Fungi as Saprobes (In Nature and On Commercial Products)

Introduction

We have studied fungi that are obligate parasites and facultative parasites on crop plants and trees, as well as in human diseases. Now we will turn our attention to the sapotrophic fungi or sapotrophs who derive their nutrition from nonliving organic material. The number of species of sapotrophic fungi far outnumbers those that are parasitic and impact our lives, daily. Unlike the plant pathogenic fungi that we just studied, some sapotrophic activities are important for our well being while others are significant because of their negative activities. However, regardless of whether these activities impact us in a positive or negative fashion, these activities all involve the fungi's ability to decompose the substrate (=the food of the fungus) on which they are growing. We will go over both the positive and negative roles that fungi play as they decompose.

Sapotrophic Fungi as Recyclers in Nature

It has already been emphasized, several times, the significance of sapotrophic fungi in nature. By breaking down complex organic compounds such as proteins, carbohydrates, fats, etc., into their most basic elements, it enables these elements to be reused as new generations of organisms are borne. In a subtropical or tropical forest, the weight of branches and leaves, alone, that are decomposed may be up to 60 tons/acre/year. Thus, if not for decomposition by fungi, we would be up to our ears in dead branches and leaves, not to mention dead animals. Although decomposition plays a significant role in nature, its mechanism is poorly known to the nonscientist and often, even to the scientist. For example, I was giving one of the lecturers in an ecology course a few years ago, and I was giving a lecture on this very topic. We were using a text book that shall remain nameless, where I was looking for a reading assignment for the students. Looking through the table of contents, I found a chapter that was titled "Nutrient Cycling" and it only had a few things to say about decomposers"In most soils the main decomposers are litter feeding invertebrates. Termites are a major component of this so-called macrofauna, but in montane forests earthworms replace them."  Reading further, I thought I would surely find some mention about bacteria and fungi as decomposers, and I did. However, not in the way that I had expected: "There are five families of lower termites. All have protozoan symbionts in their guts which decompose cellulose. The single higher termite family, Termitidae, dominate many rainforest soils and have bacterial gut symbionts which perform the same role. One of its subfamilies, Macrotermitinae, is widespread in rainforests, and uses mainly fresh plant material to cultivate subterranean fungus combs or 'gardens'. The fungus breaks down both cellulose and lignin and the termite feed on the food bodies that it produces." What is stated here is basically correct. The various species of termites do have bacteria or protozoan in their gut and that is why they can digest wood. Also, some termites do have fungus gardens that they cultivate and utilize them as food, but this book misses the most important concept, which is that the actual decomposition of organic material, in nature, requires fungi and bacteria. This is not to say that earthworms and termites are not important in the process of decomposition of organic material, only that they are not responsible for the actual process of decomposition! They are instead what we call detritus feeders or detritivores, but not decomposers. I'll define these terms later. In ecology classes, where the topic revolves around interactions between organisms and their environment, the roles of fungi and bacteria as decomposers would seem to be an integral topic. However, in the ecology textbook that I quoted from, above, the role of fungal decomposition in nature was not covered because of a lack of understanding of the role of fungi in recycling organic material.

The concept of a food pyramid illustrated below can be used to offer a simplified explanation of how nutrients are recycled by bacteria and fungi as they decompose organic compounds. In a food pyramid, the plants can be referred to as the producers since they are the primary source of food. The rest of the food chain is composed of consumers, which are either directly or indirectly dependent upon plants for their food.   Consumers may be divided into categories: The primary consumers are herbivores whose diet is composed of plant material. The secondary consumers are carnivores whose diet is composed of herbivores. Finally, there are only a few organisms that are tertiary consumers, whose diet is composed of carnivores. For example, vultures and other carrion feeders would fit in this latter category. On each level of the food chain, bacteria and fungi will act to break down the waste material, dead bodies, or disposed body parts such as hair and leaves. The material being decomposed are complex organic compounds that will eventually be broken down into simple inorganic compounds that can then be recycled as nutrients for plants. You should note that both bacteria and fungi are important as decomposers, but during this discussion I will usually only refer to fungi when I am talking about decomposition.

Diagrammatical representation of food pyramid showing recycling of material to simple inorganic material that can once again be reused by plants (=producers).

Although what I have just summarized is correct, decomposition is much more complex than what has just been described. Today, we'll talk about decomposition in this sense and the roles that various organisms, such as earthworms and termites have in the process of decomposition as well as fungi. Lets first go over some terms that will be relevant to the process of decomposition:

  1. Decomposers: Organisms that break down organic compounds which eventually leads to mineralization: Conversion of organic to inorganic forms as a result of decomposition. The term mineralization is important here to separate decomposers from other organisms. This is what fungi and bacteria are able to do that cannot be done by any other organisms.
  2. Detritivores: Other organisms important in decomposition, but which are not decomposers. These are organisms that feed on detritus. Term usually refer to small organisms, such as earthworms and some insects. Although fungi can begin decay of detritus immediately, the detritivores are an intermediate group of organisms that breaks down the material to be decomposed into a smaller pieces. Although these organisms break down the size of the detritus, breakdown of the complex organic compounds to simpler compounds do not take place here. However, these organisms accelerate the process of decomposition by breaking the organic compounds into smaller pieces and thereby decreasing the surface area of the substrate that can be acted upon by the fungi. This is not to say fungi cannot decompose the detritus without help, only that they can do it more rapidly if the detritus pass through the detritivores first. There is a succession of organisms that go through the detritus before decomposition is completed.
  3. Microbivores: The microorganisms feeding on decomposers. Since fungi also have bodies that are composed of organic compounds, when they die they must also be decomposed by other microorganisms.

