Composition of wood & wood rot.
Many fungi, both Ascomycetes and Basidiomycetes, live on wood and obtain their nutrients from it.
Fungi are the main causes of wood decay.
The living parts of a tree are the roots, leaves and a thin layer under the bark, the rest being dead heartwood.
The wood-rot fungi mainly feed on dead wood but some will grow on live wood and may cause it to decay.
The cell walls of all plants are made of cellulose, a simple carbohydrate, and are white in colour.
Tree cells have a second wall of brown lignin inside the primary wall.
Lignin is a complex polymer and harder to break down than cellulose.
The dry weight of wood is 40-50% cellulose, 25-40% hemicellulose and 20-35% lignin.
Dry wood will not rot.
If the water content is over 28% the cells can absorb no more and the water becomes free water.
When there is free water the timber is at risk of being attacked by wood rot fungi.
Wood-decay fungi secrete digestive enzymes and other chemicals to break the wood down.
The resulting simplified products are absorbed by the fungus as food or recycled in the soil.
Some fungi attack the lignin, others the cellulose and a few can attack both.
The enzymes require free water to move in so dry wood will not rot.
Wood decay fungi may be classified by the appearance of the wood residue left.
This depends on what part of the wood the fungi attack.
White and brown rot are the two main types of wood rot.
In brown rot the wood has a brownish cubed appearance and in white rot it is white and fibrous.
Other types include dry rot which is a form of brown rot, and soft rot.
In general, live trees tend to decay from the inside out and dead trees from the outside in.
Sapwood has a very effective resistance when the tree is alive but virtually none once the tree is dead.
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Brown or cubical rot.
Brown-rot fungi attack both cellulose and hemicellulose leaving the lignin mostly intact.
These have very few hyphae and the initial attack is chemical involving hydrogen peroxide (H2O2).
This easily diffuses through the wood causing damage to areas remote from the fungus.
This is followed by the release of enzymes.
Brown-rot can be produced by treating wood with H2O2 alone.
The fibrous texture of the wood, due to cellulose, is lost early in the process.
As the wood dries it shrinks and breaks into roughly cube shaped pieces.
These are brown or dark brown in colour due to the remaining lignin.
It then crumbles easily.
Most brown-rot fungi are Basidiomycetes, frequently leathery polypore brackets or shelves.
Common genera include Schizophyllum and Fomes.
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Soft rots.
Soft rots are caused by both fungi (Ascomycetes) and bacteria.
The fungi produce cellulase which attacks cellulose and hemicellulose.
This causes microscopic cavities in the cell walls.
The enzymes only affect areas where there are hyphae, usually on the surface.
They have little effect on lignin.
The wood may become brown with cracking patterns like brown-rot or it may have a bleached look.
They grow more slowly than the other wood-rots and do not affect living trees.
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White rot.
White-rot fungi can attack both the lignin and cellulose and eventually decay the wood completely.
They produce few enzymes but have strong oxidants, including hydrogen peroxide, which attack the lignin.
Cellulose is degraded by enzymes such as cellulase.
Some attack mainly the lignin, others the lignin and cellulose either simultaneously or sequentially.
These differences result in varying appearances of the decayed wood.
When the lignin is attacked first the remaining cellulose makes the wood pale and stringy.
When both are attacked together the wood is moist, soft, spongy, or stringy and white or yellow.
Decay may be confined to the cells colonised by the hyphae.
This results initially in areas of advanced decay surrounded by normal wood.
There are thousands of Ascomycete and Basidiomycete white rot species.
They are the only organisms able to completely degrade lignin.
White-rot fungi, such as the Shiitake mushroom, are often used as food.
J.F.
