Dry Rot

Fungi are living plants, of which there are thousands of species, and over a dozen are known to cause deterioration of timber. Wood-decaying fungi reduce the weight of the wood, spoil its appearance and take away its strength. True dry rot is the name given to the decay of timber brought about by one particular species of wood-rotting fungi, merulius lacrymans.

The term ‘dry’ is descriptive of the dry and friable conditions to which the rotten wood is reduced. Dry rot is often a symptom of neglected maintenance or the consequence of faulty design or construction of buildings, because the fungi thrives only in conditions of dampness and poor ventilation.

Dry rot cannot develop in wood containing less than about 25 per cent moisture, and the optimum moisture content for its growth is probably between 30 and 40 per cent.

The characteristic signs of the decay by which the dry-rot fungus can be identified are: Rust-red dust caused by gathering spores from a fruiting body indicate an advanced attack of some duration. The spores only accumulate in still, unventilated conditions.

A covering of matted fungal strands external to the timber occurs as thin sheets of silvery-grey or mouse-grey appearance, tinged here and there with lilac patches; bright yellow patches may also occur. This type of fungal hyphae is known as mycelium.

In damp, humid conditions the mycelium grows rapidly. It is snowy white, rather like cotton wool, but where the edge of such mycelium comes into contact with drier air or exposure to light, it becomes bright yellow.

The specific name, lacrymans, refers to the characteristic it shows in damp conditions when in active growth. Innumerable globules of water sparkle in the light of a torch like a large number of teardrops – lacrymans means ‘weeping’. The generic name, merulius, refers to the bright-yellow colouration which occurs on the mycelium, similar in colour to the beak of a male blackbird (Merula).

Wood decayed by the mycelium shows deep transverse and longitudinal fissures and the wood breaks up into cubes, sometimes of large dimensions. Such cracking is seen on the surface of the wood.

The wood becomes very light in weight, owing to the extraction of the cellulose by the fungal hyphae.

The wood becomes darker in colour, usually brown; it is friable when rubbed between the fingers; and the wood loses its characteristic fresh, resinous smell.

The appearance of a sporophore or fruiting body, which is thin and pancake-like, white round the edges, with the centre thrown into corrugation. The colour of the spores makes it rusty-red. When in active growth, the sporophore and the mycelium have a strong mushroomy smell.

A very important characteristic of merulius lacrymans is the ability of the fungus to produce water-carrying strands or rhizomorphs. These strands are formed from hyphae and modified to form vein-like structures. They may be as large in diameter as a lead pencil.

The importance of the rhizomorph is that it conveys the water from wood, which has decayed to dry wood, elsewhere, the strands passing over brickwork, stone or metal.

It is in the hyphae constituting the rhizomorphs that food reserves are stored, so that even if the affected wood is taken away, the rhizomorph is still capable of further growth and infecting new wood. The rhizomorphs are also able to penetrate soft brickwork and mortar.

10. November 2011 by admin
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