The Penn State Handbook for Commercial Mushroom Growers identifies thorough post-crop pasteurization as an essential best management practice to control pests and disease on a mushroom farm. According to the handbook, "After the last flush of mushrooms has been picked, the growing room should be closed off, and the room should be pasteurized with steam or steam with added sanitizing agents." This practice aims to minimize the chances of contaminating subsequent or adjacent crops when compost that may be heavily infested with pests and or disease-causing pathogens is removed from a growing room. Rooms heavily infested with flies should be steamed to kill the adults in the rooms, as well as the eggs and larvae in the substrate. Failure to pasteurize these infested rooms provides a source of adults to infest and infect newer crops. Not steaming a crop provides a continuous source of the next generation of flies on farms and in neighboring communities. The flies that emerge from the substrate later in the growing cycle are increasingly likely to contact fungal pathogens and therefore can transmit these pathogens and cause disease to early growing rooms on the same farm or across the industry. Post-crop steaming also breaks disease cycles by killing any fungal, bacterial, and virus pathogens in the substrate or those that may penetrate the wooden bed boards.
Post-crop pasteurization by steaming is an effective method of mushroom fly control because mushroom fly populations in the growing room build as the crop matures. While the flies prefer to go outside to mate, some still mate and lay eggs inside the growing room. The resulting generation of flies in the compost (that largely goes unseen) are killed by post-crop pasteurization. This includes the egg, larval, and pupal life stages of the fly. Most growers and workers notice that adult flies emerge from the substrate in large numbers as the harvesting period progresses. In theory post-crop pasteurization should kill the adult flies in the room as well. However, because the room takes time to reach the fly’s thermal death point, the flies have time to exit the room before the temperatures rise to lethal levels. It is especially important to prioritize inclusion of the flies before the room is steamed, as this would ensure the flies would be present in the growing rooms while they are being steamed. Growers should maintain tight seals around doors, windows, air handling units, and other potential ports of entry into and out of the growing rooms to prevent the easy movement of flies. Additionally, growers should prioritize filling cracks, crevices, and holes in the walls, roofs, and other structures to ensure the inclusion of flies in the growing rooms.
If a farm is plagued with Trichoderma, Mummy, LaFrance virus-infected mushroom mycelium and spores, flies, or nematodes, it is critical that compost is pasteurized before it is removed from the growing room. The suggested bed temperature for pasteurizing a room with substrate is at least 150oF (66oC) for at least 10 hours. The air temperature to accomplish this may be raised to at least 150oF (66oC) or higher if possible. Research has shown that regardless of how high the air temperature was heated, the substrate bed temperature took approximately 14 h to reach 140oF (60oC). Some farms will raise the air temperature as high as possible and then reduce it once the compost and wood reach the desired temperature.
It is well documented that some pathogens can grow into the wood and survive post-crop steaming with the substrate still in the beds. Therefore, it is sometimes suggested that once the substrate is removed and the beds are cleaned and washed down, the room then be steamed again. Steaming empty may be helpful because it takes much longer for wood to reach temperatures necessary to kill these pathogens or infected mushroom mycelium than it does for the substrate to reach these temperatures. In fact, it takes 5 to 6 times longer for a wood-like cypress to reach pasteurization temperatures than it does for the substrate. Therefore, the likelihood of pasteurizing the internal portion of wood during Phase II pasteurization is remote. Considering these factors, the best time to pasteurize wood is when the beds or trays are not filled with substrate, and the wood has been cleaned, well-moistened, and laid out. If steaming a room after removing the substrate, the suggested air temperature is at least 150oF (66oC) for at least 10 hours. This additional steaming provides a good opportunity to steam the filling nets, preferably rolling out the washed nets before steaming.
After this post-crop pasteurization, the growing room should be emptied, and the material removed is called mushroom compost (formerly called spent mushroom substrate). While mushroom compost still has some nutrients available to support mushroom growth, replacing the substrate and starting a new crop is more economical. This final post-crop pasteurization kills weed seeds, insects, and other unwanted microorganisms. After removing it from the growing room, users can consider mushroom compost to be free of living weed seeds and insects. Mushroom compost adds organic matter and structure to the soil, improves soil structure, and provides nutrients as a soil amendment.