A Google search found this:
These valuable fungi (mushrooms) are cultivated with spores prepared and grown under controlled conditions. It is best to ensure that a pure strain of fungus is isolated. As the spawn develops, it is allowed to colonize kernels of moist rye or millet. Moist rye, is Chometz, and some have therefore cautioned against using mushrooms on Pesach. However, most authorities do not consider this to be a problem since the grain is merely a carrier of the spawn and it imparts no value to the spawn. The grain decomposes and becomes inedible.