Parasitic and Wood-Decaying Fungi Explained

Fungi make a living in different ways. The mushrooms in our Library are, between them, wood-decayers, parasites and something in between. This article explains each lifestyle, with examples from British woods and fields, and shows why lifestyle shapes where each fungus grows and how it is cultivated.

Three ways to make a living

Every fungus needs to get its food from something else, because fungi can’t photosynthesise. Broadly, they do it in one of three ways:

• Decomposers (saprotrophs) feed on dead material.
• Parasites feed on living hosts, often harming them.
• Partners (mutualists) exchange resources with a living partner to mutual benefit, like the mycorrhizal fungi that live in partnership with tree roots.

The lines are blurry: many fungi can switch between lifestyles depending on circumstances. See What Is a Fungus?

Wood-decaying fungi

These fungi break down wood, one of the toughest materials in nature. Some mainly decompose dead wood; others attack living trees, usually through wounds, and continue feeding after the tree dies.

• White-rot fungi break down lignin as well as cellulose, leaving pale, soft, stringy wood. Reishi, Oyster, Lion’s Mane and chaga are all white-rot fungi, as is the artist’s bracket.
• Brown-rot fungi mainly break down cellulose, leaving brown, crumbly wood that cracks into cubes, often seen in old conifer stumps.
• Soft-rot fungi attack wood in very wet or very dry conditions, such as fence posts and timbers.

See How Fungi Decompose Wood and Recycle Nutrients and Artist’s Bracket.

Parasitic fungi

Parasites take nutrients from a living host.

• Insect parasites. Cordyceps and its relatives parasitise insects, growing inside a larva or pupa before producing a fruiting body. A tropical relative, Ophiocordyceps unilateralis, is famous for infecting ants and changing their behaviour, so that they climb vegetation and clamp onto a leaf before the fungus grows from their bodies. See The Complete Guide to Cordyceps.
• Tree parasites. Honey fungus, common in British gardens, spreads between trees via tough black cords known as “bootlaces” and can kill them.
• Plant parasites. Rusts, smuts and mildews cause many crop diseases.

Somewhere in between

Chaga infects living birch through wounds and grows inside the tree for many years, decaying its heartwood while the tree still lives. Its black conk is a sterile mass rather than a fruiting body, and it only fruits after the tree dies, a relationship between parasitism and decay. See Chaga and Birch Trees.

Reishi can act as a weak parasite of living trees, attacking the roots and base of the trunk, but mostly feeds on dead or dying wood.

The Oyster mushroom adds a twist: it can trap and digest tiny nematode worms to supplement the nitrogen that wood lacks. See How Oyster Mushrooms Grow.

Why it matters

These lifestyles shape where each fungus grows, how easily it can be cultivated and why some, like chaga, are still harvested from the wild. Decomposers such as Oyster and Lion’s Mane can be grown on sawdust or straw. Cordyceps militaris can be persuaded to grow without an insect. Chaga, which depends on years of growth in a living birch, has so far resisted commercial cultivation as a conk. See Mushroom Substrates Explained.