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Direct observation techniques were used to determine the antibiotic effects of eight actinomycete species on Helminthosporium sativum in soil and in vitro. In actinomycete-inoculated sterilized soils, the inhibition of germination of fungal spores corresponded with the degree of inhibition of the fungus produced by the same actinomycetes in Petri plate culture. The effects of the actinomycetes on the vegetative growth of H. sativum in soybean-supplemented soils varied with the individual actinomycetes and, with one exception, were identical with the effects caused by the actinomycete antibiotics in vitro. These hyphal changes in soil included: suspension of further mycelial development; lysis; characteristic morphological effects such as stunting, distortion, excessive branching and the formation of hyphal protuberances. Lysis of the fungal hyphae only occurred in the presence of soil and was shown to be due to the combined effect of the antibiotic and some unidentified soil factor. Evidence was also obtained which demonstrated the ability of the actinomycetes to produce antibiotics in unsupplemented soils. Additional proof of antibiotic activity in soil was obtained by using the antibiotic actinomycin and strains of Streptomyces antibioticus, the organism responsible for its production. In Petri plate culture, both crystalline actinomycin and the actinomycetes produced a characteristic swelling, distortion and stunting of the vegetative growth of Helminthosporium sativum. When introduced into sterile soils, identical morphological changes were evidenced by the fungus in the presence of either the pure antibiotic or the actinomycetes.

Antibiotic Activity of Actinomycetes in Soil as Demonstrated by Direct Observation Techniques