Wood is subject to several kinds of defects or blemishes that are caused by fungi and bacteria. They are forms of plant life. Many of them grow on the contents of the cells of the wood but do not attack the cell walls; their only important effect is on color. Only part of the discolorations are due to organisms and the causes of many of them have yet to be determined. Most discolorations of hardwoods are already present before the tree is cut, but sap stain or blue stain usually occurs after the lumber is sawed.

The discolorations of yellow-poplar, the tuliptree, are particularly striking. In this and several other important species most of the discolorations have been found to indicate no appreciable weakening of the wood. Browned or bleached spots or streaks, however, commonly indicate the presence of decay fungi, which dissolve the cell walls and thereby weaken or destroy the structure.

The decay fungi belong to a special group, most of which have fleshy sporeproducing bodies—toadstools, mushrooms, brackets. The gills or the pores found on the under side of the fruiting bodies of most of the species have a large surface area. On these surfaces are borne a myriad of microscopic spores that are carried by wind to start new infections. The Division of Forest Pathology has about 300 species of these fungi growing in pure culture.

Wood attacked by fungi is lowered in toughness or shock resistance and (to a less degree) in bending or crushing strength before it is appreciably softened or reduced in weight. In wood to be used for paper, decay fungi reduce the quantity and quality of pulp, depending on the fungus involved and the process to be used in pulping.

Plywood is generally quite as susceptible to decay as solid wood of the species from which it is made, although some glues hinder the passing of fungi from one layer of wood to another. Wood or paper impregnated with a high content of phenolic resin (as impreg, compreg, or papreg) strongly resists decay, but impregnation with urea resins has given less consistent protection in the tests so far made.

MOST WOOD DECAY FUNGI will grow rapidly only between 60° and 90° F. They remain alive during long periods below freezing, but can be quickly killed by heat at temperatures about 150°.

The food requirements of the fungi limit many of them. The fungi that merely discolor are generally unable to attack heartwood of any species of tree because of its lack of the sugars or other readily digested food materials that they require. Some decay fungi can attack the wood of the broadleaved species only; others are limited to softwoods; some are even limited to a particular genus of trees.

Moisture is the factor most important from a practical standpoint. Fungi cannot grow in constantly air-dry wood, even in the more humid parts of the United States. Strictly speaking, there is no such thing as dry rot. Wood must contain moisture equaling more than one-fifth of the weight of the oven-dry wood before decay or staining fungi can develop in it. Decay fungi progress rarely or slowly, if ever, at moisture contents below 25 percent (oven-dry basis). The molds that grow on the sugars and other foods present in sapwood or destroy the starch or protein glues used in bonding some wood or wood-fiber products, however, can apparently work under conditions somewhat less moist than would be required for decay of wood.

Two of the decay fungi are especially dangerous to buildings because they can conduct water from moist soil or wood and thus attack wood parts of buildings that otherwise would be too dry—but they depend just as much on a source of moisture as other fungi. Fortunately these two species are not common in the United States.

The oxygen requirement becomes a limiting factor for the fungi in some situations. Wood that is completely waterlogged decays slightly, if at all. No important decay occurs in wood that is under water.

The heartwood of naturally durable species contains chemicals that limit the growth of organisms. These are nearly insoluble in cold water, but most of them can be extracted in hot, water. Such woods as redwood, baldcypress, black locust, pitch-soaked pine, and several of the cedars commonly remain free from attack for decades of exposure to the conditions that favor decay. Unfortunately, the second-growth stands on which we now depend largely for lumber contain a larger proportion of sapwood, all of which is decay-susceptible. Building practices that were reasonably safe with the lumber of the past century may not be good enough with the lumber we have now.

The salt in ocean water also appears to have some importance in hindering the decay in the hulls of boats. Ammonium salts in the amounts used in wood as fire retardants have prevented the decay of wood in laboratory trials, although they favor the growth of some of the relatively harmless mold fungi.

FOR LUMBER ALREADY DISCOLORED as it comes from the tree, all that can be done at present is to distinguish colors that indicate decay from those that do not, in order to avoid discarding harmless discolorations.

To avoid discoloration from fungi that develop in logs, the best measure is to get the logs to the saw promptly. Where this cannot be done, fungi can be kept from entering through the ends of the logs and spots where the bark has been knocked off by prompt spraying or brushing of the exposed wood with solutions containing organic mercury salts or chlorophenols or phenates.

