The results of studies conducted during the harvest of fruiting bodies of oyster mushrooms indicate that the development of harmful micromycetes during the first wave of fruiting was less than in the second about twice. It is established that the distribution of microorganisms is also influenced by the placement of substrate blocks in the fructification premises. In the first wave of fruiting, the number of affected blocks varied depending on the tier location. The least affected blocks were found on the 3rd tier, which is the highest – 3,6 %. The yield loss on this tier was 3.1 kg of mushrooms. On the second tier, the percentage of the contaminated blocks was 10,4. The largest amount of infected substrate was observed on the 1st tier – 27,1 %. The increase in the affected blocks on the 1st and 2nd tiers is due to excessive moisture, which leads to the flow of drops from the upper bags to the lower ones. Observation during the second wave of fructification has shown, that the dissemination and development of the diseases was considerably bigger. In the 1st tier, the substrate blocks were infected with the micromycetes by 34,6 %, and the rate of disease development was 13,4 %. The smallest amount of infected substrate was found on the 3rd tier – 4,4 %, and the rate of micromycetes was 1,8 %. Substrate blocks having infection with 4-5 point development were removed and disposed. The average lesion score ranged from 2 to 3. During the first and second waves of fruiting, the frequency of micromycetes on substrate blocks increases due to the development of Trichoderma (up to 69,4 %), Penicillium (up to 15,7 %), and Aspergillus spp. (up to 8,3 %), commonly known as green mold substrate pathogens.