To control bacterial wilt (Ralstonia solanacearum, phylotype IIB/4NPB), the antimicrobial effect of Allium fistulosum aqueous extract was assessed as a preplant soil treatment. Three concentrations of extract (100, 50, and 25%, 1:1 [wt/vol]) were evaluated by in vitro inhibition assay and in vivo experiments in a growth chamber. In vitro, A. fistulosum (100 and 50%) suppressed growth of R. solanacearum. Preplant treatment of the soil with A. fistulosum extract significantly reduced the R. solanacearum populations. No pathogen was detected in the soil after treatment with 100% concentrated extract from the third day after application until the end of the experiment. A. fistulosum also significantly reduced the incidence of tomato bacterial wilt. In the untreated control, the disease affected 61% of the plants whereas, with 100 and 50% extracts, only 6 and 14% of the plants, respectively, were affected. These results suggest that A. fistulosum extracts could be used in biocontrol-based management strategies for bacterial wilt of tomato.