Archives for November 2008

What are wood rot fungi? Wood is one of the major components of building materials in residential and office buildings. In fact it’s almost impossible to miss a wooden item in a building. It is subject to attack by wood rot fungi and other organisms if it’s not well preserved.

Growth Requirements for wood rot fungi

For fungi to colonize wood, the following conditions are required:

• Favourable temperatures. Generally wood colonizing fungi have optimal growth temperature at aound 25 degrees Celsius.
• Adequate moisture. Moisture is the most critical requirement for fungi to colonize wood. Fungi will not attack dry wood (i.e., with a moisture content of 19 percent or less). Decay fungi require a wood moisture content of about 30%.
• Adequate oxygen. Most fungi require (oxygen) for growth.
• Food source. Like every other living organism, fungi require nutrients for growth. These are readily available on wood surfaces in form of dust. The wood itself is made of biodegradable compounds (cellulose, hemicellulose,lignin).

There are 2 groups of wood rot fungi. These are the wood-decaying fungi (wood-rotting fungi) and the Wood-staining fungi (sapstaining fungi).

Wood rot fungi
The wood-decaying fungi are the most damaging of all the wood destroying fungi. These fungi are prolific producers of strong enzymes that they use to breakdown complex wood components (cellulose, hemicellulose,lignin) to simple sugars that they can utilize as food.

Wood-decaying fungi

Wood-decay fungi colonize the sapwood and heartwood of most tree species. These fungi grow inside the wood and/or on wood surfaces. On the wood surface they appear as fan-shaped patches of fine, threadlike, cottony growths or as rootlike shapes. The color of these growths may range from white through light brown, bright yellow, and dark brown. The spore-producing structures (fruiting bodies) of the fungus may take the form of mushrooms, shelflike brackets, or flattened, crustlike structures. Fine, threadlike fungal strands called mycelia grow throughout the wood and excrete enzymes that digest parts of the wood as food. By breaking down the cellulose, hemicellulose,and lignin wood the strength and other properties of the wood are destroyed.

The rate of decay and extent of deterioration depend on the duration of favorable conditions for fungal growth. Decay will stop when the moisture content is lower than the fungu’s requirements. Decay slows down significantly if the temperature of the wood is either too low or too high. Early decay is more easily noted on freshly exposed surfaces of unseasoned wood than on wood that has been exposed and discolored by the weather. Wood decay fungi are generally grouped into three major categories: brown rot, white rot, and soft rot.

Brown Rot

Brown rot fungi such as Poria monticola and Serpula lacrymans break down primarily the cellulose component of wood for food,leaving a brown residue of lignin. Wood severely infested with brown rot fungi is greatly weakened even before decay is visible. Advanced stages of brown rot infestation are characterised by:

• The dark brown color of the wood
• Excessive shrinkage
• Cross-grain cracking
• The ease with which the dry wood substance can be crushed to a brown powder.

Brown rot fungi are probably the most important cause of decay of softwoods used in aboveground construction in North America. Brown rot-decayed wood, when dry, is sometimes called “dry rot.”

A few fungi such as Serpula lacryman can decay relatively dry wood by using water-conducting strands (rootlike structures called rhizomorphs) that can carry water from damp soil to wood in lumber piles or buildings. These fungi can decay wood that otherwise would be too dry for decay to occur. They are sometimes called the “dry rot fungi” or “waterconducting fungi.”

White Rot

The white rot fungi, Phellinus megaloporus and Poria contigua, break down both lignin and cellulose in wood. They have a bleaching effect that may make the damaged wood appear whiter than normal. Affected wood shows normal shrinkage and usually does not collapse or crack across the grain as with brown rot damage. However, the infested wood loses its strength gradually until it becomes spongy to the touch. White rot fungi usually attack hardwoods, but several species can also cause softwood decay.

Soft Rot

Soft rot fungi such as Chaetomium globosum usually attack very wet wood, causing a gradual and shallow (3-4 mm) softening from the surface inward that resembles brown rot. The infested wood surface darkens and becomes very soft, hence the name soft rot.

Wood-staining fungi

Unlike the wood-decay fungi, wood-staining fungi are only a cosmetic problem. They tend to grow on the surface of wood. Examples of wood staining fungi include Ceratostomella spp. and Diplodia spp. These fungi penetrate and discolor sapwood, particularly of softwood species. Typical sapstain, unlike staining by mold fungi, cannot be removed by brushing or planing. Sapstain fungi may become established in the sapwood of standing trees, sawlogs, lumber, and timbers soon after they are cut and before they can be adequately dried. The strength of the wood is not greatly affected, but the wood may not be fit for use where appearance is important (such as siding, trim, furniture, and exterior millwork that is to be clear-finished).

Superficial Wood Colonizing Fungi

Superficial wood colonizing fungi such as Fusarium spp and Penicillium spp., first become noticeable as green, yellow, brown, or black, fuzzy or powdery surface growths on the wood surface. The colored spores they produce can usually be brushed, washed, or surfaced off. On openpored hardwoods, however, the surface molds may cause stains too deep to be easily removed. Freshly cut or seasoned wood stockpiled during warm, humid weather may be noticeably discolored with mold in less than a week. Molds do not reduce wood strength, but they can increase the capacity of wood to absorb moisture, thus increasing the potential of attack by decay fungi.