The warm, damp weather conditions of autumn are ideal growing conditions for fungi. If you want to see some colourful examples, pop down to your local woods this month.
One species which really stands out is the Fly Agaric. Fly Agaric are the iconic toadstools, made familiar through illustrations in fairy tale books. When you find one of these toadstools under a tree you almost expect to see a pixie sitting on top of it.
They're attractive to the eye, but they're also extremely poisonous. In fact they contain two poisons. One is muscimol, which has hallucinogenic effects (groovy) and the other is muscarine which causes violent sickness and possibly coma (very ungroovy). These poisonous effects, and more especially the psychoactive properties might well explain some of the fairy tale connections.
Look for them around the base of Birch Trees, on a warm day after it's been raining.