**9. Relationship between Understanding Fungal Decay and Developing More Rational Wood Protection Methods**

Humans have long labored to limit the risk of decay and prolong the useful life of timber. Initially, they did so without even knowing that fungi were the causal agents, but emerging knowledge led to the development of a range of biocides that could protect timber from decay. Interestingly, many of these advances, such as the development of creosote preceded that understanding. As a result, wood protection largely remains a combination of effective designs that limit moisture, coupled with the use of naturally durable timbers or impregnation with biocides where moisture control is not possible. However, the types of biocides being used, and their method of application, is changing as concerns emerge about the use of all chemicals in the environment. These concerns have created opportunities to develop more rationale methods for protecting timber based upon our understanding of wood science and fungal physiology.

Current efforts to protect wood without biocides have focused methods such as thermal modification and chemical wood modification with techniques such as acetylation. Both techniques were explored in the middle of the 20th Century but were dropped at that time because they were not considered economical, but their use is now being resurrected. Thermal modification and acetylation function to limit water uptake by wood, but there may be more subtle approaches to wood protection under in some environments. For example, our improved understanding of the roles of free radicals in the degradation process has also been exploited experimentally through the use of anti-oxidants to protect wood [76]. The use of many plant-based extracts follows a similar path but uses a less specific approach since these treatments are often mixtures of compounds. While still not commercially used, they illustrate the potential for using our knowledge of microbial decomposition mechanisms to identify more rationale prevention strategies.

#### **10. Coatings for Bio-Based Materials and the Need for Better Protection against Fungi**

While coatings have long been successfully used to impede and in some cases prevent fungal degradation while also limiting UV degradation, it is critical to understand both their function and limitations. The long-term effectiveness of any coating is dependent on its ability to prevent water penetration leading to coating failure and subsequent penetration by fungal hyphae or their enzymes and low-molecular weight metabolites through, around or behind the coating and into the wood. Coating efficacy is achieved via barrier technology and/or enhanced moisture exclusion. Loss of coating efficacy is often associated with increased moisture retention that can promote decay.

The general issues relating to coating failures for wood materials have been well summarized previously [77]. Decades of research at the U.S. Forest Products Laboratory showed that most paint coatings usually cracked at the wood joints or knots due to differential dimensional stability. Water enters the wood through these cracks and is trapped. Wood moisture content can then quickly reach the range suitable for fungal attack. Oil-based or alkyd-based paints contain oils that cure by reacting with oxygen to form cross-linked polymeric films. The more cross-linked the polymer, the more resistant it is to either liquid or vapor water. Latex paints form a film by coalescence of small spherical polymeric particles dispersed in water. These polymers are more flexible and not as highly cross-linked as cured oil-based paints, and generally remain more flexible with age. As a result, they provide less of a barrier to water ingress. Oil-based paint films generally provide superior initial water resistance; however, they become brittle over time.

As noted in the introduction to this chapter, moisture content changes with wood shrinkage and swelling are an issue which must be considered relative to the development of advanced coatings for wood products. Coatings must be designed to flex to accommodate changes in wood dimensions with wetting and drying. However, even under the best of circumstances, most coatings will fatigue and develop fine micro-fractures after thousands of shrinkage/swelling cycles. These fine micro-fractures are exploited by fungi that then penetrate into the wood beneath. This is particularly true in exterior environments where UV exposure weakens the coating thereby enhancing dimensional change stresses and promoting fractures (Figure 3). This can occur in seemingly impermeable barriers. For example, 1–2 mm thick polyurea coatings were penetrated by decay fungi over a four-year exposure in Hilo, Hawaii [78].

**b Figure 3.** Scanning electron micrographs showing (**a**) fungal spores and hyphae growing on the surface of a new coating surface and (**b**) fungal spores and hyphae growing on an aged coating surface containing microcracks that can be exploited by fungal hyphae (Goodell images).

While we have previously discussed the decay process, hyphae of some decay and stain fungi also have the ability to directly penetrate wood cell walls using combinations of enzymatic/catalytic dissolution and mechanical/physical force. The mechanical forces generated by fungal hyphae can drive a fungal peg through a metal foil [79], even in the absence of enzymatic or catalytic action. This force is certainly great enough to allow fungi to penetrate many types and thicknesses of polymeric coatings, and the presence of surface micro-cracks undoubtedly aids in this process. Minimizing surface defects, slowing micro-crack development and even limiting hyphal growth on the coating surface (via the presence of biocides) can all contribute to limiting fungi compromising the coating barrier. A more difficult goal will be the development of clear coatings for exterior applications. Consumers generally prefer clear finishes so that they can see the wood; however, these coatings tend to be more susceptible to UV damage that leads to coating failures and, ultimately premature replacement of the timber [10,80].

