Penetration of Sclerotia ofSclerotium rolfsiibyTrichodermaspp.

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Abstract
Penetration of sclerotia of S. rolfsii by Trichoderma isolates was followed by a direct and an indirect approach. Sclerotia were immersed in an aqueous suspension of Trichoderma and conidia (1.5 .times. 10 ml) and incubated on water agar or on soil. Periodically, sclerotial samples were surface sterilized with 1% sodium hypochlorite solution for 2 min, washed, and plated out on a selective medium for Trichoderma and on a medium that allowed germination of sclerotia only. When the sclerotia were preincubated on either agar or soil, Trichoderma isolates differed in their ability to penetrate the sclerotia. Penetrated sclerotia that showed different stages of degradation were fixed, sectioned, stained and examined with a light microscope. Hyphae of Trichoderma penetrated the rind and cortex, lysed the medullar tissue, produced chlamydospores inside and conidia outside the sclerotia, and finally underwent autolysis. Degraded sclerotia became dark in color, soft, and empty and disintegrated under slight pressure. Sclerotia that were allowed to be penetrated on agar without being degraded and that were further incubated in soil, retained their firmness and were not degraded by the penetrating antagonist. Penetration capacity is an important but not the only property required for Trichoderma isolates to be efficient biocontrol agents.

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