Molds and Stain Fungi - ACS Publications - American Chemical Society

Chapter 4

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Molds and Stain Fungi Jeffrey J. Morrell Department of Wood Science and Engineering, Oregon State University, Corvallis, OR 97331

The presence of fungi on the surfaces of both freshly sawn lumber as well as wood surfaces in buildings has become an increasing public concern. Formerly, most wood users were concerned about decay fungi that could weaken the wood, creating unsafe structural conditions; however, surface molds and stain fungi have become an increasing concern because of their potential as allergens. This paper describes the types of fungi that cause molds and stains and outlines the conditions required for growth on wood.

Introduction While concerns about the risks of mold in indoor environments have become a major public issue in recent years, these fungi have long been a component in the built environment wherever conditions are suitable for their growth (7). Fungal spores are nearly always present in the air, although the levels present and species involved can vary (2-P). As a result, creating conditions that are conducive to fungal growth will inevitably lead to fungal colonization. Molds, which have adapted to use simple sugars as a food source and are capable of rapid growth, will tend to colonize the wood first followed by decay fungi. There are a number of different conceptions of what constitutes a mold; but for the purposes of this chapter, molds are defined as those fungi that discolor the surface of wood through the production of pigmented spores. These spores 58

© 2008 American Chemical Society In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


59 can typically be brushed from the surface leaving the wood appearance unmarred. Stain fungi, which permanently discolor the interior of the wood, also produce dark pigmented structures on the wood surface and may be confused with molds. Similarly, decay fungi can produce color changes in wood that may be confused with mold. A l l of the fungi involved in these changes in appearance have two common traits. First, the fiingi involved are all growing into the wood to obtain nutrients; principally from sugars, starches and proteins stored in the ray cells. In addition, all of the fiingi require that some free water be present. Free water is usually present when the wood moisture content is 30% (by dry weight) or greater, although there is considerable debate about the ability of molds to grow at wood M C s as low as 20 percent and this value is easily achieved in most properly designed, constructed and maintained buildings. It is also important to note that moisture in wood can change markedly in a short time, particularly near the surface. Thus, wood can become wet enough to support mold growth but this moisture can be gone by the time the damage is detected, leading to the conclusion that the fungus somehow sorbed moisture from the air. Molds are all fiingi which are heterotrophic, typically filamentous organisms that obtain nutrients from a variety of materials including wood. Estimates of the number of fungal species range up to 300,000 or more species (70). While many of these species are highly adapted for special niches such as leaves of a single tree species, molds are generally cosmopolitan in nature. As a result, many species grow on a broad range of materials under a variety of environmental conditions. Nearly all mold species are members of the Fungi Imperfecti, a group of fiingi characterized by asexual spore production. This allows these fiingi to produce abundant quantities of spores in a relatively short time. A number of mold fiingi are also capable of sexual spore production. These fiingi are typically members of the Ascomycetes. In many cases, the anamorph-telomorph connections between sexual and asexual stages remain unknown. Although there are innumerable mold species, wood-based materials tend to be dominated by a few genera, including members of the Trichoderma, Pénicillium, Aspergillus, Fusarium, Alternaria, and Aureobasidium. In addition, some species of Ascomycetes, notably Chaetomium globosum, have been implicated in indoor air issues. Finally, a few mold species have drawn considerable attention, although they are not among the more prevalent species. The most notorious among these is Stachybotrys atra (chartarum). This fimgus is prevalent on cellulose containing materials, particularly paper, including that used in drywall. S. chartarum has gained considerable attention because it produces a potent mycotoxin as a byproduct of its metabolic processes. Mycotoxins can induce a variety of human health responses (11-24). They are a particular problem for people with compromised immune systems, but there remains considerable debate about the real health risks associated with casual

In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


60 exposure to this fungus. While S. chartarum is found in wood structures it is more commonly found on drywall and other paper products, and its presence is an indicator of serious moisture issues in a structure. The species present in a given wood-based material depends on a number of factors including moisture levels, temperature, and time of year. A number of early studies found that a diverse array of fungi colonized logs or freshly sawn lumber (25-29). For example, Kang and Morrell (30) studied fungal colonization of freshly sawn Douglas-fir sapwood and found that species diversity and isolation frequency increased steadily over time. Trichoderma spp. and other fast growing molds tended to predominate in this material. The type of wood based material will also influence colonization as well as fungal growth rates (31-35). The variations in building materials, environmental conditions, and capabilities of fungal species make it difficult to make global statements concerning the importance of these fungi in structures. Formerly, the presence of mold was viewed as an indicator of excessive moisture, which would eventually lead to more serious structural damage by decay fungi. This approach has clearly shifted and the presence of molds is, itself, now the primary concern and is often the cause of extensive remediation efforts.

