Fungi and other microorganisms decay wood by releasing enzymes that "digest" specific wood components such as cellulose, hemicellulose, and lignin.  The products of this decay are then absorbed as food.  

Wood-decay fungi are categorized based on components utilized and characteristics of the decayed wood.  These categories include the Soft-Rot Fungi, the Brown-Rot Fungi, and the White-Rot Fungi.

Soft-Rot Fungi: This group contains species that are capable of degrading cellulose and hemicellulose and may partially digest lignin. The soft-rots are particularly prevalent at the early stages of wood decay and in conditions of high moisture and increased nitrogen content. They therefore play an important role in the decomposition of fence posts, building timbers, window frames, and other wood components of buildings.  Wood affected by soft rot may appear wet, spongy, or pitted. There are over 300 species of known soft-rots. These include many filamentous micro fungi (the molds) such as Cephalosporium, Acremonium, and Chaetomium.  

Brown-Rot Fungi: The brown-rot fungi are also capable of degrading cellulose and hemicellulose but are unable to digest the lignin component of wood.  In this case, the lignin remains intact and appears as a brown, crumbly matrix.  Unlike the soft-rots, the brown-rot fungi are relatively few in number, comprising less than 6% of all wood-decay fungi.  All of these species are members of the Basidiomycota (the mushrooms). The brown-rots are most prevalent in conifer woods throughout the northern hemisphere.

White-Rot Fungi: The white-rots are capable of degrading all the major components of wood (cellulose, hemicellulose, and lignin).  This simultaneous decomposition causes wood to become progressively more fragile over time.  Although white-rot is caused by a wide variety of fungi, most are macro fungi that produce visible reproductive structures such as mushrooms.

What role does moisture play?                    

As with all fungi, wood-decay species require a certain amount of moisture to metabolize, grow, and reproduce.  But as a general rule, wood with a moisture content less than 20% is typically protected from decay.  At the low end of this spectrum are the dry-rot fungi, most of which are brown-rots that can remove water from the surrounding "dry" wood.  The most notorious dry-rot fungus is Serpula lacrymans, a species that causes millions of dollars of damage to wood buildings in Europe and Asia.  Here in the North America, other species (such as Meruliporia incrassata) cause similar dry rot problems.  It is also important to remember that many species of non-wood-decay species may grow at moisture levels as low as 16%.  Although these species do not cause significant rot, they can pre-condition the wood for subsequent colonization by wood-rotting species.

Wood Preservatives and Mildewcides

Wood decay is best prevented by controlling the environmental conditions that enable fungal growth and rot (i.e. oxygen, pH, temperature, moisture, nutrient source).  Unfortunately, these conditions are not always manageable and several chemical agents are therefore used in the prevention and treatment of wood decay fungi.   Chemical preservatives and mildewcide offer varying degrees of effectiveness, but all have their limitations and although manufactures may claim otherwise, there are no "silver bullets" that guarantee against wood decomposition.  Furthermore, these compounds have restricted use due to environmental risks and human toxicity. 

For several decades, one of the more popular treatments for protecting exterior surfaces has been Chromated Copper Arsenate (CCA) applied under high pressure (i.e. pressure-treated wood).  The use of CCA-treated wood is now banned for most applications due to health and environmental concerns, but  alternative products have emerged, including Alkaline Copper Quaternary (ACQ) and copper azole.  Formulations of ACQ contain copper oxide, quaternary ammonium compound (didecyl dimethyl ammonium carbonate and didecyl dimethyl ammonium bicarbonate), and boric acid.  Copper azole contains copper carbonate and the fungicide tebucanazole. 

Wood has also been preserved using organic oil-based carriers such as liquified isobutane with pentachlorophenol, iodo propynyl butyl carbamate (IPBC), copper and zinc naphthenate, and tributyl tinoxide.  In some applications, permeation into the wood interior is enhanced using increased pressure (i.e. pressure-treated wood).   For more extreme applications where painting or other surface coatings are not practical, creosote impregnation has been the application of choice (e.g. telephone poles).

Mildewcides are compounds added to cleaning agents or coatings (paints, varnishes, stains) to kill or remove fungal growth.  Like preservatives, mildewcides will degrade over time.  As a result, treated materials require maintenance or re-applications.  Examples of common mildewcides are listed below.

Some compounds are used as both a preservative and mildewcide.  The most common are borate derivatives (disodium octaborate tetrahydrate or sodium borate).  The primary limitation of borates is their relatively high solubility in water, which causes the compound to leach from wood exposed to repeated wetting.  Most borates are therefore not well-suited for exterior applications.