| Summary: | Several diseases affect the productivity of olive trees, including root rot disease caused by <i>Pythium</i> genera. Chemical fungicides, which are often used to manage this disease, have harmful side effects on humans as well as environmental components. Biological management is a promising control approach that has shown its great potential as an efficient eco-friendly alternative to treating root rot diseases. In the present study, the antagonistic activity of ten bacterial isolates was tested both in vitro and in planta against <i>Pythium schmitthenneri</i>, the causal agent of olive root rot disease. These bacterial isolates belonging to the genera <i>Alcaligenes</i>, <i>Pantoea</i>, <i>Bacillus</i>, <i>Sphingobacterium</i>, and <i>Stenotrophomonas</i> were chosen for their potential antimicrobial effects against many pathogens. Results of the in vitro confrontation bioassay revealed a high reduction of mycelial growth exceeding 80%. The antifungal effect of the volatile organic compounds (VOCs) was observed for all the isolates, with mycelial inhibition rates ranging from 28.37 to 70.32%. Likewise, the bacterial cell-free filtrates showed important inhibition of the mycelial growth of the pathogen. Overall, their efficacy was substantially affected by the nature of the bacterial strains and their modes of action. A greenhouse test was then carried out to validate the in vitro results. Interestingly, two bacterial isolates, <i>Alcaligenes faecalis</i> ACBC1 and <i>Bacillus amyloliquefaciens</i> SF14, were the most successful in managing the disease. Our findings suggested that these two antagonistic bacterial isolates have promising potential as biocontrol agents of olive root rot disease.
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