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A New Approach to Stimulate Population Proliferation of Trichoderma species and Other Potential Biocontrol Fungi Introduced into Natural Soils. J. A. Lewis, Soil scientist, Soilborne Diseases Laboratory, Plant Protection Institute, U.S. Department of Agriculture, Beltsville, MD 20705; G. C. Papavizas, plant pathologist, Soilborne Diseases Laboratory, Plant Protection Institute, U.S. Department of Agriculture, Beltsville, MD 20705. Phytopathology 74:1240-1244. Accepted for publication 24 May 1984. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/Phyto-74-1240.

Population densities of isolates of Trichoderma viride (T-1-R4) and T. harzianum (WT-6-24) increased about 10⁴- and 10³-fold, respectively, in natural soil during the first 3 wk of incubation when the antagonists were added as a mycelial preparation (sterile bran-sand-water, [1:1:2, w/w/v] inoculated with conidia and allowed to incubate 1- 3 days before addition to soil). The preparation applied consisted of 1.0% bran and 10³- 10⁴ young, actively growing propagules per gram of soil. Populations did not increase when conidia were added to soil with or without bran. An 8-day-old preparation was less effective in stimulating a population increase and a 40-day-old preparation was entirely ineffective. The number of colony-forming units (cfu) gradually decreased over 18- 36 wk and stabilized at 10⁵- 10⁶/g of soil. Addition to soil of a mycelial preparation to provide as little as 0.01% bran and 10¹- 10² propagules per gram resulted in a 10⁵- to 10⁶-fold increase in population. Proliferation in soil resulted only when hyphae were in intimate contact with or in possession of the substrate (bran). Mycelium of T-1-R4 and WT-6-24 in bran resulted in population densities of about 10⁸/g of soil in several soil types (sandy loam, loamy sand, loam, silty clay loam) of various organic matter content. Proliferation of populations of Trichoderma in soil as a result of addition of mycelial preparations was accompanied by an increase in metabolic activity as determined by CO₂ evolution and by growth of the antagonist from a food base to pieces of organic matter in soil. A wide variety of isolates of T. viride, T. harzianum, and T. hamatum as well as other antagonists (Talaromyces flavus, Gliocladium virens, G. roseum, G. catenulatum, and Aspergillus ochraceus) added to soil as mycelial preparations gave results similar to those obtained with T-1-R4 and WT-6-24. Population densities increased up to 10⁶-fold within the first 3 wk of incubation, with the rate of increase greatest in the first week.