A wood-decay fungus is a variety of fungus that digests moist wood, causing it to rot. Some wood-decay fungi attack dead wood, such as brown rot, and some, such as Armillaria (Honey fungus), are parasitic and colonize living trees. Fungi that not only grow on wood but actually cause it to decay, are called lignicolous fungi. They do not necessarily need to decay lignin in the wood to be termed lignicolous.
Wood-decay fungi can be classified according to the type of decay that they cause: brown rot, soft rot, and white rot.[1][2]
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Brown-rot fungi break down hemicellulose and cellulose. Cellulose is broken down by hydrogen peroxide (H2O2) that is produced during the break-down of hemicellulose.[1] Because hydrogen peroxide is a small molecule, it can diffuse rapidly through the wood, leading to a decay that is not confined to the direct surroundings of the fungal hyphae. As a result of this type of decay, the wood shrinks, shows a brown discoloration, and cracks into roughly cubical pieces; hence the name brown rot or cubical brown rot.
Brown rot in a dry crumbly condition is sometimes incorrectly referred to as dry rot in general. The term brown rot replaced the general use of the term dry rot, as wood must be damp to decay, although it may become dry later. Dry rot is actually a generic name for certain species of brown-rot fungus that have water-conducting strands capable of carrying water (usually from soil) that enable it to decay normally dry wood. It is also called true dry rot.
Brown-rot fungi of particular economic importance include Serpula lacrymans (true dry rot), Fibroporia vaillantii (mine fungus), and Coniophora puteana (cellar fungus), which may attack timber in buildings. Other brown-rot fungi include the sulfur shelf, Phaeolus schweinitzii, and Fomitopsis pinicola.[3]
Soft-rot fungi secrete cellulase from their hyphae, an enzyme that breaks down cellulose in the wood.[1] This leads to the formation of microscopic cavities inside the wood, and sometimes to a discoloration and cracking pattern similar to brown rot.[1][2] Soft-rot fungi need fixed nitrogen in order to synthesize enzymes, which they obtain either from the wood or from the environment.
Examples of soft-rot-causing fungi are Chaetomium, Ceratocystis, and Kretzschmaria deusta.
White-rot fungi break down the lignin in wood, leaving the lighter-colored cellulose behind; some of them break down both lignin and cellulose.[2] Because white-rot fungi are able to produce enzymes, such as laccase, needed to break down lignin and other complex organic molecules, they have been investigated for use in mycoremediation applications.[4]
Honey mushroom (Armillaria ssp.) is a white-rot fungus notorious for attacking living trees. Pleurotus ostreatus and other oyster mushrooms are commonly cultivated white-rot fungi.[4] Other white-rot fungi include the turkey tail, artist's conk, and tinder fungus.[3]