THEIR PROPERTIES

Published by University of Toronto Press TORONTO BUFFALO LONDON

 First edition

ISBN o-8020-2430-0

TA419-c38 1981 674'.00971 C81-094571 -1

Many species of primitive plants, known as fungi, live in wood. Some of these organisms use only the food that is stored in the wood (molds and sap stains), while others (wood-destroying fungi) attack the cellulose or lignin and ultimately rot the wood. Under suitable conditions these wood-destroying organisms may attack both trees and manufactured wood products. Except in tropical regions where insects are more active, fungi are the chief cause of the biodeterioration of wood. These specialized plants attack the structural elements and cause deterioration of wood (decay or rot), thus degrading its natural properties. Specific forms of deterioration can also be ascribed to bacteria, but their attack is far less extensive (Wilcox 1970; Rossell, Abbot, and Levy 1973).

Because we are concerned with the functional use and appearance of wood, the activities of all wood-inhabiting organisms are considered harmful. Estimates of losses from biodeterioration of logs and timber are higher than those from wildfire in the forest and have exceeded 2o% in North American stands. Fungi may also be responsible for extra labor and production costs when business activity is disrupted by replacement of timber that has deteriorated in service. For example, in one special case, it has been estimated that over $6ooo was expended when one decay-weakened transmission pole was replaced and power had to be curtailed. Between 85,000 and 100,000 poles are replaced annually in Canada because of decay, most of them in urban areas where costs of machines, poles, and labor can average $500 or more per pole.

For certain uses, chemicals can extend the service life of wood. Untreated wood used for poles, railway ties, bridges, and boats and in other exposed locations has been known to fail in three years. With preservative treatment, however, the service life of wood can be extended to 35 years or more. When preservatives are properly applied to wood before it is used in construction, there is little risk of subsequent damage to the environment. When properly fixed, such preservatives remain within the wood indefinitely and form an effective barrier against attack by wood-destroying organisms.

The body of a fungus is composed of slender, tubelike conductive strands called hyphae, which secrete enzymes, or biochemical catalysts, to enable the fungus to digest the walls and contents of cells. Reproductive structures, such as fruiting bodies, or sporophores, may be produced from the hyphae and are the means of identification for the many thousands of existing fungal species (Figure 8.8). There are numerous forms of decay fungi, many of which are best known as mushrooms or conks. They may appear on piled logs and timber, on timber in service, on wood buried in soil, or on trees in the forest. At maturity, they can produce millions of spores. These microscopic, seedlike bodies may be dispersed for long distances, carried in the air or in water droplets, or sometimes by insects feeding on the myceliurn -the mass of filamentous hyphae. Under favorable conditions the spores that reach moist wood germinate and develop new areas of decay. Fungal damage may also be spread by direct contact between decayed material and sound wood when hyphal strands (Figure 8.9) develop at the surface between the adjacent pieces.

The natural durability of wood, or its ability to resist fungal attack, depends on properties associated with the growing tree. Certain species, such as western red cedar and oak, are well known for their high durability, but this applies only to the heartwood because the sapwood is not durable. Decay resistance is provided by fungicidal chemicals formed as the tree grows, and these substances are deposited in the heartwood cells as the tree matures. In most woods these chemicals are more concentrated in the outer heartwood than in wood near the pith; thus, in many cases the inner heartwood tends to be reasonably low in resistance to fungal attack. The relative decay resistance of some species that grow in Canada is shown in Table 8. 1. Chapter 2 also contains an appraisal of the durability of Canadian species.

Both the development and the spread of wood-inhabiting fungi require food, air, moisture, and warmth. Food is supplied by the wood components, including cellulose, lignin, and the cell contents. While certain woods are more susceptible to attack (i.e., are less durable) than others, all species will eventually be destroyed by fungi under suitable conditions.

The supply of oxygen contained in the woody cells of forest products is usually adequate to support fungal growth. However, if it is removed, as in water-saturated wood (submerged logs), or contaminated by the introduction of fungicidal gases (fumigation), decay will be arrested.