Effects of Mold on Wood Properties While many mold species have cellulase systems (i.e. they can degrade cellulose), most are unable to attack the lignocellulose matrix, making them relatively innocuous to wood from a structural aspect. The growth of most mold fungi is limited to the sapwood, where they scavenge proteins, lipids, sugars and other readily assimilated compounds (Figure 1). The hyphae move through the cells via the pits, degrading the pectin in the pit membranes, primarily at the surface of the wood. As a result, moldy wood tends to have greater permeability than non-colonized wood. This increased permeability can affect coatings and finishes, preservative treatment, drying rates and susceptibility to rewetting once in service (36). Colonization by mold fungi has also been reported to condition wood to colonization by wood decay fungi, either by degrading toxic extractives or enhancing water uptake. Conversely, some mold fungi produce an array of antibiotic compounds that have been shown to inhibit colonization by decay fungi. The roles of these compounds in situ are poorly understood, although they are clearly toxic under laboratory conditions. Some of these fungi are also mycoparasitic and will attack and degrade the hyphae of competing organisms. A number of members of the genus Trichoderma are capable of mycoparasitism and antibiosis and have been used for inhibiting the growth of other fungi. Stain



Figure 1. Mold and stain fungi tend to preferentially colonize sapwood in most species, heartwood(center of the wood piece above) not typically colonized.



62 fungi also preferentially colonize the sapwood, where they degrade pit membranes as well as ray parenchyma. As with molds, stain fiingi increase wood permeability. Unlike molds, stain fiingi can reduce the impact strength of the wood and, with prolonged exposures, some stain fiingi can produce soft rot attack. The most important effects of molds and sapstain fiingi on wood-based materials are cosmetic, as a result of pigmented spores and hyphae on the wood surface by molds and the pigmented hyphae of stain fiingi deeper in the wood (57). While most mold damage can be eliminated by brushing or power washing, its presence can raise concerns among wood users.

Fungal Life Cycle The life cycle of a mold fungus is relatively simple (Figure 2). It generally begins with a spore landing on a wood surface. The spore imbibes water from the wood or the surrounding surface, and swells. Eventually, the spore produces a hyphal initial that begins to penetrate the substrate. The hyphae secrete digestive enzymes as they grow and these enzymes degrade and solubilize the simple carbon compounds present on or in the wood that then diffuse back to the fiingi. Although hyphae will be found throughout the wood, they tend to preferentially colonize parenchyma cells because these cells contain the stored nutrients. Once the fungus has obtained a sufficient amount of nutrients, it will begin to produce spores, usually asexually (Figure 3). In some cases, continued growth will lead to sexual reproduction, although this does not occur with all fiingi and many mold fiingi are known primarily by their asexual states. The total time from germination to sporulation can be as little as 24 hours, although the time varies with fungus, substrate and environmental conditions.

Fungal Requirements for Growth Like most living agents, fiingi require nutrients, adequate temperature, oxygen and water to survive. They may also have specific requirements for pH, vitamins or other compounds, but the former four requirements are the most critical for successful colonization. The stored nutrients in the ray cells provide an abundant resource for fiingi. These cells contain proteins, lipids and carbohydrates and most wood inhabiting fiingi preferentially colonize these cells. The process of lumber manufacturing ruptures many of these cells, making access to the nutrients even easier. Pollen and other organic debris falling on the wood surfaces further enhance this nutrient mix and possibly increase the level of the essential nutrient nitrogen.



Figure 2. Example of a typical life cycle of a fungus with both asexual and sexual reproduction.



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Figure 3. Example offungal hyphae and spores of the Graphium stage of a blue stain fungus Ophiostoma picea in ponderosa pine sapwood.

As a result, the area colonized by a given fungus before it produces spores can be quite limited. For example, Xiao (38) noted that the Graphium stage of Ophiostoma picea extended only a few cells inward from the wood surface, but was still capable of producing a fructification. Other species clearly more extensively colonize this substrate. Mold fungi tend to be primarily present in the sapwood of most species, although limited growth also occurs in the heartwood. Limited fungal growth in the heartwood reflects the tendency for stored compounds in this region to be converted to phenolic compounds that provide a less nutritious substrate and are, in some cases, toxic to fungi. Temperatures for fungal growth can be quite extreme, with fungal colonization occurring at or near freezing temperatures (5°C) and well as up to 40°C and beyond. Most fungi, however, grow best between 20 and 28°C, coincidentally, the temperatures found in most homes. Temperature is rarely a limiting factor for mold attack for long periods of time (i.e. eventually it will warm). Oxygen is essential for aerobic respiration, however, most fungi tolerate very low oxygen tensions. For example, soft rot fungi and many heart rotting



