Moisture - For fungi to destroy wood, free water must be present in the cell cavities. Free water is
usually found when the moisture content is 25 to 30 percent or higher. Decay stops at 20 percent or
less moisture content. Thus, one effective way to control wood decay is to keep the moisture con-
tent of wood below 20 percent. Lumber properly installed in buildings has a moisture content of 8
to 15 percent, while that cut from freshly harvested trees may have a moisture content of well over
100 percent, based on the oven-dry weight of the wood. Wood in soil contact almost always has a
moisture content of 20 percent or more.
4.2.1 Mold. Molds (Figure 4-2) discolor the surface of both hardwoods and softwoods.
Molds are predominately different shades of green, black and occasionally orange or other light
shades. Although the fungus hyphae does penetrate into the wood, the surface discoloration can
generally be planed or even brushed off. On hardwoods some shallow staining may result. Strength
other than toughness is not seriously affected by molds, but the permeability of the wood may be
Control measures include reducing the moisture content of wood to below 20 percent, or the use of
approved fungicides applied as dips or sprays. It should be noted, however, that some molds can
develop on air-dried wood with a moisture content in the 20 percent range if the relative humidity is
90 percent or more.
18.104.22.168 Biological. Sapstain caused by fungi (Figure 4-3) is the blue to black,
gray or brown darkening of the sapwood of both hardwoods and softwoods. The dark color is due
to the deep penetration of large masses of fungal hyphae. Some sapstains may produce relatively
bright colors such as red, purple, and yellow. Logs, as well as green lumber, poles, piles, and vir-
tually all wood products, are susceptible. In warm weather the sapwood of some species can dis-
color in less than a week. It is generally agreed that sapstains do not seriously reduce wood strength
except for toughness. Therefore, the wood should not be used where it will receive significant
repeated jars, jolts or blows. However, it should be recognized that decay could easily accompany
intense sapstain and thus affect other strength properties.
Like the molds, sapstains increase the permeability of wood.
Two good methods to control sapstain in lumber cut from stain free logs are kiln drying lumber im-
mediately after sawing, or dip treating it in an approved fungicide as it leaves the sawmill.
Peeled round products such as poles and piling cannot be reliably protected by ordinary dipping or
spraying. These superficial treatments will keep the surface bright, but seasoning checks will expose
the interior wood.
22.214.171.124 Chemical. Stains caused by fungi should not be confused with a gray to
brown chemical or oxidation stain which develops on hardwoods during prolonged warm, wet