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Experimental Evaluation of Hypotheses Explaining the Nature of Soil Fungistasis. T. S. Hora, Botany and Plant Pathology Department, Colorado State University, Fort Collins, CO 80523, Present address of senior author: Soilborne Diseases Laboratory, Agricultural Research Service, U. S. Department of Agriculture, Beltsville Agricultural Research Center-West, Beltsville, MD 20705; Ralph Baker(2), and Gary J. Griffin(3). (2)Botany and Plant Pathology Department, Colorado State University, Fort Collins, CO 80523; (3)Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. Phytopathology 67:373-379. Accepted for publication 17 September 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-373.

The quantity of nutrients available for germination of fungal propagules in soil has been a subject of controversy. A bioassay was used to detect the available nutrients and/or inhibitors in 17 soils (pH 5.1-8.6) at various intervals after moistening. To allow diffusion of soil solutions, washed agar disks were placed aseptically on sterile Nuclepore membrane filters resting on soil. After 22-24 hr of incubation, agar disks were removed and tested for ability to support germination of four test fungi. Both increases and decreases in germination of the test fungi were detectable in the bioassay. Increased germination was best attributed to enhanced nutrient status of agar disks resulting from exposure to soil. Decreased germination was attributed primarily to inhibitory substances. Sterile filtrates of soil: water (1:1, w/v) extracts diluted to 1:50 supported germination of conidia of Verticillium albo-atrum which have an exogenous organic carbon requirement for germination. The results are best explained by hypotheses explaining the nature of soil fungistasis based on interactions between inhibitors and nutrients in soil.

Additional keywords: spore germination, volatile fungistatic factor, Gonatobotrys simplex, Penicillium chrysogenum, Fusarium solani.