 (15-fold slower). Once inside the taproot, R. solanacearum colonized, to some extent, almost all regions of the resistan t and susceptible plants. However, colonization occurred sooner in the susc eptible than in the resistant cultivars, as measured by viable cell counts of bacteria in the midstems. Rates of multiplication and maximum bacterial cell densities were also greater in the susceptible than in the resistant c ultivars. Growth experiments utilizing xylem fluid from infected and uninfe cted plants indicated that neither antimicrobial activities nor reduced lev els of growth-supporting nutrients in the xylem fluids were responsible for the reduced bacterial multiplication in the resistant cultivars. Quantific ation of EPS I in the infected plants, using an enzyme-linked immunosorbent assay, revealed that the bacterial populations in the susceptible cultivar produced greater amounts of EPS I per plant than those in the resistant cu ltivars. Immunofluorescence microscopy using antibodies against either EPS I or R. solanacearum cells revealed that bacteria and EPS I were distribute d throughout the vascular bundles and intercellular spaces of the pith in t he susceptible cultivar, whereas in the resistant cultivars, bacteria and E PS I were restricted to the vascular tissues.