Influence of Radiation on Growth

Generally speaking, light tends to check the growth of fungi and prolonged

exposure to sunlight may even kill the vegetative mycelium of

many species. Light has an important influence on the formation and development

of the fruit bodies of the higher fungi; few species are able to

form normal fruit bodies in total darkness and one finds many queer abortive

growths on pit props in mines. Agaris tend to form long, branching

stalks with little or no cap. Polypores often form only flat poroid growths

which may be difficult to recognize. Even short exposure to light of no intensity

may initiate fruiting in some species. Prolonged exposure to ultraviolet

radiation is fatal to exposed mycelium, but penetration of these rays

into wood is slight and exposure to ultra-violet is not therefore an effective

method for sterilizing samples of wood for culture experiments.

Fungi appear to be insensitive to X-rays and can survive exposure to

radiation of moderate intensity without suffering any noticeable damage.

Dielectric heating by means of a high frequency current is now used extensively

for setting of glue in joints and it has often been suggested as an effective

means for sterilizing timber and walls infected with the dry rot fungus.

Under laboratory conditions the intense and localized heating that can

be obtained so quickly has proved most effective for sterilizing timber infected

with insects or fungi, but there are many practical difficulties to be

overcome before this method can successfully be employed for the treatment

of large-sized timbers in ships or thick walls in actual buildings.

Metabolic Products of Wood-Rotting Fungi

Fungi are continually building up products at the same time as their

enzymes are decomposing the cellulose and other substances in the wood

in which they are growing. Some of these so-called metabolic products

remain inside the hyphae of the fungus, others are secreted into the surrounding

medium. It is this second group of substances that are of particular

interest to the student of wood preservation for they can affect the performance

of wood preservatives with which they come into contact. Organic

acids are probably the commonest products of the metabolism of

wood-rotting fungi. It is well-known that fungi vary greatly in their susceptibility

to different fungicides.

The preservation of wood against fungal decay usually involves protecting

it against a wide range of possible enemies — unlike the protection

of a plant against a specific parasite when one deliberately chooses a

chemical that is known from laboratory tests to be particularly deadly to

the parasite concerned. While we generally have to give blanket protection

against any fungus that may come along, there are cases where we can use

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our knowledge of the resistance or susceptibility of particular fungi to

practical advantage. We would not, for instance, use copper sulphate for

preservation of pit props in mines where Poria spp. are known to be prevalent,

or attempt to treat an outbreak of Lentinus lepideus with brushapplied

applications of creosote. This problem of choosing the preservative

in relation to the type of destructive organism involved assumes greater

importance if we attempt to protect timber against the fungi that bring

about so-called "soft rot".

Physiology of Fungi that Cause "Soft Rot"

We have coined the term "soft rot" to describe the decomposition of

wood by cellulose-destroying micro-fungi which is characterized under the

microscope by the longitudinal penetration of the secondary cell walls by

the fungal hyphae. This type of breakdown is generally found on the surface

of timber exposed to persistently damp conditions and is much more

common in hardwoods than in softwoods, at least at normal temperatures.

It assumed economic importance only in special instances where for some

reason the attack by normal wood-rotting Basidiomycetes is inhibited, as

in the louvres of water-cooling towers. Generally speaking, the attack is

confined to the surface layers of the wood and one may reasonably compare

the method of attack to the damage done by the gribble — which results,

in a wasting away of the surface layers. It will obviously be of

greater significance where the softened surface is eroded away, e. g. by

falling water, or when it occurs on very thin material such as plywood. For

this reason we shall find that it will be more difficult by treatment with

conventional wood preservatives to protect ply-wood that is in contact

with the ground than solid timber, in which slight surface softening has no

practical significance.

In preliminary studies on the physiology of the moulds that cause soft

rot we have established that they grow very much more readily on hardwoods

than on coniferous woods and that their ability to attack wood is

greatly stimulated by the addition of nitrogen and phosphates. Temperature

seems to have an even greater effect on the rate of decay by these moulds

than it has on the wood-rotting Basidiomycetes. In the limited number of

tests that we have been able to make it appears that some of the cellulosedestroying

moulds are more resistant than most of the Basidiomycetes to

many of the chemicals used in wood preservation, and that in situations

where this type of attack is important it may be necessary to modify the

treatment given or again, as in the case of the gribble, to combine chemical

treatment with some form of physical protection such as a paint or synthetic

resin finish afford.

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