Ralstonia solanacearum causes bacterial wilt of tomato and limits the crop production, and antagonistic microorganisms use to suppress the disease, of which Trichoderma and Pseudomonas species are the most effective agents to control bacterial wilt. In the present study, attempt was made to isolate these two microorganisms to evaluate their effectiveness to control R. solanacearum, the causal agent of bacterial wilt disease of tomato under greenhouse conditions. Thus R. solanacearum, Pseudomonas and Trichoderma spp. were isolated from wilted and healthy tomato plants grown from farmer's field in Ziway and Meki, Oromia Ethiopia. The virulence of the pathogen and the antagonistic effect of the bacteria and fungi were evaluated against R. solanacearum in vitro and in vivo condition. Based on the in vitro results the best two isolates were selected to show their antagonistic effect under greenhouse condition in single and combined designs. The result showed the pathogenicity test of the isolates were evaluated under greenhouse condition, and isolate AAURS1 showed highest virulence (75%) followed by isolate APPRCRS2 with pathogenicity of 50%. With regard to antagonism test, isolates AAURB20 and AAUTR23 showed the highest inhibition against R. solanacearum with inhibition zone of 16 and 15 mm, respectively. Among the treatments co-inoculation was more effective and reduced disease incidence by 13.33% and increased the bio-control efficacy by 72.22% when compared with individual treatment and negative control. The isolates significantly increased the plant height and dry weight by 72.33 cm and 12.18 g, respectively. Thus, the combined use of the biocontrol agents significantly reduced the incidence of tomato bacterial wilt disease. However, their performance should be evaluated using other yield parameters under field conditions to produce healthy tomato seedling to minimize the use of chemicals and reduce environmental pollution.
Key words: Biocontrol, Pseudomonas, Ralstonia solanacearum, Trichoderma.