Moisture is most important in the development of fungi and, in combination with air, governs the extent of decay in wood. Infection of wood through germination of spores or by penetration of the mycelium requires both a moist surface and a humid environment. Once the hyphae penetrate the surface, a balance of oxygen and moisture is needed to maintain the respiration that supports the enzymic decomposition of the cellular material.

In living trees the moisture-air balance is significant in that the heartwood can support decay organisms (causing heart rot), whereas the sapwood remains free of decay as long as the bark is intact because the sapwood is saturated with moisture. Once the tree is felled and the log is converted into wood products, air replaces some of the water in the sapwood. Deterioration caused by fungi can occur and will continue until the wood dries to a moisture content below 20%. Wood is considered immune to decay as long as its moisture content remains below 20%, although fungi present in the wood will not necessarily be killed under that condition; certain fungi can remain dormant for very long periods of time and revive when sufficient moisture again becomes available.

Fungi can tolerate a wide range of moisture content in wood and grow rapidly in wood containing approximately 50% moisture; the exact amount of growth depends on the species of fungus and on the density of the wood. The denser woods are more resistant to decay because there is more wood substance in these species and less void space than in the less dense woods. Under high-moisture conditions, with more water occupying the cell lumens, dense wood becomes more saturated than less dense wood. Consequently less oxygen is available for fungal growth, and, as a result, the denser wood has better decay resistance.

Fungi grow more rapidly in warm weather than in cold weather and become dormant under freezing conditions. The optimum temperature for most wood-destroying fungi is around 22'C (70'F). Although a small group of fungi called thermophiles is still active above 35'c (95'F) (Cooney and Emerson 1964; Smith and Afosu-Asiedu 1972), Most species are inactive at this temperature. Fungi are killed by prolonged exposure to temperatures above the maximum at which their growth ceases, particularly in a humid atmosphere. Conditions in commercial dry kilns using temperatures above 65'c (150'F) will kill woodinhabiting fungi.

Wood-inhabiting fungi can be broadly classified into different types, depending on their capacity to break down and digest the lignincellulose complex of cell walls. One very large group includes molds and sap stains (Figure 8. 1o) that cause little or no damage to structural elements because they obtain nourishment mainly from stored food (sugars and starches) contained within the cells. These fungi do not reduce strength significantly and, in some species, appear to inhibit decay to some extent (Shields and Atwell 1963; Hulme and Shields 1972). Mainly they produce unsightly discolorations in shades of blue-black through green and yellow to pink.

The discoloration resulting from the growth of molds (including species of Aureobasidium, Chaetomium, Cephaloascus, Penicillium, and Trichoderma) is largely superficial and can often be removed by brushing or planing the wood. In contrast, sap-staining fungi (including species of Ceratocystis, Fusarium, Stemphylium, and Chlorosplenium) penetrate deeply, and the stains are harder to remove.

Seen under the microscope, the discoloration is often associated with characteristic dark-colored hyphae, which develop extensively in the wood rays, where reserve food is most abundant. The hyphae pass from cell to cell, mainly through the pits. In some cases where they penetrate cell walls, the normally thick hyphae become constricted to fine threads within the tissue (Roff 1964).

Spores of both molds and sap-staining fungi are produced in copious numbers, either directly from the mycelium or sometimes from the ends of erect, microscopic, hairlike structures that give a fuzzy appearance to the myceliurn mat. Under warm, humid conditions (particularly if unseasoned wood is freshly cut), mold and sap stain can become visible within one day, the initial rate of development being many times that of decay-causing fungi.

The significance of bacteria in the breakdown of woody tissue is becoming more evident, particularly in wood piles used in the support of buildings. Many such piles have a high moisture content or are submerged in water, conditions conducive to bacteria attack that is dependent on the presence of free water in the cells. Under these conditions, material in the cell walls and in the pits is selectively degraded, and the wood becomes more porous (Ellwood and Ecklund 1959).

Most wood-decaying fungi show a marked preference for a specific type of wood. Knowledge of this preference, plus a general appreciation of their appearance and the damage they cause, can greatly assist in estimating the severity of the damage that has ensued.

