While we in Minnesota can find fungi of one sort or another all year round, Autumn is the season when more fungi fruit than at any other time.
Many fungi are quite particular about the material they grow on.
Some are saprophytic, breaking down dead and decaying material for energy. This association benefits the fungus by providing food from the plant and benefits the plant by extending the reach of its roots, providing more nutrition, especially phosphorus. Since the purpose of the fruiting body is to reproduce by producing and distributing spores, fungi fruit when conditions are favorable for spore dissemination and germination. There are many ways that spores can be disseminated, but germination of spores usually requires moisture. In Autumn, not only is it often rainy, but the weather is cooler.