A wood-decay or xylophagous fungus is any species of fungus that digests moist wood, causing it to rot. Some species of wood-decay fungi attack dead wood, such as brown rot, and some, such as Armillaria (honey fungus), are parasitic and colonize living trees. Excessive moisture above the fibre saturation point in wood is required for fungal colonization and proliferation. Fungi that not only grow on wood but permeate its fibrous structure and actually cause decay, are called lignicolous fungi. In nature, this process causes the breakdown of complex molecules and leads to the return of nutrients to the soil. Various lignicolous fungi consume wood in various ways; for example, some attack the carbohydrates in wood and some others decay lignin. The rate of decay of wooden materials in various climates can be estimated by empirical models.
Wood-decay fungi can be classified according to the type of decay that they cause. The best-known types are brown rot, soft rot, and white rot. Each produce different enzymes, can degrade different plant materials, and can colonise different environmental niches. The residual products of decomposition from fungal action have variable pH, solubility and redox potentials. Over time this residue will become incorporated in the soil and sediment, so can have a noticeable effect on the environment of that area.
Brown-rot fungi break down hemicellulose and cellulose that form the wood structure. Cellulose is broken down by hydrogen peroxide (H2O2) that is produced during the breakdown of hemicellulose. Because hydrogen peroxide is a small molecule, it can diffuse rapidly through the wood, leading to a decay that is not confined to the direct surroundings of the fungal hyphae. As a result of this type of decay, the wood shrinks, shows a brown discoloration, and cracks into roughly cubical pieces, a phenomenon termed cubical fracture. The fungi of certain types remove cellulose compounds from wood and hence the wood becomes a brown colour.
Brown rot in a dry, crumbly condition is sometimes incorrectly referred to as dry rot in general. The term brown rot replaced the general use of the term dry rot, as wood must be damp to decay, although it may become dry later. Dry rot is a generic name for certain species of brown-rot fungi.
Brown-rot fungi of particular economic importance include Serpula lacrymans (true dry rot), Fibroporia vaillantii (mine fungus), and Coniophora puteana (cellar fungus), which may attack timber in buildings. Other brown-rot fungi include the sulfur shelf, Phaeolus schweinitzii, and Fomitopsis pinicola.
Brown-rot fungal decay is characterised by extensive demethylation of lignins whereas white-rot tends to produce low yields of molecules with demethylated functional groups.
There are very few brown rot fungi in tropical climates or in southern temperate zones. Most brown rot fungi have a geographical range north of the Tropic of Cancer (23.5° latitude), and most of these are found north of the 35° latitude, corresponding to a roughly boreal distribution. Those brown rot fungi between latitudes 23.5° and 35° are typically found at high elevations in pine forest regions, or in coniferous forest regions such as the Rocky Mountains or the Himalayas.
Soft-rot fungi secrete cellulase from their hyphae, an enzyme that breaks down cellulose in the wood. This leads to the formation of microscopic cavities inside the wood, and sometimes to a discoloration and cracking pattern similar to brown rot. Soft-rot fungi need fixed nitrogen in order to synthesize enzymes, which they obtain either from the wood or from the environment. Examples of soft-rot-causing fungi are Chaetomium, Ceratocystis, and Kretzschmaria deusta.
Soft-rot fungi are able to colonise conditions that are too hot, cold or wet for brown or white-rot to inhabit. They can also decompose woods with high levels of compounds that are resistant to biological attack. Bark in woody plants contains a high concentration of tannin, which is difficult for fungi to decompose, and suberin which may act as a microbial barrier. The bark acts as form of protection for the more vulnerable interior of the plant. Soft-rot fungi do not tend to be able to decompose matter as effectively as white-rot fungi: they are less aggressive decomposers.
White-rot fungi break down the lignin in wood, leaving the lighter-colored cellulose behind; some of them break down both lignin and cellulose. As a result, the wood changes texture, becoming moist, soft, spongy, or stringy; its colour becomes white or yellow. Because white-rot fungi are able to produce enzymes, such as laccase, needed to break down lignin and other complex organic molecules, they have been investigated for use in mycoremediation applications.
There are many different enzymes that are involved in the decay of wood by white-rot fungi, some of which directly oxidize lignin. The relative abundance of phenylpropane alkyl side chains of lignin characteristically decreases when decayed by white-rot fungi. It has been reported that the oyster mushroom (Pleurotus ostreatus) preferentially decays lignin instead of polysaccharides. This is different from some other white-rot fungi, e.g., Phanerochaete chrysosporium, which shows no selectivity to lignocellulose.
Honey mushroom (Armillaria spp.) is a white-rot fungus notorious for attacking living trees. Pleurotus ostreatus and other oyster mushrooms are commonly cultivated white-rot fungi, but P. ostreatus is not parasitic and will not grow on a living tree, unless it is already dying from other causes. Other white-rot fungi include the turkey tail, artist's conk, and tinder fungus.