In warm weather, lumber of many species, if not kiln-dried or quickly air-dried, is commonly stained by fungi within a few days after it is sawed. This can be controlled by dipping the lumber in a toxic solution not more than 24 hours after sawing. The same fungicides are used as for logs, but at lower strengths; the cost for materials is only 15 to 20 cents a thousand board feet of lumber dipped. Such dipping, if followed by good open piling to dry the lumber, reduces to a minimum the molding and staining and also the decay that sometimes gets started during seasoning.

To prevent decay in storage or use, the most generally practicable method is to keep the wood dry all the time or for so much of it that decay fungi never have a chance to get started. Until lumber is dry, it should not be solid-piled or built into parts of structures in which further drying is slow, unless it has been dipped promptly after sawing in a stain-control chemical solution.

To avoid decay in buildings, roof leaks must be avoided. Exterior walls must be so constructed that there is a minimum chance for water to enter at joints and be trapped in the wall. Where wood is on concrete laid on soil, there should be a dampproofing layer in or on the concrete; all embedded stringers should be of a decay-resistant wood or impregnated with a preservative.

Buildings without basements are subject to a special decay risk. During cold weather, moisture evaporating from the soil under the building may condense on the cold surface of the sills and joists, and stay long enough to let decay fungi get started. This sweating can be prevented by placing ventilating openings in the foundation wall on opposite sides of the building. Under test buildings where the vents have been too few or too small to keep the wood dry, the moist condition has been relieved by simply laying a cover on the soil under the building. Heavy roll roofing (55 lbs. or more per 108 square feet) rolled out on the soil and lapped but not fastened, was very effective. A 3-inch layer of slag or gravel in the soil, though apparently somewhat less efficient, was also helpful.

Where wood must be used in contact with soil or water, it should be either heartwood of one of the highly durable species or else it should be impregnated with a good preservative. Even in the best species, the heartwood from young trees or from the central heart of old trees is likely to be rather decay-susceptible. No sapwood of any species should be used in contact with soil without thorough preservative impregnation. Treatment of sills and first-floor joists of low buildings is a desirable—although not a necessary—precaution.

Impregnation is best accomplished by pressure treatment at a commercial treating plant. Wood of ordinary lumber thickness can be reasonably well impregnated without pressure if given a hot bath followed by a cold bath. Dip or brush treatments have some place in wood members exposed to rain or occasional moisture, as in porches and window sash and frames, if it is too difficult to get impregnated lumber locally. Water and fungi enter through exposed end grain more readily than through sides; preservative treatment of ends of members is therefore especially profitable. If untreated ends are exposed, no treatment is worth much. Paint can be of value for decay prevention if it is unusually well maintained, with no cracks at the joints. If wood is painted when green, its drying out may be delayed and the decay hazard actually increased.

In the special case of boat construction, only heartwood of durable or moderately durable species should be used. These would include the woods mentioned previously; also teak and mahogany; white or chestnut oak, but not red or black oak; and dense Douglas-fir and dense southern pine. Seasoned wood should be used so far as possible. Leakage, especially of fresh water or rain water, into the boat must be minimized. Ventilation must be provided for all parts of the hull. More attention must be paid to ventilation when the boat is laid up than when it is in use. Except for the interior trim, preservative treated wood is needed if durable wood is not used, but it is difficult to employ with full effectiveness in boats because the cutting, fitting, and fairing so often expose parts of the wood that have not been penetrated by the treatment.

Decay of wood used in aircraft is easily avoided. The cases of damage reported in service have nearly always been due either to failure to put drain holes at the lowest points or carelessness in allowing them to become clogged. Out-of-doors storage in crates that admit rain also caused damage.

The life of plywood bonded with protein glue has been greatly increased in moist situations by the use of chlorophenols or phenates in the glue. The resistance of fiberboard to deterioration by molds can be increased similarly by the use of chlorophenates, which in this case must be added to the fiber during manufacture as well as to the laminating glue.

Often, when one replaces decayed members of structures, he leaves some of the old decayed material in contact with the new wood. This is an invitation to trouble. It should never be done where moist conditions may continue.

Charles F. Brannan

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