Abstract:The influence of substrate infestation with Trichoderma pleurotum and T. pleuroticola isolates on yielding of two commercial strains of Pleurotus ostreatus was investigated. The examined P. ostreatus strains growing on substrates infested with Trichoderma isolates showed a considerable yield decline. T. pleuroticola isolates were found to exert a greater unfavorable impact on P. ostreatus yields than T. pleurotum isolates. The performed experiments demonstrated that the response of the examined P. ostreatus strains to infestations with T. pleurotum and T. pleuroticola isolates was similar.