Induced fungal decay
A special way of giving grown wood an unusual structure is to infect it with a parasitic fungus by storing it in a humid environment (fungal decay). The fungus penetrates the layers of the wood and changes the nature of the cells. This process creates individual patterns and shades of colour. The wood treated in this way is then excellently suited for the production of all kinds of design objects. In order to stabilise the wood structure weakened by the fungus, resins or plastics are usually introduced into the material by special vacuum processes.
A special icing process applied to beech wood leads to results similar to those obtained with fungal decay. After the wood has been soaked, it is iced and then dried. The result is a very light wood with an almost black grain. This result, which also occurs very rarely in nature, is called ice-beech.
Natural durability is the inherent capability of wood to tolerate and resist fungal decay and insect attacks, such as woodboring beetles and termites, and marine organisms. This protective feature is attributable to specific biological compounds, called extractives that are toxic to wood-destroying organisms. Along with the tree’s growth, the sapwood converts into heartwood and this brings to physical and chemical changes to the wood. As a result, the permeability decreases while the natural durability increases. Thus, the extractives responsible for natural durability are mainly present in the heartwood, although they may also be contained in small amounts in the sapwood. Different chemicals have been isolated from the heartwood of naturally rot-resistant trees and have shown to be protectants, including polyphenols, lignans (e.g. gmelinol, plicatic acid), flavonoids (e.g. mesquitol), tropolones (e.g. hinokitiol and other thujaplicins), sesquiterpenoids (e.g. α-cadinol). The natural durability varies between tree species, geographic regions, environmental conditions, growth stage, and increases with the age. Thereby, some trees are more resistant to fungal diseases and insects and their timber lasts longer than other trees. Notably, the timber of these trees remain durable for a long-time period, even around a century, thereby they have been used as a reliable building material for centuries. Since the young trees do not produce enough protecting chemicals, some trees grow with a hollow, rotten trunk at an early age. However, the stands of old-growth trees are more naturally durable than second-growth stands. Tree species that have significant natural durability include Lagarostrobos franklinii (Huon pine), Intsia bijuga (Ipil), Eucalyptus species (Ironbark), Podocarpus totara (Totara), Vitex lucens (Puriri), Agathis australis (Kauri), trees of the Cupressaceae family, such as Chamaecyparis obtusa (Hinoki cypress), Thuja plicata (Western red cedar), Thujopsis dolabrata (Hinoki asunaro), Juniperus cedrus (Canary Islands juniper), Cedrus atlantica (Atlas cedar), Chamaecyparis taiwanensis (Taiwan cypress), and many other species belonging to this family.
According to the EN 350:2016 standards by the APA – The Engineered Wood Association, the durability of wood and wood-based products to fungal decay can be classified into five categories: very durable (DC1); durable (DC2); moderately durable (DC3); slightly durable (DC4); and not durable (DC5). And the durability to insect attacks can be categorized as durable (DC D); moderately durable (DC M); and not durable (DC S). Generally, the heartwood of durable tree species is considered as very durable, whereas the sapwood of all tree species is considered as not durable and is the most vulnerable.
A wide selection of timber preservation has been developed to give the wood an improved durability and to protect it from decay. The wood can be treated according to the purpose (biological protection, e.g. fungi, insects, marine organisms) and the environment (interior, exterior, above ground, in ground, in water) of its use. Timber preservatives include chromated copper arsenate (CCA), alkaline copper quaternary (ACQ), copper azole (CuAz), borates, sodium and potassium silicate, oil-based preservatives, such as creosote and pentachlorophenol, light organic solvent preservatives (LOSP), propiconazole-tebuconazole-imidacloprid, epoxy resins, wood acetylation, natural or biological preservation, such as treatment with heat (thermally modified wood), mud, tung oil, impregnation using biopolymers from agricultural waste (biological modified timber), covering wood with copper sheathes, etc. Treatment of timber with natural extractives derived from rot-resistant trees, such as hinokitiol, tannins, and tree extracts, is another promising environmentally-friendly wood preservation method. The more permeable is the wood, the easier is it to treat. According to the EN 350:2016 standards, the treatability of woods can be categorized in four levels: (1) easy to treat; (2) moderately easy to treat; (3) difficult to treat; and (4) extremely difficult to treat.
Over the years a lot of concerns have arisen regarding the arsenic and chromium contents of the CCA. In 1986, the U.S. Environmental Protection Agency (EPA) has recognized arsenic as a human carcinogen. The water contamination with arsenic and its compounds is a serious public health issue, and their release to the environment and soil pollution is another environmental problem. Different regulatory interventions have been undertaken worldwide to restrict their use in wood industry, especially in timber for residential use. By the end of 2003, the U.S EPA and the wood industry agreed to discontinue the use of CCA in treating timber for residential use. Its use is also prohibited in Canada, Australia and European Union.
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