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The Possible Role of Competition between Trichoderma harzianum and Fusarium oxysporum on Rhizosphere Colonization. Alex Sivan, Former graduate research student, Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, The Faculty of Agriculture, Rehovot, 76100, Israel; Ilan Chet, Professor, Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, The Faculty of Agriculture, Rehovot, 76100, Israel Phytopathology 79:198-203. Accepted for publication 16 August 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-198.

Soil was enriched with chlamydospores of Fusarium oxysporum f. sp. vasinfectum and F. oxysporum f. sp. melonis and amended with increasing concentrations of glucose and asparagine. Maximal germination of chlamydospores was obtained in soil amended with 0.4 mg of of glucose and 0.08 mg of asparagine per gram of soil. Addition of conidia of the biocontrol agent Trichoderma harzianum (T-35) significantly (P = 0.05) reduced the chlamydospore germination rate of both Fusaria. However, in soils amended with concentrations higher than 0.3 and 0.06 mg/g of soil of glucose and asparagine, respectively, the inhibition was nullified. Chlamydospore germination of F. o. melonis and F. o. vasinfectum in melon and cotton rhizosphere soil were significantly inhibited after soil or seed application with T-35. As in the case of the glucose and asparagine, addition of an excess of seedling exudates increased the germination rate and eliminated the inhibition. Moreover, a continuous application of germinating cotton seed exudates to a soil infested with F. o. vasinfectum planted with cotton and treated with T-35 significantly reduced the disease control capability of the antagonist. A seed treatment with T-35 in a constantly humid soil resulted in high population densities of the antagonist on the developing rhizosphere. Plants grown from seeds treated with T-35 had roots with lower levels of Fusarium spp. in their rhizosphere than roots from plants from untreated seeds. The greatest density and the largest reduction in levels of Fusarium were detected on the lower 4 cm of the roots. Numbers of Fusarium in the rhizosphere were inversely proportional to the number of conidia of T-35 applied to soil. On the other hand, as the concentration of the pathogen in soil increased, T-35 counts on root segments decreased. Trichoderma had little effect on the survival of Fusarium spp. in nonrhizosphere soil. Inhibition of germination may therefore have resulted from competition.