Physiology of wood-destroying fungi

The most favourable condition for the growth of fungi in wood seems

to be when the cell walls in addition to being fully imbibed are covered

with a film of liquid water in which free diffusion of the enzymes and the

products of their action can take place, but where some air spaces remain

in the cavities through which diffusion of gases can take place; in practice

this usually means a moisture content of 30 to 40 per cent. The less dense

the wood the higher is the optimum moisture content for growth of any

particular fungus. If the wood is in the form of sawdust the fungi can tolerate

— in fact demand — a very much higher moisture content, one of the

order of 200 per cent of its dry weight.

Some fungi can survive prolonged periods of desiccation and can remain

alive, though in a dormant condition, for many years in air-dry wood.

The fungi that occur on fences and exposed woodwork are much more resistant

than those that cause dry rot in buildings. In some tests still in progress

at the Laboratory we have found that Lenzites trabea is still able to

revive and grow vigorously after ten years in wood kept at 12 per cent

moisture content at 77 °F, whereas the mycelium of Merulius lacrymans in

wood died after six months' storage under the same conditions In practice

one cannot assume that even prolonged storage under dry conditions can

free wood from latent infection which may revive if the wood is rewetted.

For this reason it is vitally important that timber used in boat building and

for the construction of vats should be completely free from even traces of

infection by wood-rotting fungi when it is used.

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Temperature Relations

In common with other plants fungi grow more quickly in warm weather

than in cold. Though the optimum temperature varies considerably between

one species and another, it generally lies between 75 and 85 °F, and speaking

very roughly the rate of growth is twice as fast at 60° and four times as

fast at 80° as it is at 40 °F. The practical implications of this are obvious —

while it may, for example, be safe to leave beech logs lying on the ground

in the woods for months during the winter, it may be unwise to leave them

for as many weeks in summer. Far more rapid extraction of logs of perishable

species is necessary under tropical conditions than during the winter in

the temperate zone if sound timber free from dote is to be obtained. Fungi

can make little or no growth at temperatures around freezing point and there

is of course no risk of any deterioration below freezing point.

Though growth may be slow at temperatures between 32 and 40 °F,

we not infrequently find quite extensive decay in the wooden linings of

cool stores used for storage of fruit, where condensation on the walls may

wet the wood. It is always wise to treat — preferably by impregnation with

an odourless wood preservative — any woodwork used-in the lining of

cold stores.

Few wood-rotting fungi can grow at temperatures exceeding 100 °F,

but many of them can survive quite long periods of exposure to temperatures

considerably higher than this. In order to ensure that the most resistant

fungi are killed it is necessary to bring the wood up to a temperature

of 150 °F, throughout its thickness and to hold it at this temperature for at

least 75 minutes. Any time required for heating of the wood throughout its

thickness must of course be added to this period. It is much easier to kill

fungi by heat when the moisture content of the wood and the atmospheric

humidity around it are high. Heat sterilization of infected timber should

therefore be carried out in a saturated atmosphere.

The effectiveness of heat in killing deep-seated latent infection in

large-sized timber may be of practical significance in the creosoting of

Douglas fir in large dimensions, into which creosote cannot penetrate

deeply. Some Douglas fir cut from large over-mature trees contains traces

of incipient decay and if the fungi responsible are not killed by heat during

the pressure treatment they may continue to develop and cause internal decay

in service after moisture has penetrated throughout the timber. Certain

fungi such as Merulius lacrymans, are much more sensitive to heat and

wood infected by them can be sterilized by exposure to temperatures of

110 to 120 °F. Though the fruiting bodies of wood-rotting fungi may be

shrivelled by frost, their mycelium is usually very resistant to low temperatures

and it is evident that native species of fungi are never likely to be in 148

jured by exposure to naturally occurring cold conditions in Europe.