Pleurotus ostreatus, commonly known as “oyster mushroom”, is an edible fungus economically important worldwide. The green mould, caused by Trichoderma pleuroti and T. pleuroticola, is a very important fungal disease, and it is commonly controlled by the use of fungicides. The yeast-like fungus Aureobasidium pullulans is a biocontrol agent naturally found throughout a wide range of habitats. The effect of A. pullulans L1 and L8 strains on P. ostreatus, T. pleuroti and T. pleuroticola was studied in in vitro assays. Both yeast strains resulted compatible with P. ostreatus growth, and effective in reducing the T. pleuroticola and T. pleuroti colony growth. The inhibitory effect of L1 and L8 was similar in the majority of the Trichoderma-A. pullulans combinations on agar plates. Both strains were more efficient than Trichoderma in substrate colonization, and produced volatile and nonvolatile metabolites which reduced Trichoderma growth. When the activity of L1 and L8 was tested against the green mould disease of P. ostreatus under controlled conditions similar to those of a mushroom farm, only L8 was effective in controlling the disease. It showed an effect similar to that of the fungicide prochloraz against T. pleuroticola, the less aggressive pathogen, and lower than that of the fungicide against T. pleuroti, the most aggressive. The antagonism was the result of mechanisms like antibiosis and competition for space and nutrients, whereas the direct attachment of A. pullulans with hyphae of the pathogens did not play a role.
