To find sustainable alternatives to the application of synthetic chemicals for oomycete pathogen suppression. Here, we present experiments on an Arabidopsis thaliana model system in which we studied the antagonistic properties of rhizobacterium Paenibacillus polymyxa strains towards the oomycete plant pathogens Phytophthora palmivora and Pythium aphanidermatum. We carried out studies on agar plates, in liquid media and in soil. Our results indicate that P. polymyxa strains significantly reduced P. aphanidermatum and P. palmivora colonization in liquid assays. Most plants that had been treated with P. polymyxa survived the P. aphanidermatum inoculations in soil assays. The antagonistic abilities of both systems correlated well with mycoidal substance production and not with the production of antagonistic substances from the biocontrol bacteria. Our experiments highlight the need to take biofilm formation and niche exclusion mechanisms into consideration for biocontrol assays performed under natural conditions.