Fungi in Forest Products

by CARL HARTLEY ONE WAY in which wood can be made to give better service to the user is to increase its durability. As a matter of fact, wood is already more lasting than is needed for many uses-it remains unharmed in air that is moist enough to cause serious corrosion of steel, and most of the timber that comes out of buildings that have been in service for centuries is practically as good as when it went in. Nevertheless it is true that decay fungi destroy considerable quantities of wood in some situations, and the uncertainty they cause is sometimes more serious than the actual total damage. The labor cost of replacements is high. Because an increasing part of our lumber supply is from young stands with a large proportion of sapwood, losses from decay will increase unless more precaution is taken to prevent them.

One way to get reliable service at low cost is to select wood intelligently. During the war, species were put to exacting uses, particularly in production of training planes and gliders, where maximum strength with minimum weight were prime considerations. In yellow birch, sweetgum, and yellow poplar for plywood, and in the poplar that also went into aircraft frames when spruce was scarce, material of the best texture and grain was in great demand.

Discolorations not previously studied led to the rejection of much of the wood that otherwise would have been accepted, because of the suspicion that the discolorations indicated weakening by incipient decay. Microscopical and cultural studies were conducted on them; we learned that, for those species and also for beech, only the brownish tints indicated infection with decay fungi. Most of the conspicuously discolored wood, sometimes pink, purple, green, or even nearly black, was found to contain no decay organisms. Thousands of strength tests showed that most such discolorations indicated no highly important decrease in strength.

Studies were also made on Sitka spruce, western hemlock, noble fir, and mahogany. Hundreds of sets of specimens demonstrating the acceptable and objectionable discolorations were prepared for use by Government inspectors and men in the wood-working industries. With their help, it was possible to use safely much valuable material that was badly needed and would otherwise have been rejected. We estimated that the information on yellow poplar made possible a 20-percent increase in the aircraft grades accepted, at a time when the production of poplar veneer was far below the requirements.

The black streaks in wood that extend in both directions from nails are familiar to everyone who has worked with wood. Most of the darkening of wood at nail holes comes from the chemical interaction of the iron with the tannic acid of the wood; small quantities of iron tannate, a common component of black ink, are formed. Such darkening is generally harmless except where appearances are important. It is frequently observed, however, that decay by fungi is more common at nail holes, and decay at those places may be important because it weakens the joint. This localization of decay is probably due mostly to water that condenses on the nail or penetrates the wood by following the nail, but it may be that the iron in the nail sometimes has an additional effect.

In a study of the darkening and molding of mahogany-faced aircraft plywood near nails used during the gluing process, it was found that iron inactivated the toxic extractives on which the wood depends for its resistance to fungi, thus allowing the fungi to develop. A similar but less pronounced effect was found with white oak heartwood. In redwood and in nondurable woods, the iron had no consistent effect on decay by the test fungi used. Since the wood and the extracts tested were heat-sterilized after the iron was added, further tests in which there is no heating are needed before final conclusions can be drawn.

Another precaution that would avoid some failures of wood in service is to use only seasoned lumber for building. Since the beginning of the, war, too much of the lumber intended for buildings has been shipped green, and some of it is still shipped without seasoning. Green lumber is physically less desirable and, besides, may contain the first stages of decay, developed during transit or storage. Decay is not always easy to recognize, but lumber that is reasonably free from sapstain or blue stain is usually free from decay, since the stain fungi develop under the same conditions that permit decay fungi to start. For uses in which great strength is required, wood that is heavily blue-stained should be avoided. In the sub- structure of a basementless building on a moist site, or in a wall assembly in which vapor barriers will hinder drying, lumber that has not been dried may further cause the spread of decay.

Even when mills consider it necessary to ship green lumber, it has been possible to reduce materially the danger that serious infection will develop in it before it has a chance to dry out. The chemical dips that were previously used before open piling have been shown to delay the establishment of heavy stain infections or of decay fungi in bulk-piled green lumber for weeks or months, and sometimes as long as a year. The concentrations found in wartime experiments to be most effective are somewhat stronger than those used on open-piled lumber, but it was found that lower concentrations, which are less likely to irritate the skin of men who handle the lumber, can be made effective by adding rather large quantities of borax to the solutions.

