Fire blight, caused by the bacterium Erwinia amylovora is the most devastating plant disease in apple and pear fruit production. The most effective agent to control the diseaseis still streptomycin, an aminoglycoside antibiotic. Increased incidence of resistant strains and streptomycin residues found in milk, honey and pork meat led to growinginterest in alternative control agents. Furthermore, streptomycin application in organic farming is strictly forbidden and in many countries generally not permitted. Thus, for more than thirty years, intensive investigations on „Biological Control Agents“ have been conducted. Because of their ability to inhibit the pathogen's growth, these epiphytic microorganisms are a biological alternative to antibiotics. The interaction between so called „antagonists“ and E. amylovora occurs on blossoms, which are the main infection sites of the bacterium. ^For an efficient antagonist not only growth-inhibition is required,but also the ability to colonize blossoms, to establish high populations in the stigma exudates, and the fact that they are not phytotoxic. Therefore, only few antagonistsare commercially available yet. Furthermore, efficient assays to facilitate the selection of potential antagonists are needed. Hence, this master thesis contains the establishment of an „In-vitro-Assay“ for investigating the antagonistic abilities of environmental microorganisms.Selected antagonists can be tested in further greenhouse- and field experiments. This assay comprises four selection steps, starting with a „Double Layer Assay“ with King's B medium in order to test a great quantity of isolated environmental microorganisms because of their growth-inhibiting effect on E. amylovora. ^Next, the environmental isolates' growth behavior needs to be examined in a synthetic medium called „Stigma Based Medium“, which is comparable with the chemical composition of stigma exudates. After that, environmental isolates can be identified through „16S rRNA-Sequencing by Sanger“ and through „MALDI-TOF“ by analysing the ribosomal protein profile. The final step is a „Competition Assay“ to quickly evaluate the inhibitory effect of potential antagonists against the pathogen in Stigma Based Medium. Eventually, a „Detached Flower Assay“ can be conducted.