*7.3. Brown Rot Fungi*

After the initial colonization stages, brown rot fungi primarily utilize the carbohydrate polymers of the wood cell wall, although they affect all three polymers. Brown rot fungi are considered to be more commercially important from a structural damage perspective because they tend to attack softwoods which represent the bulk of timber used in North American and European construction. However, they can also attack hardwoods and will readily do so in some environments [35]. The resulting decayed wood has a brownish, fractured appearance (Figure 1). The most important aspect of brown rot decay is the tendency for these fungi to produce degradation far in advance of the point of hyphal growth at the cellular level, and to depolymerize the carbohydrates much faster than they can be utilized. This results in very dramatic changes in many timber properties even when the macroscopic visual appearance of the wood is little changed. The result is very rapid losses in properties such as tension or bending at very low mass losses [26,36].

**Figure 1.** Classic advanced brown rot fungal decay in a softwood. Note the checks (cracks) which cut across the grain of the wood giving the wood a cubical appearance. In early stages of brown rot decay, the wood can appear unchanged from undecayed wood, yet significant mechanical property loss may be sustained even in early brown rot decay stages.

Brown rot fungi have a more limited suite of enzymes involved in the depolymerization and deconstruction of cellulose compared to white rot fungi, and they possess no peroxidase enzymes for lignin depolymerization. However, brown rot fungi rapidly attack and degrade cellulose while also using a non-enzymatic mechanism to depolymerize lignin to allow access to the cellulosic components. The lignin then rapidly repolymerizes, leaving a modified lignin that long led researchers to conclude that brown rot fungi had little effect on this polymer. This unique oxygen radical-based chemistry known as the "chelator-mediated Fenton" (CMF) system functions in advance of enzymatic action to deconstruct wood polymers [37]. CMF chemistry is more complex than conventional Fenton chemistry and allows brown rot fungi to generate powerful hydroxyl radicals within the wood cell wall rather than next to the fungal hyphae (which would kill the fungus). CMF chemistry causes lignin to depolymerize and then repolymerize as small, discrete irregular masses, separate from the cellulose [38,39], thereby opening the wood cell wall to further deconstruction. This type of chemistry is being studied as an efficient mechanism for lignin processing in future biorefineries. This later aspect may be of importance for modifying lignin for use in polymeric coatings and is reviewed further in the section on biotechnological applications.
