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A wood-decay fungus is a variety of fungus that digests moist wood, causing it to rot. Some species of 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. Various lignicolous fungi consume wood in various ways; for example, some attack the carbohydrates in wood and some others decay lignin.
Brown-rot fungi break down hemicellulose and cellulose. Cellulose is broken down by hydrogen peroxide (H2O2) that is produced during the breakdown of hemicellulose. 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 a generic name for certain species of brown-rot fungus.
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.
Soft-rot fungi secrete cellulase from their hyphae, an enzyme that breaks down cellulose in the wood. This leads to the formation of microscopic cavities inside the wood, and sometimes to a discoloration and cracking pattern similar to brown rot. 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 rots break down lignin and cellulose and commonly cause rotted wood to feel moist, soft, spongy, or stringy and appear white or yellow.
White-rot fungi break down the lignin in wood, leaving the lighter-colored cellulose behind; some of them break down both lignin and cellulose. 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.
Honey mushroom (Armillaria spp.) is a white-rot fungus notorious for attacking living trees. Pleurotus ostreatus and other oyster mushrooms are commonly cultivated white-rot fungi, but P. ostreatus is not parasitic and will not grow on a living tree, unless it is already dying from other causes. Other white-rot fungi include the turkey tail, artist's conch, and tinder fungus.