The antagonistic ability of eleven bacterial isolates against Sclerotium cepivorum, causal agent of onion white rot disease (OWR), was determined in vitro and the best five isolates were identified as Bacillus pumilus (BP1 and BP2) and B. marinus (BM1, BM2 and BM3). The in vitro antagonistic efficacy ranged between 11.1 to 68.9% depending on the concentration of the bacterial cell suspension and number of applied sprays under greenhouse and field conditions. It was found that using double sprays with B. pumilus (BP2) in the greenhouse experiment gave the highest efficacy in reducing OWR (93.8%) at the concentrations of 10 and 20 ml/l. In case of field experiment there was no significant deference between number of sprays or the tested bacteria in reducing infection percentage of OWR. The significant difference was reported only between the concentrations of the bacterial suspension during the two growing seasons 2008/2009 – 2009/2010. In case of using one spray during season 2008/2009, the most effective bacteria in reducing OWR was B. pumilus (BP2) giving 64.7 and 61.6% efficacy when used at the concentrations 20ml/l and 10 ml/l, respectively, followed
by B. marinus (BM3 ) which gave 58.2% at the concentration 20ml/l. On the other hand, when two sprays of bacterial suspension were used B. marinus (BM1) and B. pumilus (BP1) gave the highest efficacy in reducing OWR reached 63.7 and 63.4%, respectively. During season 2009/2010 B. pumilus (BP2) was the most effective bacteria reached 80.0% efficacy followed by B. marinus (BM3 ) recorded 76.9% when both used at the concentration 20ml/l for one spray. When two sprays were used, B. marinus (BM2) gave the highest percentage of efficacy in reducing OWR (87.3%). Data showed that applying the tested bacteria at any concentration, either once or twice, significantly increased onion bulb yield compared with control treatment during the two growing seasons.