Steam-off (also known as post-crop pasteurization) is an important part of maintaining low pest populations. The Best Management Practices for Mushroom Farming manual includes a thorough post-crop steaming process after harvesting is complete. “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.”
The goal of this practice is to minimize the chances of contaminating subsequent or adjacent crops when substrate, which may be heavily infested with pests and/or disease-causing pathogens, is removed from a house. Post-crop steaming procedures will kill adult flies as well as their eggs and larvae in the substrate. A second steaming process once the house is empty is also recommended to ensure the growing room itself is treated.
Failure to treat the substrate and the house may provide a source of adult flies to infest newer crops as well as a continuous source of the next generation of flies around the farms and neighboring communities. Post-crop steaming also breaks disease cycles by killing any fungal, bacterial and viral pathogens in the substrate or those that may penetrate the wooden bed boards.
If a farm is plagued with Trichoderma, nematodes, Mummy, LaFrance virus infected mushroom mycelium and spores, or other pests then steaming off both before and after the substrate is removed from the house is critical. Some pathogens are capable of growing into the wood and survive post-crop steaming with the substrate still in the beds. Therefore, it is imperative the room be steamed again once the substrate is removed and the beds are cleaned and washed down. Steaming empty is necessary because it takes much longer for the wood to reach temperatures necessary to kill pathogens or infected mushroom mycelium than it does for the substrate. In fact, it takes five to six times longer for a wood like cypress to reach effective kill temperatures. The likelihood of eliminating these organisms in the internal portion of the wood during a Phase II pasteurization is remote. Considering these factors, the best time to steam treat wood is when the beds or trays are not filled with substrate and the wood has been cleaned, well moistened and laid out.
The suggested temperature for steaming off a room with substrate is at least 150°F (66°C) for at least 12 hours. When steaming a room after the substrate has been removed, the suggested temperature is at least 150°F (66°C) for at least 24 hours. Research has shown that regardless of how high the air temperature is heated, the substrate temperature takes approximately 14 hours to reach 140oF (60°C). Some farms will raise the air temperature as high as possible, then reduce it back down to a lower temperature once the substrate and wood are both up to the desired temperature.
These final post-crop steaming processes kill insects and mushroom pests that negatively affect mushroom production. Following the best practices procedure for the post-crop sanitation will reduce the risk of contamination and disrupt the pest breeding cycle.