Decay fungi are thread-like organisms which feed on organic matter- including wood. They grow by extending and branching out microscopic tubular structures called hyphae (plural of hypha), which they extend into wood to consume material in the wood cell wall structure. As the microscopic fungal structure- called a mycelium- grows, it develops a root-like appearance.

Decay in its early stages is called incipient decay and will not be seen by inspectors. Incipient decay is sometimes difficult for even specialists using microscopes to see. By the time decay becomes visible to the unaided eye, wood may have lost 50% or more of its strength. For this reason, developing the ability to recognize conditions which can cause decay and making clients aware of the importance of correction is very important.

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Fungal Mycelium are sometimes visible, more often not.

FUNGAL REPRODUCTION

Fungi reproduce by producing single-celled spores, which are like microscopic seeds. Spores are tough, able to resist extreme conditions of temperature and humidity and under adverse conditions may go dormant for long periods, sometimes many years. Spread primarily by air currents, they collect on horizontal surfaces including the upper surfaces of logs and large amounts are in soil.

Spores are often introduced into bark and sapwood by insects such as bark beetles and into logs (dead wood) by a variety wood-destroying insects. Spores which have attached themselves to the outer bodies of beetles are rubbed off as beetles move through wood. Spores then germninate when conditions are right.

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Spores from various fungal species

GERMINATION and COLONIZATION

Spores resting on wood require moisture content higher than the Fiber Saturation Point (FSP) of the wood species upon which they rest, typically between 27% and 30%. Once sufficient water and the right temperature are available, spores germinate and develop by extending a hyphal tube. As more spores germinate, fungi multiply to form a colony. With the right conditions present, colonies can expand quickly.

FUNGAL DECAY of WOOD

Decay fungi feed on the cellulose and lignin of which wood cell walls are composed. The hyphae secrete a substance which dissolves at least part of the wood cell being fed upon, dissolving it into a form in which it can be absorbed.

The photo to the right shows a hypha from decay fungal mycelium extending into the interior of a wood cell.

Most decay fungi consume cellulose, some consume lignin and some are capable of consuming both. Extractives stored in heartwood often contain chemicals and compounds toxic to decay fungi, so heartwood cells are generally more resistant to decay than sapwood cells.

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Typical Wood Content

cellulose (50%*)
lignin(23% – 33%)
hemicellulose
extractives

ENVIRONMENTAL REQUIREMENTS for DECAY

Decay fungi require certain conditions to actively decay wood:

TEMPERATURE

Fungi will be active at temperatures between 50° and 90° F with the optimum at 70° to 85° F. Activity tapers off and wood is generally safe at temperatures below 35° and above 110° F, although killing them requires about 140° F.

MOISTURE

Fungi will not attack dry wood, but decay activity starts at wood Moisture Content (MC) levels of around 20%.

Although at moisture levels of 20% or more wood materials will be consumed and hyphae will grow, the germination of spores (sporulation) requires wood to be at or above the Fiber Saturation Point (FSP), generally 27% to 30% depending on wood species. Spores must absorb moisture from wood to germinate, and at wood moisture levels below the FSP, wood exerts too much suction on water.

In short, decay will begin at 20% MC, but rapid expansion of the fungal colony won’t start until about 30% MC. Once thriving colonies are established, the optimum MC of wood is between 40 and 80%, with upper limits between 100 and 250% possible depending on wood species.

Two types of fungi are capable of extending mycelial cords across dry, inert material to reach water for use in the decay process, allowing them to decay dry wood. In North America, one is limited to the Eastern U.S. and the other has limited capabilities.

Moisture meters cannot be used to pinpoint the exact point at which wood becomes at risk from decay because that point will vary with the type of wood, type of fungi and with air vapor pressure.

OXYGEN

Fungi require oxygen to survive. Logs are sometimes protected in sawmills by keeping them water-saturated.

DORMANT FUNGI

Without adequate temperature, moisture, oxygen or food, decay activity will stop, but fungi will not die, they’ll simply become dormant. Once conditions are right, decay will resume.