BIOCONTROL OF SOIL-BORNE PATHOGENS BY STREPTOMYCETES

Soil-borne pathogens such as Pythium, Fusarium, Rhizoctonia or Sclerotinia spp. cause severe epidemics and extensive crop losses. Disease management employs cultivation of resistant varieties and use of fungicides. However, these strategies meet serious problems, such as evolution of virulent pathogen strains, environmental pollution or decreased diversity of non-target organisms. Furthermore, recent legislation restricts the use of synthetic chemicals and favors the use of biological control products. Until now, only a small number of microorganisms have been formulated as biocontrol agents. Streptomycetes are common inhabitants of rhizosphere. They have been exploited in farmaceutical industry for the production of antibiotics, but very little is known about their potential in agrobiology applications as plant growth promoters or biocontrol agents. In our study, Streptomyces spp. strains isolated from diverse plant species were studied for their plant growth promoting and biocontrol potential against selected fungal pathogens. Strains were selected based on their in vitro antagonism against several pathogenic agents of root rot. These strains were successively tested for their plant growth promoting activity in vitro and in vivo, with particular reference to tomato, lettuce, corn salad, rocket and onion. Five strains were identified in in vitro antagonistic assay, that showed inhibitory effects against at least five diverse pathogens. The ability of selected strains to colonize seed coats was confirmed by SEM. They were further studied in vitro for their effects on seed germination and radicle and hypocotyl growth. The activity of individual strains depended on plant species used in the test and according to their performance the best combinations of strain-plant species were identified. 3 Streptomyces strains were investigated also in vivo to evaluate their promoting activity on seedling emergence and plant growth. All three strains improved substantially seed germination of three selected plant species. The strains were reisolated from surface-sterilized plant roots to confirm their ability to colonize root tissues. The ability of these strains to control plant diseases in vivo will be determined successively by greenhouse pathogenicity tests.