In addition to living in symbiosis with plants and decomposing dead organisms, mushrooms can also gain energy by acting as parasites.

Parasites are organisms that gain their energy by feeding on other living organisms. Many fungi live as parasites on tree trunks or in other plants. For example, many polypore fungi cause significant damage to pine and spruce trees.

Most plant diseases are caused by fungi. Many fungi that cause plant diseases are microscopically small.