Induction of strains of Ralstonia solanacearum that do not produce extracellular polysaccharides (EPS) in soil and effect on disease suppressiveness

Abstract

To suppress bacterial wilt of tomato caused by Ralstonia solanacearum with soil administration, soil conditions that influence the production of extracellular polysaccharides (EPS) in R. solanacearum were surveyed, and the prevention of this disease by strains that do not produce EPS (EPS^- strains) was investigated. The culture conditions, using either liquid medium or sterilized soil were investigated to induce the phenotype conversion (PC) in relation to the reduction or loss of EPS production in R. solanacearum. EPS^- colonies were frequently detected under unfavorable culture conditions, such the use of a rather alkaline medium (pH8 and pH8.5), higher temperature (37℃) or still culture rather than shaking culture. The culture conditions under which the EPS^- colonies were detected with a high frequency varied depending on the strains tested. In the case of culture conditions of the bacterium using either sterilized or non-sterilized soil, EPS^- colonies were also observed with a high frequency in all the cases, including, acidic soil, higher temperature, and soil moisture exceeding the maximum water holding capacity. Disease severity was evaluated after the EPS-producing strains were inoculated either alone or simultaneously with EPS-non-producing strains by mixing into soil where subsequently tomato was planted. When EPS-producing strains and EPS-non-producing strains were simultaneously inoculated, the damage was less serious than in the case of inoculation with EPS-producing strains alone. When the ratio of EPS-non-producing cells to EPS-producing cells increased, the effect on disease suppression was more pronounced. If the phenotype conversion of R. solanacearum living in soil could be induced with a high frequency by soil administration, bacterial wilt disease may be effectively suppressed.