Much deterioration in service can be avoided by using decay-resistant wood in the places where it is needed. It has long been known that heartwood from certain species of trees, as, for example, black locust, Osage orange, bald cypress, redwood, and some of the cedars, is highly resistant to decay, and that a number of other species furnish moderately durable wood. Laboratory studies on shipbuilding timbers have confirmed previous opinions as to the general superiority of white oak to the red and black oaks. They also indicated that among the white oaks, chestnut oak is more resistant than true white oak. Studies completed recently in cooperation with the Soil Conservation Service show that there are large differences between different strains of black locust. The shipmast and flowerfield varieties, both in field experience and in accelerated laboratory tests, were decidedly more resistant to decay than ordinary locust.

It has also been found that there are large differences between trees of different ages and between parts of the same tree. In the white oaks, locust, western red cedar, and Douglas fir, and in most other resistant species on which tests have been made, higher resistance or a larger amount of the extractives on which resistance depends has been found in wood of older trees, and in the outer rather than inner heartwood. The central part of the heart at the base sometimes has very little decay resistance. These findings open the way for selecting more resistant wood when it is needed for situations that especially favor decay, such as tank staves, boat ribs, and wood that is placed in contact with the soil.

One of the advances toward making wood more useful has been the increase in the amount of wood treated with fire-retardant chemicals. These usually contain ammonium salts, which in weak solutions many fungi can use as a source of nitrogen, an element of which there is little in untreated wood. It was therefore feared that treated wood would be more susceptible to decay than untreated wood. Laboratory tests made with pure cultures of three common decay fungi on pine sapwood, however, showed that while the treatments did favor some of the mold fungi, they actually hindered or prevented decay in wood not subject to leaching. In contact with soil, deterioration of treated wood was no more rapid than of untreated. Fiberboard containers are sometimes subjected to moist conditions. The weakening of the board due to the moisture is frequently aggravated by the attack of micro-organisms. The paper layers of which fiberboards are built up are bonded with glue. When urea-formaldehyde resin is used in the glue, it is extended with several parts of starch. This mixture is susceptible to attack by fungi, which sometimes cause the fiberboard to delaminate; the fiber itself is also affected by fungi. Tests showed that this could be much delayed by the use of 2 percent sodium pentachlorophenate in the fiber, or "furnish." Loss in tensile strength in moist exposure was reduced to less than one-third of that in untreated board, and a higher concentration should give still better results.

Preliminary tests indicated that the contents of moist packages might be protected against mildew by volatile fungicides incorporated in the packing material. Of 40 different chemicals tested, the familiar paradichlorobenzene used for moth control appeared most promising; it prevented mold both on wood and on other materials exposed with it.

Plywood and Modified Woods

Protein glues used in bonding plywood were found to be destroyed by either molds or bacteria; the latter were able to attack the glue in submerged specimens in which delamination had been previously attributed to direct effect of moisture, but require much more moisture than do the molds. This points to the possibility of better protection by adding preservatives that are bactericides as well as fungicides.

The synthetic resins coming into use for gluing generally resist fungi and bacteria, when not extended with other materials, although most of them give little or no evidence of any lasting toxicity to fungi. Hot-pressed phenol- and resorcinol-formaldehyde and melamine resin glues applied in liquid form have hindered effectively in laboratory tests the passage of decay fungi from one wood ply to the next.

Cold-pressed urea resins were less effective, and cold-pressed resorcinol resins still less so. When a hot-press phenolic resin was used in the sheet form in which it is often applied, fungi spread through the glue lines with little hindrance, though two sheets used together proved very effective. The number of representatives tested in each type of glue was small, and additional tests may alter these findings. Phenolic resin used as a varnish had value in preventing the surface molding of wood, if enough coats were applied to give a continuous cover. Resin glue lines materially delayed the decay of the wood under experimental conditions in special types of plywood in which the veneers used were only 1/5o inch thick, but not in specimens made of 1/16-inch veneers.

Woods that have been modified by impregnation with resins or otherwise are of interest for use as face plywood and for other purposes. Both compressed wood and pulp impregnated with phenol-formaldehyde resin (compreg and papreg) have shown a high degree of resistance to decay fungi in laboratory tests. Acetylated wood has also been highly resistant in preliminary trials of Forest Service material.