ANTAGONISTIC BACTERIA OF POTATO AS A BASIS FOR BIOLOGICAL CONTROL OF SOILBORNE PATHOGENS J LOTTMANN 1, H HEUER 2, K SMALLA 2 and G BERG 1 1 Universität Rostock, Mikrobiologie, Gertrudenstrasse 11a, D-18051 Rostock; 2 Biologische Bundesanstalt, Institut für Biochemie und Pflanzenvirologie, Messeweg 11/12, D-38104 Braunschweig, GERMANY Background and Objectives Beneficial bacteria can influence plant growth and health by two ways: a direct beneficial effect on plant growth will be achieved by the bacterial production of phytohormons, e. g. indole-3-acetic acid (IAA) and an indirect effect will be achieved by biological control of plant pathogens. The beneficial bacterial populations in the two microenvironments on underground organs rhizosphere (root) and geocaulosphere (tuber) of potato were analyzed. The target pathogens of biocontrol activity were Verticillium dahliae and Erwinia carotovora. The objectives of our research was to characterize the beneficial bacteria of potato and to find out new powerful antagonists. Materials and Methods Potatos cv. "Désirée" were grown in 1996 and 1997 in a field trial in Groß Lüsewitz near Rostock (Germany). Population densities based on colony forming units were determined at different developmental stages of plant. Representative bacterial isolates were screened for their ability to produce IAA and to inhibit the phytopathogens in vitro. Antagonistic bacteria were identified by their fatty acid pattern (FAME) and BIOLOG. Data of colony forming units and percentages of antagonistic and IAA-producing isolates were proved by analysis of variance (Fa. BioMath, Germany). Results and Conclusions The bacterial population densities were relatively stable during growth season: The population in the rhizosphere was 2,5 x 108 cfu g-1 fw and in the geocaulosphere significantly lower: 2,9 x 107 cfu g-1 fw, on average. The percentage of IAA-producing isolates was 32 on average and significantly higher than the percentage of antagonistic bacteria. Most of the identified isolates belong to the group of fluorescent pseudomonads like Pseudomonas chlororaphis, P. fluorescens and P. putida. Altogether, 28 different bacterial species were determined. Significantly more bacteria were found with antifungal activity against Verticillium dahliae than bacteria with antibacterial activity. Some isolates were able to produce IAA and had antagonistic activity to both of the phytopathogens. These bacteria are logical candidates for evaluation as biological control agents.