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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.
147
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
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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.
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