65 basidiomycetes continue to growth at 2 to 5 ppm oxygen. B y comparison, the air we breathe contains approximately 20% oxygen (200,000 ppm). Oxygen can become limiting in very waterlogged wood, as moisture fills the wood cell lumens and excludes air. This phenomenon is often observed in very wet wall cavities. For example, wet insulation pressed up against either a stud or drywall paper can create wet pockets that limit fungal growth. Growth does occur at the edge of the pockets because oxygen can diffuse inward to support fungal growth. For practical purposes, however, oxygen is rarely a limiting factor in mold growth in a structure. The most important factor in mold related issues is water. Wood is considered to be immune from fungal attack at moisture levels below 20% (wt/wt). By its nature, wood is hygroscopic and will tend to sorb moisture. This moisture is initially sorbed by the hydroxyl groups in the wood polymers (primarily cellulose and hemicellulose) and is termed bound water. As these sites are occupied, the water eventually accumulates in the interstitial spaces and cell lumens and is termed free water. The point where free water begins to accumulate in the wood is termed the fiber saturation point (fsp). The fsp for most wood species lies between 25 and 32% (39). Fungi generally require free water to colonize wood, but there are numerous reports of mold growth at lower moisture levels. The reasons for these anomalies are probably several fold. First, biological organisms are inherently variable and it is very possible that some fungi have evolved more efficient strategies for sorbing moisture. These fungi, however, appear to be the exception rather than the rule. The more likely explanations for apparent fungal growth in seemingly dry wood reflects the relationship between water and wood. Wood in humid environments will sorb moisture to specific levels at a given temperature and relative humidity (39). For practical purposes, wood under nearly saturated relative humidity conditions will reach moisture levels between 17 and 21% M C . These levels still do not allow free water to accumulate in the wood and would therefore be marginal for fungal growth. However, i f the temperature of the wood were to drop slightly, while the amount of moisture in the air remained the same, then liquid condensation would occur on the wood surface and any fungal spores present (there are always spores present) could sorb moisture and initiate growth. This process could continue, resulting in wet wood with associated fungal attack. However, one can imagine another scenario where wetting occurs, the fungi grow and sporulate, and no further wetting occurs. The moisture present in the wood continues to diffuse into the wood which equilibrates to moisture levels below 20 %. Examination of the now moldy wood at a later time reveals fungal growth on wood at or below 20% M C leading to the conclusion that the fungus grew on very dry wood. This scenario commonly occurs on kiln dried lumber in containers. The container heats up, evaporating water from the wood (which is still at 17% M C ) . Condensation



66 occurs as the container cools, allowing for fungal growth. Some exporters add moisture sorbent material to containers to preclude this possibility. As noted earlier, however, growth of fungi in residential structures is rarely the result of short term condensation, but rather occurs because of plumbing leaks, failure to caulk around windows or other building elements or design flaws that allow moisture to build up to the point where condensation or water intrusion is inevitable. In all these cases, it is important to remember that water can enter wood as a liquid or gas, but it can only exit as a gas. Thus, wood will invariably dry far more slowly than it wets, allowing some organisms to grow. Although a large industry has evolved to deal with moisture intrusion and related mold issues, prevention is far less expensive and more effective.

Significance of Molds The presence of mold and stain fiingi on the surfaces of building materials is clearly disconcerting to the homeowner. The responses to mold can vary, but generally small patches can be cleaned with dilute bleach or soap. These treatments will not completely kill the fiingi, but will help reduce the spore load and brighten the surface (40). Fungal growth is also common on framing lumber, but this colonization generally ceases when the wood is seasoned and is sealed in by the sheathing and drywall. Thus, any spores present on these surfaces are unlikely to invade the inhabited space to impact the occupants. Larger areas of mold must be approached more carefully since careless cleaning can disperse spores, leading to more widespread contamination. The use of a professional remediation specialist may be advisable under these circumstances. In all cases, the first step in remediation is to remove the source of moisture. Failure to remove moisture will allow fungi to reinvade the substrate, negating the value of the treatment.

Conclusions Concerns about molds have become a prominent building issue and are unlikely to go away. In the end, molds problems are intimately associated with moisture problems and careful ρ planning is required to limit moisture peneration and accumulation in a structure. Practices that include designs to avoid wetting, ventilation to limit moisture accumulation and proper construction can all limit the potential for both moisture intrusion and mold.


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Molds and Stain Fungi - ACS Publications - American Chemical Society

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