Activities of Decomposers, Detritivores and Microbivores

The process of decomposition begins with non-living organic material and involves many species of bacteria and fungi, and detritus feeders.  It is impossible for any one species to decompose all of the various organic material found in nature. Instead, there must be a succession of species that decomposes the many different organic compounds of plants and animals. The organic material may be in the form of dead organic material that is continually shed by living organisms: skin, hair, feathers, horns of various animals, leaves, fruits, flowers, etc., or when an organism dies, the entire body of the plant or animal will serve as a source of food for decomposers and detritus feeders.

There are numerous factors that will affect the rate at which decomposition will take place. For example, plant material will decompose more slowly than animals because plant cells are surrounded by cell walls. When decomposition occurs in plant material, it can only take place one cell at a time, whereas animal decomposition can occur on the entire organism at one time. Decomposition of material will also occur more rapidly if there are many "small" pieces of a substrate than if there was one large piece, of the same substrate  that is comparable in mass, because of the greater surface area that is exposed for decomposition. Reduction of a large mass of substrate into smaller pieces is usually accomplished by detritivores, such as insects and earthworms. When detritivores eat, they reduce the substrate into smaller pieces, which will accelerate decomposition. There are many other factors that can affect the rate of decomposition, but you get the idea.

The many little pieces of substrate on the left will decompose more rapidly than the single large substrate, of comparable mass, on the right because there is more surface area exposed to the action of decomposition in the pieces of substrate on the left.

Succession of Decomposers

When decomposition begins on any given substrate, a number of decomposers will be required to completely decompose it. In nature, substrates that will be decomposed are usually made up of numerous organic compounds. These may range from simple to very complex compounds. Most of these compounds will require the action of a specific digestive enzyme in order to break it down to a simpler material. Thus, many species of decomposers will be required to completely decompose a given substrate since no single species will be able to produce all of the digestive enzymes that would be required to completely decomposed the substrate. In decomposing the substrate there is order to what compounds will be decomposed first. Decomposition begins with simplest compounds and will end with the decomposition of the most complex.

Generally substrate will have some material that will not require digestion and may be absorbed as is, e.g., simple sugars and amino acids, may be absorbed without digestion. These simple compounds will be immediately attacked by the so-called "sugar" fungi and bacteria. So, during composition, initially there will be a population explosion of "sugar" decomposers. Why? Since the simple compounds can be directly absorbed into the fungus, production of digestive enzymes are not necessary. The digestive enzymes are not being produced by the fungus all the time. The enzymes are produced only when the complex compounds, e.g., cellulose, proteins, fats, etc., which they act upon are present. It would be a waste of energy to keep making an enzyme if it is not going to be utilized. So, in the time that is necessary for the various fungi to produce the digestive enzymes to decompose the larger organic compounds, the population and growth rate of "sugar" fungi will have increased dramatically. However, as the simple compounds are used up, these "sugar" fungi will decline. There is still a lot of organic material in the soil, but these can’t be utilized by the "sugar" fungi because they lack the digestive enzymes to break down these complex compounds.

The complex organic compounds will be broken down by the next generation of fungi, those species of fungi that have been making the enzymes necessary to break down specific types of complex organic compounds. If we are talking about plants, the first generation of decomposers would usually be species of fungi that will be able to digest simple carbohydrates, e.g., sugars, starches, etc and small proteins. A wide variety of fungi are still able to utilize these these compounds, but far fewer in number than the "sugar fungi".

Eventually these compounds will be exhausted and a decline of the these fungi will occur and the next generation will be species that are capable of digesting still more complex compounds such as cellulose, fats and oils. Decomposition will occur more slowly and fewer species of fungi are capable of breaking down these compounds than in the previous groups. These fungi will also decline as the compounds on which they are feeding are depleted.

The next generation of fungal species would digest the most complex compounds, such as lignin, waxes and tannin. These compounds are the most resistant to decomposition and take the longest to breakdown, and only a handful of species may decompose these compounds. Lignin is perhaps the most evident in this last group of compound as demonstrated by leaf skeletons. Leaf skeletons are the remnants of leaves in which only the lignin that make up the veins of the leaves have not been decomposed.

Leaf Skeletons: Remnant of leaves that is near completion of decomposition. Mostly the veins, which is composed of lignin, remains. Arrow points to a magnification of the area in square where a patch of the leaf surface still remains.

 

It can be seen then that there are several waves of decomposers that must comes in before all of the organic compound in any organism is totally broken down. Individual species of decomposers can only produce enzymes that can decompose a limited number of complex compounds, and so there must be a diverse number of species present in any substrate before the chemically complex tissues of a plant or animal corpse can be completely decomposed.

