Fungal decay

Cellar rot fungus

Coniophora puteana and C. marmorata are the commonest cause of wet rot in buildings that have become soaked by water leaks, for example, soil moisture or plumbing leaks. The fungal spores are ubiquitous and germinate readily, so they are likely to occur whenever suitable conditions arise. The hyphae are initially white, then yellow to brownish in colour, remaining off-white under impervious coverings. C. puteana forms rhizomorphs that are initially yellowish when young, later becoming brown to black at maturity. They never extend from the infected wood.

The sporophore rarely occurs in buildings and consists of an olive green to brown fruit body with a paler margin, thin skin and warty surface. C. marmorata fruit body is pinkish-brown and has a smooth to lumpy surface.

The fungus causes considerable shrinkage to wood, and cracking may occur, not unlike that caused by dry rot. The rotted wood is dark brown with dominant longitudinal cracks and infrequent cross grain cracks. Freshly colonised wood usually shows a yellow colouration.

Poria fungi

These fungi generally attack softwood in buildings. They require a higher moisture content than dry rot, but are tolerant of occasional drying and so are normally associated with roof leaks.

This group includes Amyloporia xantha, Antrodia vaillantii (Fibroporia vaillantii) and Poria placenta. They are commonly called white pore fungi or mine fungi. Antrodia vaillantii can cause extensive damage in buildings.

The mycelium of this group of fungi forms white or cream sheets or fern-like growths, which may discolour brown on contact with iron. The rhizomorphs may be up to 3mm in diameter, seldom thicker than twine, white to cream in colour, remaining flexible when dry, and they do not extend from their foci of infection.

The sporophore is rare in buildings; it is a white, irregular lumpy sheet, 1.5 to 12mm thick, covered with distinct pores, sometimes with strands emerging from its margins. Spore bearing surfaces are white to pale yellow, occasionally with pink patches (Poria placenta only).

The decay damage to wood is similar to that caused by S. lacrymans, but the cubing is somewhat smaller, less deep and lighter in colour. When decayed the wood crumbles between the fingers, it is not as powdery as that attacked by S. lacrymans, but slightly more fibrous and gritty.

Phellinus contiguus

This fungus attacks both softwoods and hardwoods and is commonly found on the external joinery of buildings. The mycelium may be found in wood cavities, or around the sporophores in the form of tawny brown tufts. The mycelium does not form rhizomorphs as in the case of S. lacrymans. The sporophore is occasionally found in buildings and is thick, tough, elongated, ochre to dark brown in colour and covered in minute pores.

The decayed wood shows no cuboidal cracking like that attacked by S. lacrymans and C. puteana, and does not powder in the same way as wood decayed by the dry rot fungus. Instead, it bleaches and eventually develops a stringy, fibrous appearance.

Donkiporia expansa

D. expansa attacks hardwood and, as it particularly prefers oak, it is largely associated with older buildings constructed with this wood. However, once the fungus colonises hardwood in a building it may spread to the adjacent softwoods.

This fungus tends to occur in much wetter conditions and is particularly found on wood where there has been persistent water leakage. The mycelium of the fungus is yellow to reddish brown, forms a thick felted growth, often shaped to the contours of the wood, and exudes drops of yellowish-brown liquid. The mycelium does not form strands like dry rot. The sporophore is thin, leathery, plate or bracket-shaped, or thick, hard and woody. The brown and buff-coloured hymenium is cinnamon-brown or fawn colour with numerous minute pores, often comprising several layers.

D. expansa can cause more extensive damage to oak than any other fungi found in buildings, often attacking the ends of beams embedded in damp walls. Damage may be confined to the interior of the beam and not noticed until the typical bracket-shaped fruit bodies appear. It is often associated with death-watch beetle attack.

The decayed wood becomes bleached and is reduced to a lint-like consistency, leaving stringy white fibres. It is easily crushed, but does not crack.

Pleurotus ostreatus

Commonly called oyster fungus, this species belongs to the family Hymenomycetes (Agaricales). The mycelium is whitish and forms a woolly mat; rhizomorphs are not formed. The sporophore is a white-gilled, grey-capped mushroom with an off-centre stalk.

It is occasionally found in buildings and usually associated with decay of panel products. Decayed wood-board lightens in colour; in particle boards, the chips tend to separate.

Asterostroma species

This fungus usually attacks only softwoods in buildings, is commonly found on joinery, such as skirting boards, and is often limited in extent.

The mycelium is white, turning cream or buff colour. Hyphal strands form occasionally with a rough appearance. These remain flexible when dry, and some can cross masonry over a long distance. The sporophore sheet is very thin and hardly distinguishable from mycelial sheets. Pores are not present. The damaged wood becomes bleached and develops a stringy fibrous appearance.

Paxillus panuoides

This fungus prefers very damp conditions. It causes decay similar to that caused by cellar rot, that is, deep longitudinal fissures with some fine cross-cracks. In the early stages wood is stained a vivid yellow. Wherever mycelium is present, but in advanced stages, the decayed wood becomes soft and cheesy.

