A discharge plasma under water is a promising candidate to reduce the risks of bacterial infection in a hydroponics culture system. We developed a system using plasma under solution and the performance of the developed system was evaluated using tomato seedlings in the recirculation system and Ralstonia solanacearum which causes wilt disease, followed by a serious crop loss. The 15 L culture solution was heavily contaminated with R.solanacearum. After that, the discharge plasma treatment was continued for 100 min. We prepared another experimental section that was a control. The culture solution in control was contaminated with R.solanacearum without plasma treatment. Seedlings were monitored and disease index was evaluated. The result shows that the disease index of discharge group increased on the 6th day, and was 20 % on the 8th day and after. In contrast, the disease index of control increased on the 4th day and continuously increasing to be 100 % on the 8th day. The density of R. solanacearum of control was above 107 CFU/mL, in contrast, that of discharge group was below 102 CFU/mL on the initial day.