"one mushroom species in five is poisonous"? 20% ??? That seems like a crazy high estimate to me, at least if you mean deadly poisonous to humans. In the USA there are only a few species of amanita, galerina, a few of the hundreds of species of cortinarius, maybe some gyromitra and a handful of others I can think of that will kill you. Among the many thousands of mushroom species in the USA, there are only a few dozen known deadly poisonous ones. It's a really tiny percentage. Of course that doesn't mean that the others are edible, just not gonna kill you...