producing mutants of the suppressive strains and antibiotic-resistant mutants of the pathogen strains. These mutants are being used to quantify the relative roles of antibiosis and resource competition in biocontrol and to determine the potential fitness of antibiotic-resistant strains of the pathogen. Additionally, the antibiotic produced by one of the suppressive strains has been partially purified and characterized ( Eckwall 1994), and strategies for purifying antibiotics produced by other strains are under consideration. Finally, information on the diversity in antibiotics produced among isolates from the suppressive soil is being used in the development of suppressive strain combinations that model the naturally disease-suppressive soil.
The use of naturally occurring pathogen-suppressive strains of Streptomyces offers tremendous promise for the control of potato scab and other plant diseases. In our trials, Streptomyces strains effectively colonized the soil, reduced pathogen populations in the soil, and significantly reduced levels of potato scab multiple years after inoculation. Future efforts to enhance the levels of biocontrol obtained using Streptomyces to control plant diseases will focus on the development of strain combinations for biocontrol, the integration of biocontrol with crop rotation and other crop management strategies, and the selection of a variety of strains that are optimized to different physical conditions. Additional work here and in other programs is investigating the abilities of new, recently isolated Streptomyces strains to control a large number of different pathogens on many different crops. As biological strategies for controlling plant disease become a more integral part of pest management, streptomycetes promise to play an important role.
Support for this research was provided by the Minnesota Agricultural Experiment Station, the Legislative Commission on Minnesota Resources, the Red River Valley Potato Growers Association, and the USDA North Central Region Integrated Pest Management Program.
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