Bactericidal or bacteriostatic effect of chlorine dioxide (ClO₂) on the plant pathogenic bacteria occurring in hydroponics was examined. In distilled water, 0.1 ppm ClO₂ showed an effect on Pseudomonas cichorii (bacterial rot of lettuce), Pectobacterium carotovorum subsp. carotovorum (bacterial soft rot of lettuce), Pec. c. ssp. odoriferum (causing similar symptoms on lettuce as are caused by ssp. carotovorum), and on Ralstonia solanacearum (bacterial wilt of tomato) in a minute. In hydroponic culture fluid, 0.5 or 1.0 ppm ClO₂ showed an effect on them in a minute. Thus, ClO₂ is a potential control agent or a disinfectant for the diseases occurring in hydroponics.