F. M. FREIMOSER<sup>1

1</sup>Federal Department of Economic Affairs, Education and Research EAER, Agroscope, Institute for Plant Production Sciences IPS, Schloss 1, P.B., 8820 Wädenswil, Switzerland.

Fungi are the most important plant pathogens in soil and cause rot or wilt diseases by colonizing roots. Soilborne fungal pathogens are also notoriously difficult to control, because they can grow in soil and persist even in the absence of a suitable host. Infestations by aggressive, soilborne, fungal pathogens can constrain crop production and often the only resort is crop rotation, fallow, or an abandonment of the cropland. Due to the complexity of the soil microbiome, the detection of low pathogen numbers is often difficult and their management by fungicides is rarely effective and in many countries (including Switzerland) not permitted. Naturally occurring yeasts are an unused resource for the management of soilborne, plant pathogenic fungi. Our research therefore focuses on antagonistic and beneficial yeasts and their application for the control of plant pathogenic fungi in soil. We have quantified the antagonistic activity of 40 yeast isolates (a representative selection of a larger collection obtained from soil, leaves, and flowers) against a diverse group of 16 filamentous fungi (antagonists, pathogens, and saprophytes belonging to the Zygomycota, Basidiomycota and Ascomycota). This initial screen of over 600 interactions identified weakly and strongly antagonistic yeast isolates, as well as species with differential competitiveness against different filamentous fungi (e.g., plant pathogens and known antagonists). Further characterization of the most promising species (growth characteristics, metabolic activities, rhizosphere competence, and antagonistic activity in soil) links the in vitro results on the competitiveness of the 40 yeast species to specific phenotypic characteristics and the behavior in a complex environment. These studies are the foundation for elucidating modes of action and for identifying the most promising species for the management of soilborne fungal pathogens.