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Decay of Wood

[Subject Prev][Subject Next][Thread Prev][Thread Next] Decay and discoloration of wood is caused by fungi - simple plants that contain no chlorophyll. Fungi derive their energy while consuming the carbohydrate and lignin components of wood. Fungi that degrade wood may be classified as decay, soft-rot, stain, and mold.

Molds occur only on the exposed wood surfaces. Causing surficial discoloration of wood, molds do not affect its structural properties.

Staining fungi cause bluish or blackish discoloration of wood. Being detrimental to the appearance and value of wood, staining does not seriously affect the strength or physical integrity of wood.

The soft-rot fungi mostly act in very wet environments, and are known for their relatively slow and clandestine action. Soft rot may result in failures of the wet-use or partially embedded structures.

The decay fungi (brown rots and white rots) cause significant softening, weakening, and, eventually, complete deterioration of wood.

Wood attacked by the brown rot has distinctive brownish or reddish color, and develops cross-grain checking. When dried, the wood affected by the brown rot breaks into cubical pieces. The brown rot fungi are known for the selectivity of their action, i.e. for the tendency to affect only certain layers of wood. Brown rot is often referred to as "dry rot", but that is a definite misnomer. Like most of the fungi, brown rot needs moisture, and does not develop in dry conditions.

White-rot fungi may give wood a bleached or whitish look, but may also have a limited effect on the overall coloration. Wood attacked by white rot may retain its shape without shrinking or checking, but becomes spongy and structurally inadequate.

Another type of wood-attacking fungi is Poria Incrassata. The devastating effect of that type of fungi on wood structures also remains concealed up until the moment of the structural failure. As most of other fungi, Poria Incrassata needs moisture to survive and grow. However, that fungus has the ability to "built" its own water-supply system pumping moisture from the adjacent soil. The living colonies of Poria Incrassata leave distinctive whitish residue on the surfaces of the attacked wood.

For a long time, Poria Incrassata was considered to exist only in the moist climates of the southeastern and northwestern United States. However, it was recently found to attack many homes in Southern California (refer, for example, to the Orange County Register, August 31, 1998).

The most important prerequisite for the development of fungi and fungi-inflicted deterioration is the high moisture content of wood. Other fungi-inhibiting conditions are temperatures in the range of 70-90F, oxygen, and acidic environment.

Additional information in regard to the fungi-related deterioration of wood may be found in several publications, for example, in "Forest Products and Wood Science" by John G. Haygreen and Jim L. Bower (The Iowa State University Press/AMES).

Vyacheslav "steve" Gordin, S.E.
Irvine, CA

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