True wood-destroying fungi possess enzyme systems that hydrolyze cellulose and other polysaccharides of woody cells into glucose and other simple nutrients. Many species in this group cannot attack the lignin portion of the cell walls, but after they have removed the accessible carbohydrates the remaining material may be dry, shrunken, often darker in color, and broken into small, crumbling, brick-shaped pieces. This condition is known as brown rot, or brown cubical rot (Figure 8. 11). This term also includes the condition incorrectly referred to as dry rot, which results from similar fungal processes.

Under wet conditions, deterioration of the surface of wood resembling brown rot may occur. This condition, called soft rot, is caused by certain microorganisms, including bacteria and fungi, in the superficial layers of the wood. It does not proceed as rapidly as other types of fungal rot.

Other wood destroyers not only attack cellulose, but also can break down lignin by means of oxidizing enzymes. These rots are more varied and are usually lighter in color than the surrounding wood. Known as white rot, they can be fibrous or stringy, or the affected portions may occur in discrete pockets or concentric bands surrounded by firm wood. The pockets may appear pitted, honeycombed, mottled, or as a soft, spongy mass.

From the scores of fungi usually present in rotting wood, certain species have been selected for the following discussion because their pattern of attack is characteristic of the rot type to which they belong and because they are economically important in the forest or in wood products.

 Fomes pini Karst. (white pitted rot, white pocket rot, conk rot)

This fungus causes one of the most widespread diseases in coniferous forests. It causes a white rot in heartwood (Figure 8. 14), which in the advanced stage appears as small, spindle-shaped cavities, often filled with fibrous material. It may involve the entire heartwood portion or be confined to a few annual rings. In the early stage of attack (incipient decay), the wood has a pinkish to reddish-purple stain, which is called red heart stain in many woods. Wood with advanced white pitted rot is relatively weak, but in the incipient stage it is as strong as unstained material (Atwell 1948; Kennedy and Wakefield 1948). The decay does not advance in service, and except where strength requirements are critical, red-heart-stained lumber is widely accepted in permanent construction. Wood with white pitted rot has also been used as decorative panelling.

The bracketlike fruiting body develops on trees. It is perennial and has a gray-brown upper surface with yellow-brown on the underside.

Fomes igniarius (L. ex Fr.) Kickz: (white trunk rot, white heart rot)

Fomes igniarius is a major cause of losses in hardwood forests. It produces a white rot of heartwood that is yellowish to white in color, has a soft and punky texture, and contains irregular patterns of dark lines. The incipient stage appears as gray-yellow and brown to gray-black zones in the wood. The presence of this stain does not indicate significant weakening of the wood, according to work done at the Forest Products Laboratories of Forintek Canada Corp. The perennial sporophores appear on trees and on down timber. They are woody and generally hoof-shaped with a brown undersurface; the upper surface is black and deeply fissured when old.

Stereum sanguinolentum (Alb. and Sch. ex Fr.) Fr. (red heart rot, red-brown stain)

Stereum sanguinolentum is widespread in coniferous forests and in down timber. It causes a white rot that is light to red-brown in color, dry, and somewhat stringy in texture; in logs it is usually a circular mass about the pith. Red heart rot is particularly damaging in the true firs (Figure 8.15). Incipient decay develops as a firm red-brown stain, often in streaks; on the end grain it usually appears as rays extending from a stained area. The decay does not advance in wood in service, and the stained wood is not significantly weaker than unstained material (Roff and Whittaker 1963); nor is the pulp strength likely to be affected. The annual sporophores occur profusely, appearing as thin, leathery sheets, often with the upper part curled to form a narrow shelf. When bruised, a fresh fruiting body exudes a blood-red stain.

Lenzites spp. (brown pocket rot)

Two Lenzites species, L. trabea Pers. and L. saepiaria Fr., are similar in growth and appearance, and both occur throughout the world. These brown cubical rots occur in softwoods, and less frequently in hard woods, in dead timber in the forest and in wood in service. Damage is extensive in exposed wood in buildings, poles, posts, and ties; the decayed wood is broken into small, cubical pieces, sometimes with thin strands of brown mycelium present (Figure 8.16). The fungi survive for very long periods in air-dry wood.

