Mold is a type of fungus that plays a significant role in the natural decomposition of organic matter. Its microscopic spores are constantly present in the air. Wood, a common organic material used in construction and everyday items, becomes a readily available food source for these spores when the right environmental conditions align. Mold growth on wood is a biological mechanism where the fungus colonizes the material to access stored nutrients.
The Necessary Conditions for Growth
Mold growth on wood requires three primary elements: a food source, a suitable temperature, and sufficient moisture. Wood provides the necessary carbon-containing organic material. Mold spores can thrive across a broad temperature range, approximately 30 to 120 degrees Fahrenheit, but growth accelerates rapidly between 70 and 90 degrees Fahrenheit.
Moisture is the controlling factor, as mold cannot absorb nutrients without it. The lower limit for sustained mold growth on wood is a moisture content (MC) of approximately 16%. Growth accelerates significantly when the wood’s MC rises above 20%. This moisture level can be reached through direct water contact (leaks or flooding) or when the surrounding air’s relative humidity (RH) consistently exceeds 80%.
The Biological Process of Colonization
The process begins when a dormant, airborne spore settles onto the wood’s surface. If the surface is damp and conditions are favorable, the spore activates and begins germination. This involves the spore absorbing moisture and swelling until a tiny, thread-like filament, called a hypha, emerges.
The initial hypha grows by extending its tip into or across the wood, forming a network of branching filaments. The mold obtains its energy by secreting powerful hydrolytic enzymes from the tips of the hyphae directly onto the wood surface.
These enzymes, such as cellulase and hemicellulase, act like external digestive juices. They break down the complex biopolymers in the wood into simpler sugars and nutrients that the fungus can absorb. As the hyphal network expands and consumes the organic material, it forms a visible, fuzzy or slimy mass known as the mycelium, signaling a successful colony establishment.
Why Wood is the Ideal Substrate
Wood is structurally composed of three main polymers—cellulose, hemicellulose, and lignin—which together provide a rich source of carbon for fungi. Cellulose forms the main structural component of the wood cell walls. Hemicellulose is a more complex, branched polysaccharide that is less chemically stable than cellulose.
Fungi often target the less stable hemicellulose first, along with simple sugars and starches present in the wood’s extractives, for initial growth. Lignin acts as a structural “cementing agent” that binds the cellulose fibers together, making the wood rigid. While mold fungi can break down these components, their primary damage is usually superficial, unlike wood-rotting fungi which penetrate deeper.
The physical structure of wood also contributes to its susceptibility because it is a porous, hygroscopic material. This porosity allows wood to easily absorb and retain moisture, creating the moist environment necessary for spore germination and growth. The capillary action within the wood cells helps draw water inward, further facilitating fungal colonization.