In the late 1980’s the deve lop ment of a severe epidemic of green mold caused by Trichoderma spp. was not ed in the commercial production of Agaricus bisporus (champignon) in the U nited Kingdom, North America, Spain and Holland, which caused extensive economic losses. The parasitic fungi isolated from the edible mushroom belonged to four biotypes, Th1, Th2, Th3 and Th4 of T. harzianum. However, among these biotypes, only Th2 (since c lassified as T. aggressivum f. europaeum) and Th4 (T. aggressivum f. aggressivum) were identified as the fungi causing problems in Agaricus production. In general, mushroom compost hosts both aggressive and innocuous is olates of Trichoderma, which are not morphologically distinguishable. Abo ut four years ago, a problem with green mold became apparent in the production o f Pleurotus ostreatus in Northern Italy, which eventually developed to a c risis situation in the South two years later and threatened to seriously comprom ise the Pleurotus market. This study was initiated to: isolate and identif y the aggressive fungi, then morphologically, physiologically and genetically characterize the isolates, dete rmine the source and phases of infection, and study methods of control. Samples were obtained from different phases of compost preparation at the locality of a major producer and supplier of compost to the mushroom industry in Southern Ital y, and microbial counts were conducted. Although the presence of Trichoderma was detected in the initial stages of composting, this value was reduced to zero from the phase of pasteurization to seeding with Pleurotus. Trichoderma infestations were noted in the packaged Pleurotus bales at various time s during the incubation phase (7-15 days after seeding) and after shipping to th e mushroom greenhouses, where the pathogen infestations greatly reduced the qual ity and quantity of the mushroom yield, as well as the number of potential harvest cycles. Preliminary r esults from the morphological and genetic characterization of Trichoderma isolates parasitic to Pleurotus indicated that they are different from bot h T. aggressivum forms parasitic to Agaricus, and the majority of the isolates probably belong to the species T. harzianum. In vitr o confrontation plates were performed with 26 isolates of aggressive Trich oderma obtained from compost, three Trichoderma isolates used in biolog ical control and 12 varieties of Pleurotus. No inhibitory effect was obse rved between any of the Trichoderma isolates with Pleurotus, althou gh some growth inhibition was caused by the biocontrol isolates of Trichoderm a on some of the aggressive isolates. The temperature optimum for Pleurotus growth was at 28 ℃, whereas Trichoderma grew well at a wider range (20- 28 ℃), and exceeded the growth rate of Pleurotus by three times at 25 ℃. T he pH optimum for the growth of Pleurotus was alkaline (pH 8-9) whereas Trichoderma preferred acidic-neutral pH (5-7) . Various commercial fungicides used in agriculture (procloraz, thiabendazole, dichloran, benomyl, p r opiconazole, thiofanatomethyl) were tested against the aggressive and biocontro l isolates of Trichoderma, as well as the different varieties of Pleuro tus to determine dose response curves and combinations that would inhibit spo re germination, mycelial growth and subsequent sporulation. Both procloraz and thiabendazole, which are pesticides allowed in e dible mushroom production, were found to control the growth of the aggressive Trichoderma isolates and did not have a negative effect on Pleurotus. In the late 1980’s the deve lop ment of a severe epidemic of green mold caused by Trichoderma spp. Was not ed in the commercial production of Agaricus bisporus (champignon) in the United Kingdom, North America, Spain and Holland, which caused extensive economic The parasitic fungi isolated from the edible mushroom belonged to four biotypes, Th1, Th2, Th3 and Th4 of T. harzianum. However, among these biotypes, only Th2 (since c lassified as T. aggressivum f. europaeum) and Th4 ( T. aggressivum f. Aggressivum) were identified as the fungi causing problems in Agaricus production. In general, mushroom compost hosts both aggressive and innocuous is olates of Trichoderma, which are not morphologically distinguishable. Abo ut four years ago, a problem with green mold became apparent in the production of Pleurotus ostreatus in Northern Italy, which eventually developed to ac risis situation in the South two years later and threatened to seriously comprom ise the Ple urotus market. This study was initiated to: isolate and identify y the aggressive fungi, then morphologically, physiologically and genetically characterize the isolates, dete rmine the source and phases of infection, and study methods of control. Samples were obtained from different phases of compost preparation at the locality of a major producer and supplier of compost to the mushroom industry in Southern Ital y, and microbial counts were conducted. Although the presence of Trichoderma was detected in the initial stages of composting, this value was reduced to zero from the phase of pasteurization to seeding with Pleurotus. Trichoderma infestations were noted in the packaged Pleurotus bales at various time s during the incubation phase (7-15 days after seeding) and after shipping to th e mushroom greenhouses, where the pathogen infestations greatly reduced the quality and quantity of the mushroom yield, as well as the number of potential harvest cyclessults from the morphological and genetic characterization of Trichoderma isolates parasitic to Pleurotus indicated that they are different from bot h T. aggressivum forms parasitic to Agaricus, and the majority of the isolates likely belong to the species T. harzianum. In vitr o confrontation plates were performed with 26 isolates of aggressive Trichoderma isolated from compost, three Trichoderma isolates used in biological control and 12 varieties of Pleurotus. No inhibitory effect was obseved between any of the Trichoderma isolates with Pleurotus, althou gh some growth inhibition was caused by the Biocontrol isolates of Trichoderm a on some of the aggressive isolates. The temperature optimum for Pleurotus growth was at 28 ° C, whereas Trichoderma grew well at a wider range (20-28 ° C), and exceeded the growth rate of Pleurotus by three times at 25 T he pH optimum for the growth of Pleurotus was alkaline (pH 8-9) Tritonoderma prefe rred acidic-neutral pH (5-7). Various commercial fungicides used in agriculture (procloraz, thiabendazole, dichloran, benomyl, pr opiconazole, thiofanatomethyl) were tested against the aggressive and biocontro l isolates of Trichoderma, as well as the different varieties of Pleuro tus to determine dose response curves and combinations that would inhibit spo re germination, mycelial growth and subsequent sporulation. Both procloraz and thiabendazole, which are pesticides allowed in e dible mushroom production, were found to control the growth of the aggressive Trichoderma isolates and did not have a negative effect on Pleurotus.