Wood with incipient decay may appear with yellowish to yellowbrown patches or sometimes with blackened areas. The incipientdecayed material, like that caused by other brown cubical rots, is seriously weakened, gives low pulp yields, and should be avoided. The annual sporophores are abundant. They may be semicircular or narrowly shelflike with a velvety, cinammon-brown upper surface (Figure 8.17). The yellow-brown lower surface has radiating gills or irregular pores (Figure 8.18).

Merulius lacrymans (Wulf) Fr. and Poria incrassata (B. and C.) Curt. (dry rot, building rot)

Both Merulius lacrymans and Poria incrassata cause a similar type of brown cubical rot, mainly on coniferous timbers in damp, unventilated situations. These two species, detailed below, cause the most extensive losses from decay in wooden buildings in North America. Once established, the decay can spread rapidly. Early infection is often invisible or may be seen as a superficial, thin, silky gray mycelium with patches of yellow, or as a fan-shaped, lilac-colored mycelial mat. Characteristic thick strands, which are brown to black, may develop on the wood. Because they can also be found in adjacent brickwork, the decay is very difficult to eradicate (Figure 8. 19).

M. lacrymans occurs primarily in eastern Canada and the northern United States. The typical sporophore is a thick, pale gray, platelike or bracket structure with a wrinkled surface that may exude water droplets. In time the conk becomes rusty red as a result of the myriads of spores produced. These conks appear in areas adjacent to the decay.

P. incrassata sporophores are similar in shape and are pale olive-gray with a pale yellow margin when young. With age, the sporophore becomes crustlike and brown to black in color, sometimes with masses of orange mycelium. This fungus is reported more commonly in the Pacific coast region of North America and in the southern United States.

Wood that is kept dry - that is, below 20% moisture content - will not deteriorate because of fungi and will be protected for as long as dry conditions last. Similarly, growth of terrestrial, wood-deteriorating fungi will cease on wood stored underwater because no oxygen is available. Under outside service conditions, however, it is not always possible to keep the wood either dry enough or wet enough to control decay (Figure 8.2o). To obtain satisfactory service life, the wood must be treated with protective chemicals. Although a preservative may be effective against fungi, it cannot adequately protect wood in service unless the treating solution penetrates deeply into the outer zone of the timber in sufficient concentration to inhibit fungal growth.

A good preservative must be a strong fungicide, possess good penetrating qualities, be leach resistant, and should not adversely affect such properties of wood as swelling, shrinkage, and strength. Treated wood should have a clean appearance, resistance to glowing or visible combustion, low mammalian toxicity, and low cost. It should be water-repellent and fire resistant, and it should not change color as a result of preservative treatment. The treated surface should also take paint well. No single preservative now in use can fulfill all these requirements.

The modern wood-preserving industry started in the eighteenth and nineteenth centuries when major efforts were made to prevent wood decay in British warships. British railway companies began treating their crossties soon after the patent for pressure-impregnating wood with coal-tar creosote was obtained by Bethell in 1883. The first treating plant of this type in the United States started operations in 1865. In Canada treated crossties were used by about 19o6, and the first commercial treating plant for this purpose was built in 1911 in eastern Canada. However, the first operation in Canada using a treating cylinder was established in 191o at Dominion Mills, Vancouver, for the treatment of wood paving blocks.

The wood-preserving industry has continued to expand and has contributed a great deal to Canada's economic growth. The chief advantages of using treated timber are the substantial savings in labor costs for replacing untreated structures that have become defective and the reduced maintenance costs of treated wood in service.

Specifications covering the quality and types of treatments for poles, crossties, piling, and other wood products are given in standards issued by the Canadian Standards Association (1974) and the American Wood Preservers Association (1974).

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Freshly cut lumber from unseasoned logs may become stained by fungi if it is not kiln dried or air seasoned within a short time. The lumber must be treated with fungicide if kiln-drying facilities are not available and warm, humid conditions prevail in the seasoning yard. Usually the lumber is immersed for a brief time in aqueous sodium pentachlorophenate, sodium tetrachlorophenate solution, or an emulsion of copper8-quinolinolate before it is stacked in the yard for seasoning. Alternatively the lumber may be treated with one of several commercial antistain products to provide superficial protection during the drying period. Unseasoned lumber may also be protected against decay by the boron-diffusion process, but some sodium pentachlorophenate must be added to retard development of staining fungi and molds.