Fungi are a type of plant without chlorophyll. This means they cannot manufacture food by photosynthesis. Fungi obtain their nutrients by excreting enzymes into their immediate surroundings. These enzymes dissolve nutrients which are then absorbed by the fungus to provide its energy. Wood is composed of cellulose and lignin. Wood rotting fungi are highly specialised to dissolve these components. Most fungi cannot do this.
A fungus begins its life as a microscopic seed like spore which is so tiny it can be readily carried on the wind or in water, or by animals including insects. When a spore lands it will remain dormant until conditions are suitable for germination. This typically requires a temperature between 5°C and 40°C, an air supply and a wood moisture content of at least 20%.
When conditions are right the spore germinates and a thin hair like thread called a hypha emerges. These are too small to see with the naked eye. They will grow into the wood excreting enzymes and absorbing dissolved wood nutrients as they go. In good growing conditions the hyphae will spread in all directions to form a network called a mycelium. At this stage the mycelium will be visible to the naked eye and may become dense enough to form a mat on the wood substrate. Over a period of time the fungus will continue to grow until it forms a “fruiting body”. In wood these can be large external growths typical of a decaying tree stump for example. They can also be much smaller delicate structures les obvious to see. In either case the “fruiting body” produces spores in vast numbers which may give the appearance of a very fine dust. The spores are released to disperse and repeat the life cycle.
Brown Rot Fungi
This group of fungi are the most important wood destroying fungi. As the brown rot mycelium spreads, excreting its enzymes, it preferentially attacks the cellulose component of the wood in preference to the lignin. The increased proportion of lignin remaining causes the wood to take on a browner appearance and to develop a cubic cracking appearance.
Brown Rot Fungi are further differentiated into two types.
Wet Rot is the most common Brown Rot and as its name suggests it tends to thrive in particularly damp condition often associated with buildings where leaks occur, or in wood used outdoors.
Dry rot, is a specialised type of Brown Rot. Contrary to popular belief it cannot live in dry wood, however, it has special water conducting hyphae which can transport water from decaying wood to a dry area, making it suitable for fungal growth.
White Rot Fungi
White Rot fungi dissolve both the cellulose and the lignin of wood so the brown colour does not occur. The wood tends to take on a white fibrous appearance as it decays. White rots tend to attack hardwoods more readily than Brown Rots but will also attack softwoods.
Soft Rot Fungi
This fungus is typically found in very wet conditions and is a major problem for telegraph poles and wood used in lakes or other aquatic applications.
Staining fungi and moulds do not digest wood fibres so they do not cause structural damage. They live on stored carbohydrates in wood cells and cause unsightly staining which can seriously devalue wood due to its appearance. They are also a visual warning that conditions are good for fungal growth, so where staining fungi are seen, wood destroying fungi will not be far behind.
Bacteria are only a problem in extremely wet conditions where wood is immersed in water and where oxygen levels are low. Fungi cannot survive in these conditions but bacteria may be able to cause damage to wood under these conditions.
There are two primary means of preventing fungal damage in wood.
1. Keep the wood dry. That means below 20% moisture content. Design of timber structures is crucial in this respect. Water traps or high humidity in structures are perfect growing conditions for fungi so should be avoided wherever possible.
Construct wooden structures to shed water and keep voids well ventilated.
2. Where the wood cannot be kept permanently dry, make the wood unpalatable to fungal attack by introducing preservatives. When a fungus excretes its enzymes and absorbs nutrients the wood preservative is also absorbed preventing the fungus from growing.