Viscosinamide-producing Pseudomonas fluorescens DR54 exerts a biocontrol effect on Pythium ultimum in sugar beet rhizosphere

Growth inhibition of the root pathogen Pythium ultimum by the biocontrol ag ent Pseudomonas fluorescens DR54 inoculated on sugar beet seeds was studied in a soil microcosm. Plant emergence was followed, together with bacterial rhizosphere colonization, antibiotic production and effects on fungal grow th. P. fluorescens DR54 inoculation of the P. ultimum-challenged seeds impr oved plant emergence after 7 days compared to a control without the biocont rol strain. At this time, P. fluorescens DR54 was the dominating colony-for ming pseudomonad in rhizosphere soil samples from inoculated seedlings as s hown by immune-staining with a strain specific antibody. Viscosinamide, a c yclic lipopeptide, which has previously been identified as a major antagoni stic determinant produced by P. fluorescens DR54 and shown to induce physio logical changes in P. ultimum in vitro, could be detected in the rhizospher e samples. The impact of P. fluorescens DR54 on the growth and activity of P. ultimum was studied by direct microscopy after staining with the vital f luorescent dyes Calcofluor white and fluorescein diacetate. P. fluorescens DR54 caused reduction in P. ultimum mycelial density, oospore formation and intracellular activity. Further, Pythium oospore formation was absent in t he presence of P. fluorescens DR54. A striking effect on zoospore forming i ndigenous Fungi was also observed in microcosms with P. fluorescens DR54 an d, thus, where viscosinamide could be detected; a large number of encysted zoospores were seen in such microcosms both with and without P. ultimum inf ections. In vitro studies confirmed that purified viscosinamide induced enc ystment of Pythium zoospores. (C) 2000 Federation of European Microbiologic al Societies. Published by Elsevier Science B.V. All rights reserved.