The Nature of the Widespread Soil Fungistasis

Abstract

Fungistasis of Conover loam, muck, and hardwood forest soils to conidia of Glomerella cingulata, Fusarlum oxysporum f. lycopersici, and Penicillium frequentans was demonstrated by indirect methods such as agar disks, double agar layer plates, and cellophan folds, as well as by direct placement of spores on or in soils. All attempts to extract toxic substances from soil with water or organic solvents failed. Fungistatic volatile substances could not be demonstrated in soil. Redox, pH, and osmotic conditions were not responsible for soil fungistasis. Various lines of evidence led to the conclusion that the so-called widespread soil fungistasis as observed by indirect methods is the result of production of antibiotics by soil microbes growing on the surface of the assay media, and is not due to a reserve of toxic substances in soils. The possibility is suggested that individual fungus spores serve as nutrient microsubstrates in soil and stimulate the rapid growth of soil microbes on their surface or in their immediate vicinity, and that this results in the production of sufficient fungistatic substances to prevent spore germination. Preliminary evidence in support of this suggestion are results with concentrated 50% ethariol washings from teliospores of Ustilago zeae which markedly stimulated growth of mixed soil microbes and of pure cultures of Streptomyces sp. and Pseudomonas sp., known antibiotic producers, in agar media.