An organic hydroponic system that we developed has potential to control root diseases including bacterial wilt of hydroponically grown tomato. In inoculation tests with Ralstonia solanacearum during tomato plant cultivation in conventional inorganic hydroponics and in our organic system, many of the tomato seedlings in the conventional system wilted and died, but none of the seedlings in the organic hydroponics wilted or developed any symptoms, suggesting that the organic system can suppress this bacterial wilt disease. Interestingly, a rhizosphere biofilm, formed only on roots in the organic hydroponic system, may be responsible for the suppression of the bacterial wilt.
The rhizosphere microbial community in a hydroponics system with multiple parallel mineralization (MPM) can potentially suppress root-borne diseases. This study focused on revealing the biological nature of the suppression against Fusarium wilt disease, which is caused by the fungus Fusarium oxysporum, and describing the factors that may influence the fungal pathogen in the MPM system. We demonstrated that the rhizosphere microbiota that developed in the MPM system could suppress Fusarium wilt disease under in vitro and greenhouse conditions. The microbiological characteristics of the MPM system were able to control the population dynamics of F. oxysporum, but did not eradicate the fungal pathogen. The roles of the microbiological agents underlying the disease suppression and the magnitude of the disease suppression in the MPM system appear to depend on the microbial density. F. oxysporum that survived in the MPM system formed chlamydospores when exposed to the rhizosphere microbiota. These results suggest that the microbiota suppresses proliferation of F. oxysporum by controlling the pathogen's morphogenesis and by developing an ecosystem that permits coexistence with F. oxysporum.
Ralstonia solanacearum, causing bacterial wilt disease, just after culturing in medium could not be detected in 35 g/L saline solution from 2 days after addition. After keeping R. solanacearum in sterile water for more than 1 month at 25°C, the pathogen survived in 35 g/L saline solution for more than 10 days. For killing the salt-tolerant pathogen, it is necessary to bring it into higher concentrate saline solution. The addition of sugar (glucose or sucrose) into 35 g/L saline solution killed the salt-tolerant pathogen within 4 days. Furthermore, the addition of acids (citric acid or acetic acid), alkali (sodium bicarbonate or sodium carbonate) or surfactant (sodium dodecyl sulfate) into 35 g/L saline solution resulted in killing the salt-tolerant pathogen within 1 day. Especially, addition of 20 mg/L sodium carbonate into saline solution was sufficient to kill the salt-tolerant pathogen within 1 day.
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