Forest Pests: Insects, Diseases & Other Damage Agents

Wood Decay Forest Health Guide for Georgia Foresters Written by Terry Price, Georgia Forestry Commission Adapted for the web by the Bugwood Network

Decay in Living Trees and Wood Rot of Lumber

There are many species of fungi that can cause decay in living trees. Of primary importance to the timber producer and the homeowner, are the various types of heart decay (rot) within the living tree. Heart decay may take place in any tree that has been exposed to infection as a result of injury, broken branches, fire scars, or cultural measures such as improper pruning. Some decay fungi are active only in the butt, while others are capable of causing decay throughout the length of the bole and larger branches. The decay fungi reproduce by means of fruiting bodies (conks) that develop on old branch stubs, wounds, or cracks in the bark caused by windshake, heat or cold. These fruiting bodies are applanate, hoof-shaped, or bracket-like, and may vary greatly in size, color and texture (Figure 145). The spores of the fungus are produced by these fruiting bodies.

Figure 145 photo from USDA Forest Service - Missoula Archives

Another wood decay fungus that is associated with standing living trees is red ring decay also known as red heart rot which is caused by the fungus, Phellinus pini. Red ring decay infections occur through dead branch stubs. It produces white pockets of decay in the heartwood of living conifers. Incidence is greater in trees above age 65. The endangered red-cockaded woodpecker selects older trees that have been infected with the fungus in which to construct nest and roost cavities. Cavities have been found in longleaf, loblolly, shortleaf, pitch, slash, pond, and Virginia pines. Studies done in the South found that cavity trees ranged in age from 62-149 years.

Wood decay is often described as white or brown rot. Organisms causing white rot break down both the lignin and cellulose causing the wood to lose its color and appear whiter than normal. Brown rot fungi feed on the cellulose in wood that is a component of the cell wall leaving the brown lignin constituent in the cell wall. This causes the wood to crack across the grain to form brown cubical pieces. Wood affected by white rot does not crack across the grain as in brown rot.

Wood decay fungi require moisture for survival and growth. This is the reason most decay fungi will not grow in dry wood. However, the fungus Poria incrassata is capable of rotting wood by developing root-like structures called rhizomorphs. These rhizomorphs can conduct water over 25 feet from a moisture source to the wood. Wood that has a moisture content above 20% will be susceptible to wood decay organisms. Stumps left around house foundations are often sources of wood decay in structures. Rhizomorphs will spread from the stump or soil to sound wood bringing with them the necessary moisture for the fungus to decay wood in use. This type of decay is often referred to as dry rot but in reality wood kept dry will never rot.

Infestations of Poria can begin in dirt filled porches, damp crawl spaces and basements where wood is in contact with the soil or moist concrete or damp bricks. Initially, yellowish mycelial fans grow over the surfaces of joists and sub-floors or wood in unexposed areas (Figure 146). The root-like rhizomorphs may be seen on foundation walls, framing, sub-flooring and other surfaces. The rhizomorphs are dirty white when young but turn brown to black as they age.

Figure 146 photo from USDA Forest Service Archives

Wood infested with Poria eventually dries and shrinks. This causes cracking and/or depressed areas in painted woodwork that may be the only evidence of a Poria infestation.

Keeping wood dry will prevent it from decaying. Untreated wood should not be in direct contact with the soil or concrete as it will absorb moisture and be subject to wood decay and termites. Decay fungi grow best at temperatures between 70 and 90 degrees F and wood moisture content above 20%.

Certain wood species naturally resist attack by decay organisms. Examples of species with natural decay resistance include redwood, cedars, walnut, Osage orange, white oak, and black locust. At a minimum these species can have a service life in excess of 10 years. Most of the historic log structures in the South were constructed of the durable, decay resistant heartwood of relatively large, slow-grown trees, whereas most modern log homes are constructed of logs from small diameter, fast-grown trees consisting of sapwood that is susceptible to decay fungi.

Wood in use that is subjected to continual wetting or that is in contact with the ground should be pressure treated. Pressure treated wood is very resistant to attacks by decay fungi and insects. The pressure treating process is done by commercial facilities where the wood is placed in large cylinders and the preservative chemicals are forced deep inside the wood using high pressure. Chromated copper arsenate (CCA) has been the primary wood preservative for many years.

Wood that has been pressure treated with CCA can last for 30-40 years. All CCA treated wood will have a stamp indicating the level of preservative treatment (ie- below or above ground use). In recent years consumers have become concerned about the presence of arsenic in CCA treated wood thus forcing the industry to develop alternative chemicals.

Effective December 31, 2003, the use of CCA treated wood was restricted to industrial applications and not residential. Some exceptions are roundstock products such as poles, building posts and piles. There are some exceptions where sawn lumber can be treated with CCA. A few of these examples are wood used in highway construction, utility pole crossarms, wood used in salt water habitats and wood used as permanent foundations.

Some alternatives to CCA treated wood are:

• Alkaline copper quat (ACQ)
• Copper azole
• Acid Copper Chromate (ACC)
• Copper dimethyldithio carbamate (CDDC)

Other chemicals used to protect wood are known as water repellents (WR) and water repellent preservatives (WRP). These products are penetrating wood finishes that enable wood to repel water thus inhibiting the growth of decay fungi. A WRP is simply a WR that has been enhanced with a fungicide, which inhibits the growth of mildews and decay fungi.

Landscape timbers are specialty products that are sold for use in flower gardens and other landscaped areas. These timbers are by-products left over from the manufacture of plywood. Although they are sometimes advertised as being treated for decay, they have not been pressure treated. They have been dipped or soaked in a wood preservative. Their life expectancy ranges from 5-7 years.

General precautions should be adhered to when using pressure treated wood:

• Do not use treated wood in places where the preservative may come in contact with food including animal feed.
• Do not use treated wood for cutting boards or countertops.
• Do not use pressure treated wood where it may come into contact with drinking water.
• Avoid frequent or prolonged inhalation of sawdust from treated wood. A dust mask and goggles should be worn when working with treated wood.
• Avoid prolonged skin contact with treated wood.

Another treatment method for wood involves the use of borates. Borate treated wood is a less toxic method of treating wood. The product Timbor® is a water diffusible borate that moves through the water in unseasoned (green) wood from areas of high concentration on the surface to one of lower concentration within the wood. Penetration is best when wood moisture content is high. Bora-Care® is a borate product that is specially formulated for use in treating seasoned (dry) wood. Both products are derived from the natural element boron (borax). Full-scale use of borates as wood preservatives in the U.S. has been limited because they are water-soluble and will leach from wood when it is re-wetted.

Wood can be pressure treated with borates and is being promoted widely on the Caribbean market. Protecting wood from termites is a challenge in areas where the water table is near the surface of the soil. Chemicals applied to the soil in such areas are soon leached away from the structure. Borate-pressure treated wood provides protection from decay and termites and eliminates the need for termite protection by soil treatments.

Molds and stain fungi are often mistaken for decay. These fungi may discolor wood by producing spores that grow on the surface of wood. The presence of molds and stains on wood should not be alarming but they are signs that conditions are favorable for decay fungi. Oftentimes people will store potatoes in crawl spaces underneath the house. Fungi that grow on the potatoes will often sporulate on the surface of floor joists. If molds and stains do appear preventive steps should be taken to increase air circulation in crawl spaces and basements. The presence of mold or stain fungi should be a warning sign that conditions may be developing for a potential wood decay problem.

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