What Is Wood Decay?
A close-up look at wood through a microscope reveals that it consists of many thick-walled cells that are like hollow tubes running through the wood. The arrangement of the cells varies to give different types of wood different properties, such as appearance, strength, or resistance to decay.
Decay is caused by microscopic thread-like fungi that attack the thick cell walls of wood. To live, decay fungi need four things: favorable temperatures, moisture, air, and suitable food material. If the other conditions are right, fungi get their food from wood. Controlling their growth usually results from depriving fungi of the one condition that is simplest to control—water. It is important to know that water by itself does not decay wood. Moist wood is more likely to decay because the spores (like seeds of a flowering plant) from which decay fungi grow and attack wood cells are everywhere in the environment. If provided with enough moisture, these fungi will destroy cell walls and weaken wood.
In the advanced stages of decay, fungi produce fruiting bodies (mushrooms) on wood. Each fruiting body produces billions of spores that when released are blown about by the wind. If they land on moist wood, they may begin growing and start the decay process again. You can see why the homeowner must remain vigilant.