Screening Tests Designed to Discover Antibiotics Suitable for Plant Disease Control

SUMMARYScreening methods aimed at finding organisms that produce stable antibiotics which inhibit a range of phytopathogens are described. Agar plates, each containing colonies of 30–50 soil microorganisms, were sprayed with four mutually nonantagonistic phytopathogenic fungi. After a period of incubation, organisms that were antagonistic to the four phytopathogens, as indicated by zones of inhibition, were isolated. The degree of antagonism exerted by these organisms on 10 phytopathogens was assessed in tests in which individual phytopathogens were sprayed on agar plates that had been streaked four days previously with a single antagonistic organism. The antagonists that were the most inhibitory were grown in five liquid media. The stability of the antibiotics in culture centrifugates was assessed with respect to pH, heat, and storage. Out of approximately 70,000 colonies of soil microorganisms, an isolate of Bacillus subtilis and five Streptomyces isolates were selected for further work.