The Possible Role of Competition betweenTrichoderma harzianumandFusarium oxysporumon Rhizosphere Colonization

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Abstract
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 soli 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 rhizophere 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 inhibiton. 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 rhizophere. Plants grown from seeds treated with T-35 had roots with lower levels of Fusarium spp. in their rhizophere than roots from plants from untreated seeds. The greatest density and the largest reduction in levels of Fusarium wer e detected on the lower 4 cm of the roots. Numbers of Fusarium in the rhizophere 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 roots segments decreased. Trichoderma had little effect on the survival of Fusarium spp. in nonrhizosphere soil. Inhibition of germination may therefore have resulted from competition.