A microbial growth medium, RSM, was developed to study the role of siderophores (microbial Fe-transport compounds) in the inhibition of the take-all pathogen,Gaeumannomyces graminisvar.tritici, byPseudomonas putidastrain B10. The inorganic constituents of the medium were designed to mimic the rhizosphere while the organic composition was designed to promote rapid growth and siderophore production. The antibiosis experiments were highly reproducible and the antagonism appeared to be due to production of pseudobactin, the siderophore of B10. On plates amended with chrome azurol S,G. graminisdid not produce siderophores while other fungi did. The growth ofG. graminison plates prepared with Fe chelate buffers was inhibited at a free ferric ion concentration of 10−24.6M, although three other fungi were not inhibited, even at 10−25.5M, presumably due to their greater production of siderophores. In liquid medium amended with Fe chelate buffers, both the doubling time and the lag phase ofP. putidaincreased as the free ferric ion concentration was reduced. A wide variety of fungi and bacteria were found to grow on this medium. Because the inorganic composition of RSM is based on that of the rhizosphere, the development of this medium may be a first step towards the study of the chemistry and biology of the rhizosphere under well defined conditi
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