Coated surfaces are bombarded with fungal spores along with dust, pollen, plant sap and a host of other potential nutrient sources for a fungus. Many paint film fungi utilize these nutrients to begin growth (Figure 3) and eventually exploit coating defects to penetrate into the wood beneath (Figure 4). Once inside, they preferentially colonize the layer between the coating and the wood, leading to further coating failure (Figure 5). The extent of this problem in both clear and opaque coatings highlights the need for coating formulations that contain long-lasting fungal biocides or other protection systems that can prevent fungi from causing this type of failure. It may also be useful to explore self-cleaning systems [81]. For example, developing superhydrophobic surfaces that mimic natural phenomena such as the self-cleaning capacity of lotus leaves would reduce accumulation of surface debris that supports fungal growth.

**Figure 4.** An example of fungal growth that has developed underneath a coating film (on the wood strip below hand) by penetrating the coating surface. This type of fungal attack occurs with opaque coatings as well, but it is readily observed when clear-polymer coatings are attacked. Clear coatings typically lack adequate UV resistance leading to development of micro-cracks in the coatings that fungi can exploit. (Goodell photo).

**Figure 5.** Example of a delaminated clear polymeric coating on wood being peeled from the surface after three years of exposure in a damp exterior environment. In this example, the finish was originally clear, similar to that seen in Figure 4. Fungi were involved in darkening of the finish and also, once they penetrated the coating film, in the delamination of the coating from the wood surface. In this example, the manufacturer's use of iron in the coating formulation also promoted the extreme darkening of the coating as, in this wet environment, wood extractives were driven to the surface, and these extractives reacted with the iron in the coating to produce the intense dark coloration (Goodell photo).

#### **11. Future Needs and Opportunities**

There is a critical need for an expanded understanding of how fungi attack and exploit current wood coatings to enable development of strategies to prevent premature damage to both the coating and the substrates they are intended to protect. Integrated technologies are needed to reduce fungal attack and enhance coating performance against physical and environmental factors. For example, enhanced UV protection coupled with better matching of long-term coating elasticity with expansion/contraction characteristics of the wood will limit the development of coating defects that fungi are known to exploit. Enhanced abrasion resistance and improved ability to shed environmental residues deposited on the coating will limit the ability of fungi to grow on and into the coating. Enhanced water repellency helps coatings shed water and reduces the time that a surface remains wet enough to permit fungal growth.

Biocides such as 3-iodo-2propynyl butyl carbamate and chlorothalonil have long been a component in many coating systems both for protecting the system against microbial attack prior to application as well as limiting fungal discoloration afterwards. While biocides ranging from tin and copper compounds to thiabendazoles are effective, they are not compatible with all polymeric systems. Novel approaches such as non-biocidal treatments that discourage attack by preventing fungal attachment to surfaces and preventing the development of biofilms have shown promise in the laboratory [82], but utilization of these coatings will likely remain limited to medical applications. While agricultural applications for this type of technology are proposed, it remains to be seen if cost-effective products can be developed that meet consumer needs for long-lasting surface protection under extreme UV exposures.

At present, a majority of coatings research remains proprietary, making it difficult to accurately assess the state of the science. However, market observations suggest that better coatings systems are still needed, and that fungal and UV resistance of coating systems has not dramatically improved in

many consumer-available coatings in the past 20 years. The lack of long-lasting UV and moisture resistant coatings leads to premature replacement of functional structures, especially decking and this, in turn sharply reduces the normally positive life cycle attributes of timber. Developing a better understanding the mechanisms underlying fungal degradation of substrates, and the mechanisms involved in new biocidal and non-biocidal coating treatments will be essential for creating integrated coatings systems capable of limiting fungal attack while providing long term protection against environmental factors.

**Funding:** The first author was supported in part by the National Institute of Food and Agriculture, U.S. Department of Agriculture, the Center for Agriculture, Food and the Environment and the Microbiology department at University of Massachusetts Amherst, under project No. S1075-MAS00503. The contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA or NIFA.

**Conflicts of Interest:** The authors declare no conflict of interest with the content of this review.
