Wood

Additional notes on fungal attack of wood

Generally fungal attack can occur only when the equilibrium moisture content of the wood is greater than 20% and the temperature is below 40⁰C. Fungi attack sapwood faster than heartwood.

Some woods are naturally resistant to decay—because their heartwoods contain substances which inhibit fungal attack.

The presence of fungal fruiting bodies on the surface of wood indicates the presence of rot; but in many cases there is no visible sign of fungi. Affected wood can be soft and spongy, or brittle and powdery.

The term 'decay' when used in relation to wood specifically refers to attack by Basidiomycete fungi. These are grouped into two types which are particularly destructive to timber in buildings and other artefacts: brown rot and white rot. They are less likely to be a problem for indoor wooden objects.

Brown rot is a term which includes wet rot and dry rot—Serpula lachrymans. It attacks softwoods more commonly than hardwoods, destroying the cellulose and leaving the lignin behind. After attack, the wood is typically left stained and brown, with cuboidal cracking of the surface.

Dry rot is extremely destructive because the fungus can grow some distance away from its moisture source, producing long strands which can travel over large areas of masonry to infect new wood.

Dry rot is found only in cooler climates because it can't withstand temperatures greater than 40⁰C. It is less common in Australia than in Europe, where it causes extensive damage to buildings; but it has been found in Melbourne and Tasmania. If a dry rot outbreak is suspected, it should be dealt with immediately by a conservator experienced in dealing with dry rot.

White rot can destroy both the cellulose and lignin in wood. It is caused mainly by Ascomycetes fungi. After attack, the wood is a whitish colour and lighter in weight, with a stringy or fibrous appearance.

Soft rot occurs in wood that is continuously damp or in contact with the ground, for example, building timbers, archaeological wood, marine artefacts, or objects in exterior displays such as outdoor sculpture.

The fungus selectively attacks the cellulose from the outer surface of the wood, causing the surface to soften. After attack, the dried wood has a cracked appearance.

Soft rot can be avoided in wooden objects on outside display by ensuring that they are raised slightly off the ground on plinths or supports, so they are not in direct contact with the soil. Objects should be sheltered where possible, to ensure that water does not lie on their surfaces.

Numerous other fungi and moulds can grow on the surface of—or within—wooden objects, causing staining and possibly structural breakdown of the surface being colonised. These fungi and moulds are more commonly found on indoor objects than are the rots described above. They can cause staining, either by releasing pigments or through the presence of dark, fungal structures.

If you have a problem relating to the storage or display of wooden objects, contact a conservator. Conservators can offer advice and practical solutions.