Wood Decaying Fungi: How They Work & How to Manage Them

Wood-decaying fungi are nature’s primary recyclers, breaking down the tough components of wood. In forest ecosystems, they are necessary for nutrient cycling and creating habitats. In human environments, however, these same fungi can cause structural damage to homes and other wooden constructions.

The Mechanisms of Wood Decay

Wood’s structural integrity comes from two organic polymers: cellulose, which provides flexible strength, and lignin, which lends rigidity. Different wood-decaying fungi use distinct methods to break down these components, leading to different forms of decay. This decomposition is accomplished by secreting extracellular enzymes.

White-rot fungi are distinguished by their ability to degrade both lignin and cellulose. They possess enzymes, like peroxidases and laccases, that can break apart the lignin polymer. This type of decay is common in hardwood forests and accounts for a majority of wood decomposition in nature.

Brown-rot fungi primarily target cellulose and hemicellulose for consumption, leaving the brownish lignin behind. These fungi use a non-enzymatic and enzymatic system to break down cellulose. This type of decay is implicated in “dry rot” in buildings, a misleading term as moisture is still required for the fungus to thrive.

A third category, soft-rot fungi, operates in conditions too wet or otherwise unfavorable for white or brown rots. This form of decay proceeds more slowly, forming microscopic cavities within the wood cell walls and leading to a gradual softening of the wood surface. They are known to degrade cellulose and hemicellulose.

Ecological Significance

In natural settings, wood-decaying fungi are foundational members of the ecosystem. Their primary role is to decompose dead wood, a process that unlocks and recycles nutrients like carbon, nitrogen, and phosphorus for living plants. Without these fungi, dead trees would accumulate, locking away nutrients and hindering new growth.

The physical act of decomposition also creates microhabitats for forest biodiversity. As fungi break down logs and standing dead trees (snags), they create hollows, softened wood, and cavities. These spaces become shelters and nesting sites for organisms, including insects, amphibians, birds, and small mammals.

Furthermore, the type of decay influences the surrounding environment. The residue from brown-rotted wood, being rich in modified lignin, contributes to the stable organic matter in forest soils, aiding in carbon sequestration. White-rotted wood can support nitrogen-fixing bacteria, enriching the local environment.

Identifying Fungal Decay in Wood

Recognizing the signs of fungal decay early is important for managing its impact on wooden structures. The most obvious indicators are the reproductive structures of the fungi, known as fruiting bodies. These can appear as mushrooms, shelf-like brackets or conks, or flat, crust-like patches on the wood’s surface, signaling a well-established fungal colony.

Another sign is the presence of mycelium, the vegetative body of the fungus, which can look like a network of fine threads. These threads can aggregate into thicker, root-like structures called rhizomorphs or form silky, cotton-like sheets over timber. A musty, earthy odor is also a common indicator of active fungal growth, especially in poorly ventilated areas.

Direct changes to the wood itself are a clear sign of decay. White-rot fungi leave wood with a bleached appearance and a soft, spongy, or stringy texture. In contrast, brown-rot fungi cause wood to shrink, turn a brownish color, and crack into cubical pieces. A unique visual cue known as spalting—dark, decorative lines running through the wood—is also created by fungi.

Managing Fungal Decay in Homes

Managing wood decay in homes requires controlling the conditions fungi need to live: a food source (wood), oxygen, suitable temperatures, and moisture. Of these factors, moisture is the most practical element to control. Fungal growth begins when the moisture content of wood surpasses 20-30%.

Preventing decay is centered on keeping wood dry. Key strategies include ensuring proper drainage around the foundation and keeping gutters clear. Promptly repair any leaks from plumbing or the roof that allow water to penetrate the structure. Good ventilation in attics and crawl spaces helps prevent the buildup of humid air, and vapor barriers can stop moisture from evaporating from the soil into the structure.

When designing or building, ensure that wooden elements do not have direct contact with the ground. Using pressure-treated or naturally rot-resistant wood for vulnerable areas can provide additional protection. If an infestation is discovered, it requires professional intervention. Remediation involves removing the decayed wood, addressing the moisture source, and may include applying borate treatments to surrounding wood to prevent future growth.