wood decay

A wood-decay fungus is a variety of fungus which has the ability to digest wood, causing it to rot. Wood-decay fungi include those which attack dead wood, such as dry rot, and those which are parasitic on living trees, such as Armillaria (Honey fungus). 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. Dry rot fungi decay cellulose in wood leaving behind the lignin as the brown crumbly remains.

Dry rot refers to the decay of timber in buildings and other wooden structures caused by certain fungi. In other fields, the term has also been applied to the decay of crop plants by fungi and the deterioration of rubber. It would appear to be a paradoxical term seemingly indicating decay of a substance without the presence of water. However, its historical usage dates back to the distinction between decay of cured wood in construction, i.e. dry wood, versus decay of wood in living or newly felled trees, i.e. wet wood^[1]. In addition, the term can be used as a metaphor for grave underlying problems within a large organization (such as political corruption in government or low morale in the armed forces) that show no symptoms until a sudden, catastrophic failure, much as dry rot of wood in ships caused catastrophic failure.

The term is most commonly used in reference to the decay of building timbers. It refers to damage inflicted by either: Serpula lacrymans (formerly Merulius lacrymans) predominantly in the United Kingdom and northern Europe; and/or Meruliporia incrassata (which has a number of synonyms, including Poria incrassata and Serpula incrassata) in North America. Both species of fungus cause brown rot decay, preferentially removing cellulose and hemicellulose from the timber leaving a brittle matrix of modified lignin. Eventually the decay can cause instability and collapse in houses, wooden ships' hulls, and other wooden structures.

When applied to these fungi, the term is a somewhat misleading misnomer as both species require an elevated moisture content to initiate an attack on timber (28–30%). Once established, the fungi can remain active in timber with a moisture content of more than 30% – in the same way as other timber decay fungi using the brown rot decay mechanism.

The perpetual saturation of wood with water inhibits dry rot, as does perpetual dryness.