White rot

A wood-decay fungus is any species 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. The rate of decay of wooden materials in various climates can be estimated by empirical models.

Wood-decay fungi can be classified according to the type of decay that they cause. The best-known types are brown rot, soft rot, and white rot. Each produce different enzymes, can degrade different plant materials, and can colonise different environmental niches. The residual products of decomposition from fungal action have variable pH, solubility and redox potentials. Over time this residue will become incorporated in the soil and sediment, so can have a noticeable effect on the environment of that area.

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. The fungi of certain types removes cellulose compounds from wood and hence the wood becomes brown colour.

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.

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