Hyphal interaction between Trichoderma harzianum and Sclerotinia sclerotiorum and its role in biological control

Hyphal interactions between the mycoparasite Trichoderma harzianum (BAFC Cult. No. 72) and the soilborne plant pathogenic fungus Sclerotinia sclerotiorum were investigated in dual culture and in sterilized soil, by light and scanning electron microscopy. In dual culture, T. harzianum hyphae grew towards and coiled around the S. sclerotiorum hyphae. Dense coils of hyphae of T. harzianum and partial degradation of the Sclerotinia cell wall were observed in later stages of the parasitism. In sterile soil, conidia of T. harzianum germinated and the developing mycelium made contact with that of S. sclerotiorum, forming short branches and appressorium-like bodies which aided in holding and penetrating the host cell wall. An in vitro system was developed to test the ability of T. harzianum to control Sclerotinia wilt in cucumber and lettuce: coating seeds with T. harzianum conidia reduced the pre- and post-emergence effect of S. sclerotiorum in cucumber by 69 and 80%, respectively, and in lettuce by 46 and 72%, respectively. In the greenhouse, the disease caused by S. sclerotiorum in lettuce was reduced by treating seedlings with a peat bran preparation of T. harzianum. Despite the non- significance of the reduction in disease, Trichoderma-treated lettuce seedlings were much more developed than controls. In sunflower, significant reductions (in the range of 68 to 84%) in disease incidence were obtained by incorporating the peat bran T. harzianum preparation into the seedling rooting mixture. Hyphal mycoparasitism, rather than sclerotial parasitism, is suggested to be the mechanism by which T. harzianum controls S. sclerotiorum under these conditions.