Obviously wild mushrooms aren't grown in sterile conditions, either. However, wild mushrooms have the luxury of only needing to grow in those unlikely places w

Obviously wild mushrooms aren't grown in sterile conditions, either. However, wild mushrooms have the luxury of only needing to grow in those unlikely places where conditions are just right for them to thrive. If you hope to grow a mushroom monoculture on a rich media (e.g. grains), you'll need to avoid the other fungi and bacteria that would easily out-compete your desired species. Once the mycelia is established, it's quite robust, at least relatively speaking. Un-colonized substrate, however, will readily harbor trichoderma, cobweb mold, etc. that will spread and kill your mushrooms. On a commercial farm, you're typically not looking at the colonization phase. If you see actual fruiting bodies, these are mushrooms at the very end of the crop cycle where contamination is no longer a risk. The initial colonization is often done on pasteurized substrate in a clean environment. Strict sterility is neither economical nor required, but effort must be made to give the desired species ideal conditions for growth. And then, contamination is a significant risk for the farmer. Once the mushrooms have been cased, exposure to fresh air is an important pinning trigger and contamination is not an issue.