The mycelium is fine, soft, hairy and a dull yellow with occasional tinges of violet. The hyphae develop into fine branching strands, coloured as the mycelium; they do not darken with age. The sporophore has no distinct stalk but is attached at a particular point, tending to curl around the edges and eventually becoming fan or funnel-shaped. The colour is dingy yellow, but darkens as the spores develop. The texture of the sporophore is soft and fleshy. The gills are yellow and branch frequently. The spore print is ochreous (rust-coloured).

Lentinus lepideus

Lentinus lepideus occurs quite frequently on worked timber that has been imperfectly creosoted, particularly telegraph poles and railway sleepers. The wood decayed by L. lepideus has a strong characteristic aromatic smell resembling that of Peru balsam. The fungus is not very common in buildings. The fruiting body, rare in buildings, is a pale brown mushroom with darker coloured scales on the top and gills underneath.

The fungus mainly attacks the cellulose, leaving the lignin practically unaffected. It does not produce strands. The decayed wood typically darkens and cracks along and across the grain. The mycelium is often present in the longitudinal cracks. In buildings the cap frequently fails to develop, giving a branched structure known as a 'staghorn' growth. A rarer species, Lentinus tigrinus, is only found on building timbers previously treated with creosote.

Dacrymyces stillatus

This fungus is very common with other joinery decay fungi, such as white rots, and attacks both hardwoods and softwoods. The fruiting bodies are only 1-5cm in diameter, orange-red and gelatinous when fresh but becoming darker and harder as they dry. The fungus is usually restricted to the interior of the timber and it gives a pocket rot appearance.

Ptychogaster rubescens

The sporophore does not occur in buildings and strand formation is very rare; they are white and brittle when dry but more fragile than those of dry rot and occur only on rotted wood. The white mycelium produces fern-like outgrowths. Wood decay is typical of brown rot; the wood darkens and cracks both along and across the grain. Spores are produced directly on the hyphae in specialised parts of the mycelium. These take the form of dull, pinkish-brown cushions which are soft and tend to disintegrate when touched to give a fine powder.

Plaster fungi

Plaster fungi are likely to be found on damp brickwork or plaster in buildings. Common examples are Coprinus species (Inkcap), Peziza species (Elf Cup) and Pyronema domesticum. These fungi feed on the surface detritus or on organic material included in walls, for example bitumised felt DPCs and hair contained in old plasters.

Coprinus species belong to the class Hymenomycetes (Basidiomycotina). The sporophore of Coprinus has rather a peculiar organisation found only in this genus. The monokaryotic mycelium, without clamp connections, is branched and this produces a conidial stage in the form of erect branches, each bearing a slimy head of unicellular and uninucleate spores. In this vegetative non-fruiting (oidium) stage of the life cycle, it may cause some decay to hardwood and sapwood, for instance, to split laths. Fruit bodies are formed on the dikaryotic (binucleate) mycelium, with clamp connections. They are of a small white or cream 'mushroom' type with black gills on a thin stalk, often in clumps. Black spores are often deposited as a spore print when the fruit body matures before it shrivels and collapses.

Peziza species

This fungus, commonly called Elf Cup, belongs to the class Discomycetes (Ascomycotina). The fruiting body or ascocarp is a small, pale brown or flesh-coloured, cup-shaped structure without stalks and is up to 50mm wide. It is pliant when fresh but breaks easily with brittle fractures when stressed between the fingers. When dry it is hard.

Pyronema domesticum

The fruiting bodies are small, bright orange and wrinkled, and the cup is jelly-like. The mycelium is profuse and pinkish but otherwise resembles that of S. lacrymans.

Stain fungi

Staining of sapwood is usually blue to black and occurs on freshly felled wood with a high moisture content. The damage persists after the wood dries, but is usually of no significance when found in wood in buildings. The stain fungi cannot grow in waterlogged wood or below about 20% moisture content.

Stain fungi commonly harbour sapwood cells of moist wood that consist mainly of ray cells. For their early nourishment they depend on parenchymatous tissues as in the wood rays, and on sugars and starch in the cells. The virtual absence of blue stain fungi in heartwood seems to be attributable chiefly to a shortage of relatively easily metabolised carbohydrates rather than inhibitory heartwood activities. Stain fungi are also commonly called 'Sat stain', a blue stain.

Staining of wood under varnish (or invisibly under paint), usually blue or black, is often accompanied by black surface growths through the coating. It is sometimes accompanied by surface mould, but occurs under different conditions through moisture accumulations under coating systems. This is known as 'Sap stain' (stain in service).

Stain fungi can be identified in wood as black or blue-black streaks and patches in the wood or coating; sometimes, dark hyphae and fruiting bodies are present (tiny nodular structures erupting through the coating and producing numerous dark spores). Staining can penetrate deeply and often cannot be removed by surface planing.

Stain fungi commonly found in buildings include Aureobasidium pullulans, Cladosporium species and Sclerophoma pithyophila, most of them belonging to class Hypomycetes (Deuteromycotina). Some stain fungi, for example, Ceratocystis species, belong to Ascomycotina.

Stains are troublesome due to their objectionable appearance, disfigurement of the wood and especially of clear finishes. Early failure of the surface may be brought about by rupture caused by the fruit bodies. Discolouration caused by Sap stain that has occurred in the log may still be detectable after the drying and conversion of the timber, but this will affect only the aesthetic value of the wood. Damage of the coatings will occur only through the growth of blue stain in service.