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Abstract: Trichoderma is a ubiquitous fiingus that can be found in the air, soil, plant materials, residues, other substrates and is also known as a mycoparasite. The most serious out break of Trichoderma species on mushroom crops caused by biotype Th-2 of T. harzianum, in Ireland in 1985-86 and resulted in losses of about 3-4 million pounds in mushroom industries in U.K and Ireland (Fletcher 1990). Different Trichoderma isolates are able to secrete hydrolytic enzymes such as chitinases, Bglucanases and cellulases, which lyse the fungal cell walls and are supposed to play a basic role in the mycoparasitic activity of this fungus. The interaction between T. harzianum, T. longibrachatum, T. virens and Trichoderma sp. and Agaricus bisporus mycelia were studied. Opposite culmre and culture on top of each other, showed that none of the Trichoderma species could produce the zone of inhibition along with host mycelium. T. longibrachiatum, T. harzianum, T. virens and Trichoderma sp. had the highest colonization rates respectively. In all of species, the mushroom mycelial growth ceased upon contact with the competitor hyphae, and then parasite growth continued over the host mycelium in a radial manner. Microscopic smdies in 2 and 0.2% MEA showed differences in the interactions between Trichoderma species and mushroom mycelium. There was only contact between the Trichoderma species hyphae and A. bisporus in nutrient rich medium, but when the nutrient stams of the medium was reduced, some coiling of A. bisporus hyphae by Trichoderma species was observed and in the presence of some species hyphal lysis appeared to occur. However, lysis of host mycelia was not common in all the species. It is supposed that differences in producing metabolites by Trichoderma species have a close relationship with their colonization rate and the resulting losses. The severe interaction between T. harzianum and T. longibrachiatum with host mycelium is due to the production of lytic enzymes by these species. A study of effects of volitile metabolites showed that no Trichoderma species could produce these materials, or if they produced these metabolites had no inhibitory effects on mycelial growth of the host.
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