Fungi are the principal agents of decomposition in ecological systems, and are an unavoidable fact-of-life. Although particularly prevalent in the south, where temperature and humidity create ideal conditions, wherever you find organic matter, fungi will be close behind.
When it comes to wood and wooden structures however, these pint-sized parasites can create big problems, leaving unsightly stains or even weakening buildings to the point of structural failure.
Two kinds of major decay fungi are recognized: brown rot and white rot. With brown-rot fungi, only the cellulose is extensively removed, the wood takes on a browner color, and it can crack across the grain, shrink, collapse, and be crushed into powder.
With white-rot fungi, both the lignin and cellulose are usually removed, so the wood may lose color and appear whiter than normal. It does not crack across the grain, and until severely degraded, it retains its outward dimensions, does not shrink or collapse, and often feels spongy.
When combating fungi, the temperature and moisture content of the wood are essential to consider.
Most fungal decay can progress rapidly at temperatures that favor the growth of plant life in general. For the most part, decay is relatively slow at temperatures below 50 degrees Fahrenheit and above 95 degrees Fahrenheit. Decay essentially ceases below 35 degrees Fahrenheit or above 100 degrees Fahrenheit.
Serious decay also only occurs when the moisture content of the wood is above the fiber saturation point (about 30 percent). Fully air-dried wood usually will have a moisture content not exceeding 20 percent, and should provide a reasonable margin of safety against fungal damage.
Brown, crumbly rot, is sometimes called dry rot, but the term is incorrect because wood must be damp to decay, but may become dry later. There are also a few dry-rot fungi that have water-conducting strands; such fungi are capable of carrying water (usually from the soil) into buildings or lumber piles.
The early stages of decay are often accompanied by a discoloration of the wood, which can be difficult to recognize but is more evident on freshly exposed surfaces of unseasoned wood than on dry wood. Abnormal mottling of the wood color, with either unnatural brown or bleached areas, is often evidence of decay infection.
Late stages of decay are easily recognized, because the wood has undergone definite changes in color and properties. The character of these changes depends on the organism and the substance it removes.
If you see these tell-tale signs of decay on your wooden structures, and cannot dry the wood or turn down the temperature, researchers at the Forest Products Laboratory (FPL) offer these tips for cleaning outdoor surfaces prone to fungi, like your deck or siding. Fungus will always be among us, but detecting it, managing it, and mitigating its damage is well within our control.
For more information, please see Chapter 14 of FPL’s Wood Handbook: Wood as an Engineering Material