Can poisonous mushrooms grow on the logs I planted?
The question is justified- if you're new to mushroom cultivation how do you know with 100% certainly? Fortunately that's one of the benefits of growing your own. If you've followed cultivation guidelines you can expect that what will grow there will be what you've planted. Planting vigorous spawn in the proper substrate at the proper time should allow for solid establishment of whatever you've planted, which means it will outcompete any poisonous invaders. Plus, once you positively identify your first cultivated mushrooms, you quickly become familiar with their every characteristic under every kind of weather condition, and you will be able to easily determine if there is a foreign fungus present. But like planting anything else outdoors, you can't control everything that grows there. That said, ultimately the responsibility of correct identification is yours, and if you can't identify it FOR SURE, it's not edible!
What poisonous mushrooms should I look out for?
If you are an outdoor mushroom grower, for peace of mind and as a good business practice, nothing beats educating yourself about the "Bad Guy" fungi out there. One of the best ways to education yourself on wild mushroom identification is to join a local mushrooming group. Check our North American Mycological Association to find one near you.
There are a few groups of mushrooms you should just steer clear of because gastronomic rewards of eating them are just not worth the risk of getting it wrong. While most fungi are not deadly poisonous, certainly names of such fungi like "The Sickener," "Death Angel," "Poison Pie," and "Deadly Galerina" offer some clues as to what will happen to you if you do eat it.
Amanitas are one such group; it includes the "Death Angels," a group of three beautiful white amanita species that, if consumed, will likely kill you. Fortunately none of these look at all like any of the mushrooms you are likely trying to grow and are not found growing on wood, so you'll be able to easily steer clear of them if you are harvesting mushrooms such as Shiitake or Oyster that grow on logs.
Beware also of the collective group "LBM" or "Little Brown Mushrooms." This group has some poisonous members, including one in particular "The Deadly Galerina," which harbor the same toxin as the Death Angels. This is a mushroom that provides the most fuel for dark dreams; it's brown, it grows on wood, it fruits in the fall; could it possibly be confused with Shiitake or Nameko? Likely not, but not outside the realm of possibility for the untrained eye. The deadly Galerina fruits on very old wood; we've only found it on fallen logs that are well on their way to barklessness and host many other wild fungi as well. This kind of log would be well past it's usefulness as a shiitake log. However, for a good look check Mushroom Expertand you'll get a good idea of what they look like.
The last group to be wary of consists of members of Cortinarius. which could be potentially confused with Blewit. Some species of this group are purplish like the Blewit, but turn them over and you will see a cobwebby veil underneath the cap, or "Cortina". The gills turn rust brown and the stalk often has bits of this Cortina on it. Once again, familiarity of the Blewit will give you the best insurance as to what is NOT a blewit.
The good news is that these fungi are different enough from what you're cultivating that in even just one season you will be familiar enough with YOUR fungi that there will be nothing to worry about.