Fungal decay

There are 2 types of rots:

1. white rot; and 
2. brown rot. 

Wet rot can be either white rot or brown rot and dry rot is a typical brown rot fungus. White rot is caused by fungi that degrade all the lignin and cellulose and brown rot by fungi that do not degrade lignin. The residue left after the action of a brown rot fungus is a brown amorphous substance, rich in lignin, and which gives the rot its descriptive name. While the lignin is not substantially degraded by a brown rot fungus, it is modified and this modification is probably necessary to allow access to the cellulose, the main nutrient of the brown rot fungus. In fact, the first action of a brown rot fungus may well be lignin modification.

Generally the main fungal decay problems found in buildings are caused by brown rot fungi though some white rot organisms, for example, Phellinus contiguous and Asterostroma species, do cause some more minor difficulties in buildings. Wet rot is also called 'white rot' as it destroys both cellulose and lignin, leaving the colour of the wood largely unaltered, but producing a soft felty or spongy texture without cross cracks. Common white rots are Donkiporia expansa, Asterostroma species, Pleurotus ostreatus and Phellinus contiguus. 'Brown rots' cause the wood to become darker in colour and to crack along and across the grain; when dry, very decayed wood will crumble to dust. Many common wet rots are brown rots, such as Coniophora puteana, Coniophora marmorata, Paxillus panuoides and Dacrymyces stillatus. Wet rot fungi contain both white and brown rotters, all dry rot fungi cause brown rot. There are a large number of wet rot organisms including Coniophora puteana, Antrodia vaillantii and Paxillus panuoides.

The range of dry rot fungi is more limited and originally included Serpula lacrymans, Serpula himantioides and Leuocogyrophana pinastri. This type of decay is caused by a number of Basidiomycetous fungi of which the most important are Coniophora puteana (cerebella), 'Poria' fungi, Phellinus contiguus, Donkiporia expansa, Pleurotus ostreatus, Asterostroma species, and Paxillus panuoides.

Figure 1: Typical wet rot and dry rot decay to timbers

Figure 2: Typical wet rot (Coniophora puteana) decay to timber bench in a museum

White Rot Pleurotus ostreatus (Oyster Fungus)

Pleurotus ostreatus white rot fungi destroys both cellulose and lignin, leaving the colour of wood largely unaltered, but producing a soft, felty or spongy texture without cross-cracks. There is currently increasing interest in the biodegradation of polycyclic aromatic hydrocarbons (PAHs), since some of these ubiquitous pollutants are suspected to be human carcinogens. PAHs are components of coal tar, creosote and crude oil formed by the incomplete combustion of organic material. P. ostreatus and other white rot fungi, such as Trametes versicolor, Bjerkandera sp., have also been shown to metabolize PAHs. For instance, P. ostreatus degrades phenanthrene, anthracene, pyrene, fluorene, and benzo[a]pyrene to CO2 and water-soluble products.

Figure 3: Fruiting body of Pleurotus ostreatus to the outer skin of the collection container in a London museum

Figure 4: Fruiting body of Pleurotus ostreatus to the inner skin of the collection container in a London museum

White rot Donkioporia expansa

Donkioporia expansa occurs in very wet wood with a moisture content of about 40% to 60% and has been reported to grow in cooling towers in saturated wood with 100% moisture contents. Donkioporia expansa causes similar destructions in buildings as Coniophora puteana and Antrodia vaillantii. Among the decay fungi in buildings Donkioporia expansa is second most prevalent in Germany and third in Belgium. Surface mycelium is inconspicuous, located between the wood and the fruiting body and attached firmly, a yellowish white in colour with no stands.

Fruit bodies are perennial woody brownish, very large with a diameter of 150mm to 250mm and a thickness of 25mm and always spread out with one or more layers of tubes, resupinate. With its porous surface Donkioporia expansa can be confused with Antrodia vaillantii, however the pores are not white, but grey to ochre brown. Donkioporia expansa is commonly found in churches on oak trusses and oak structural timbers ad is associated with death watch beetle (Xestobium rufovillosum). The infestation by Donkioporia expansa is localised in the areas of water ingress and fungus does not spread to other areas due to lack of standing.

Figure 5: Fruiting body of Donkioporia expansa in the basement of a care home in Buckingham