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Title:   Author  item
                                            Samac, Deborah

                                                         - *Debby*item
                                            WILLERT, AMY - HAMLINE UNIVERSITYitem
                                            MCBRIDE, MATTHEW - UNIVERSITY OF MINNESOTAitem
                                            KINKEL, LINDA - UNIVERSITY OF MINNESOTA
  Submitted to: Applied Soil Ecology
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 8/7/2002
                                    Publication  Date: 1/15/2003
                                    Citation: SAMAC, D.A., WILLERT, A.M., MCBRIDE, M.J., KINKEL, L.L. EFFECTS OF ANTIBIOTIC-PRODUCING STREPTOMYCES SPP. ON NODULATIONS AND LEAF SPOT IN ALFALFA. APPLIED SOIL ECOLOGY. 2003. V. 22. P. 55-66.Interpretive Summary: The soil supports a diverse community of microorganisms that impact plant health. Due to the production of antibiotics, soil bacteria known as Streptomycetes have the potential to control a variety of plant pathogens. However, their value will depend upon their abilities to colonize plant surfaces and their inhibitory effects on beneficial microbes. We  investigated the ability of antibiotic-producing Streptomycetes to coloniz alfalfa plants and impact pathogenic and symbiotic microbes. Alfalfa roots were colonized readily when Streptomycetes were inoculated around the seed at the time of planting. The majority of the Streptomyces strains tested inhibited growth of the fungus causing spring black stem and leaf spot disease of alfalfa. Several Streptomyces strains decreased leaf spot symptoms when inoculated onto leaves before the pathogen. Eleven Streptomyces strains inhibited growth of the symbiotic, nitrogen-fixing  bacterium, Sinorhizobium meliloti, but four Streptomyces strains had no effect on growth of this bacterium. None of the Streptomyces strains reduced the number of nodules formed by the symbiotic bacteria, although some strains reduced plant weight. These results suggest that specific strains of Streptomyces can be selected for use as biological control agents to suppress diseases of alfalfa roots and leaves that will not adversely affect symbiotic bacteria. Development of such biological agents with the ability to control a broad range of plant diseases will give growers more options for ecologically sound and economical crop production.
                                Technical Abstract: The soil supports a highly diverse community of microorganisms that influence plant health. Antibiotic-producing Streptomycetes have the potential to impact growth of soil microorganisms and may be useful for controlling plant pathogens. We investigated the ability of Streptomycetes to colonize alfalfa (Medicago sativa L.) plants and influence the  activities of pathogenic and symbiotic microbes. Alfalfa leaves, roots, an nodules were colonized when Streptomyces strains were introduced around the seed at the time of planting. Streptomyces strain densities on leaves decreased 10- to 100-fold during an 8 week period, while densities on roots remained constant. We tested the ability of 15 antibiotic-producing strains of Streptomyces to inhibit in vitro growth of Phoma medicaginis, the causal agent of spring blackstem and leaf spot of alfalfa. The majority of the Streptomyces strains inhibited growth of P. medicaginis. In a detached leaf fassay, one Streptomyces strain decreased leaf spot symptoms and two Streptomyces strains decreased defoliation. Eight Streptomyces strains inhibited in vitro growth of S. meliloti, while four Streptomyces strains had no effect on growth. In a growth chamber assay, two Streptomyces strains reduced plant dry weight significantly compared to the treatment with S. meliloti alone but did not reduce the number of nodules significantly. These results suggest that careful selection of Streptomyces isolates for use in biocontrol of plant diseases will limit the potential negative impacts on symbiotic bacteria.


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