The use of antagonistic bacteria could be an environmentally friendly altemative to protect plants from soil-borne pathogens if the rhizosphere competence and biocontrol efficiency of the antagonists would be more predictable. In this project, we aim to test the hypothesis that the soil type and plant species influence the rhizosphere competence and biocontrol efficiency of inoculant strains. Vice versa the structural diversity of the indigenous microbial community in the rhizosphere might be affected by the inoculation either directly or indirectly due to the interaction of the inoculant with the plant, which might result in changes of the root morphology and exudation patterns, with potential impact on the rhizosphere competence of the inoculants. Amongst a collection of in vitro antagonists towards Rhizoctonia solani previously tested under growth chamber and field conditions, two strains (Pseudomonas jessenii RU47 and Serratia plymuthica 3Re4-18) repeatedly showed a remarkable ability to reduce disease symptoms and severity of bottom rot disease on lettuce caused by R. solani AG 1-IB. Both model strains produce several factors potentially contributing to their bio-control activity, which at least in the latter strain are quorum sensing regulated. Whereas strain RU47 colonized only the root surface of lettuce, strain 3Re4-18 was also detected inside of the root tissue. The unique experimental setup at the field site of the Institute of Vegetable and Ornamental Crops (IGZ) in Gro√übeeren will enable us to test the influence of soil types on the rhizosphere competence (lettuce) of both antagonists at the same site under field conditions. In the third year the colonizing ability and the antagonistic performance of both strains will be tested on lettuce and potato. Growth chamber experiments with DsRed-tagged strains will be performed to assess how the colonization patterns, the root morphology and the exudation patterns are influenced by the soil type and the plant species. A polyphasic approach comprising advanced methods will be used to unravel the complex Interactions between inoculant strains, the pathogen, the indigenous microbial community and the plant.