Let us now say that all of the "waves" of decomposers have now gone through a cycle and have broken down the different types of organic compounds into simple inorganic compounds, e.g., carbon dioxide, water, etc, which can now be utilized by plants to produce more food. It is at this point that the substrate has become mineralized. But, what about the biomass of the decomposers themselves? Although not evident, they normally make up a sizeable mass (remember the world's largest fungus?) of the biota in the soil and they, too, will be made up of complex organic compounds that will be unavailable to the ecosystem. This is more difficult to appreciate since the only visible indicator of the presence of this organic material may be their reproductive structures, such as the fruiting bodies of mushrooms, and they would only represent part of the fungi that are in the soil. However, immobilization of this material, in the mycelium of fungi, is also a positive feature. It prevents nutrients from being eroded away. The slow release of this nutrient through decomposition of the fungi by microbivores allows the soil to be "fertilized", but yet will prevent soil erosion, let us say for example, from flooding.

Let us describe the decomposing or rotting (note change in terminology) of wood, a material, which is abundant in nature and is of commercial importance. Note also that we did not include wood rot in the plant pathogen category. Technically, wood is nonliving and so the fungi that rot wood are sapotrophs.

An Introduction to Trees and the Fungi that Attack Them

This is an important topic since how we care for trees play a significant role in the amount of decay that occurs inside the tree. Theoretically, if a tree remains intact, i.e., bark has not been cut, the wood inside will remain "healthy", much like the inside of our bodies. The bark is much like our skin, and with the toxic chemicals that are produced there, serves as a barrier that will discourage decay fungi from entering. Unfortunately, it does not take much to break an opening to this barrier. Cultivated trees are often pruned and with the large scale use of weed whackers and romantic boyfriends who carve initials on trees, the wood inside the bark is certain to be exposed. Even in nature, trees will inevitably be wounded in some fashion. Lightning and wind often break off branches, and birds and insects will create wounds in trees as they are searching for food. 

When an opening is formed on a tree, the exposed wood is not immediately colonized by wood decaying fungi. Initially colonization by bacteria occur and some non-wood decay fungi will soon follow. Although these "pioneers" microbes do not injure the wood in any way, they do change the chemistry in the area that they colonize. It is only after this time that wood decay fungi activities become noticeable.

Wood is mostly cellulose, pectin and lignin.  Cellulose is a carbohydrate very similar to starch and is the main component of the cell walls in plants. As such, it functions in the structural support of plants. Pectin is the material that serves as the "glue" that holds plant cells, together (also the material used to make jellies). Lignin is a complex compound which further strengthens the cell walls of plants and is responsible for the elastic quality in wood. That is lignin gives wood its flexibility so that when bent, it will snap back to its original shape. There are also less complex compounds, such as hemicellulose and various kinds of sugars and chemicals. It is these latter constituents that differs in the various species of plants, which are in part responsible for the differences in the quality of wood in different species of trees and for the  differences in resistance to rot.

The matter in which wood is decomposed, whether it be from a living tree or of commercial wood or wood products is the same. Digestive enzymes are released by the fungus and the cellulose, lignin and other compounds are broken down and "eaten by the fungus". In a living tree, wood-rotting fungi, can be divided into different types of rots, based on the different types of enzymes that are produced. Because of time constraints, we will limit our discussion to heartwood rots or heart rots. These are the rots that occur in the older, inactive central wood of a tree called the heartwood, which is usually darker and harder than the younger sapwood that surrounds it. 

Log X-Sect2.gif (27334 bytes)
Section of log, with arrows showing bark, sapwood and heartwood.

In addition there are two types of heart rots: brown and white rots. These are named for the color of the wood after they have become rotted. Brown rots are caused by fungi that digest mostly the cellulose from the wood, but not the lignin. Because lignin is brown, the wood that remains is usually a dark brown, and can also be recognized by the characteristic brick-like pattern of the rotted wood. White rots are caused by fungi that digest mostly the lignin from the wood and not the cellulose. Because the cellulose is white, the wood that remains appears to be bleached and with a fibrous appearance.

Left Image: Brown rot, courtesy of Jim Deacon, The University of Edinburgh. Right Image: White rot, courtesy of Tom Volk, University of Wisconsin.

Fungi that are wood rotters are primarily members of a group that we commonly call the polypores. They are so called because their spores are produced in the hundreds to thousands of pores of the fruiting body. Polypores are closely related to mushrooms and their fruiting bodies are often in the form of a bracket or shelf. However, polypores come in all shapes and sizes. There are also some mushrooms that are wood rotters, but there are relatively few when compared to the polypores. 

The pores of the fruiting body as seen under a microscope where the spores are produced. Ganoderma australe, an example of a wood rotter. The spores are borne on the white, lower surface of the fruiting body. Several clusters of Phellinus gilvus, probably the most common wood rotter, in Hawai‘i. 
Microporous flabelliformis, one of the more colorful species in Hawai‘i. Pleurotus djamor, one of the mushrooms that can cause wood rot, in Hawai‘i.

Polypores cause a great deal of damage when they attack the heartwood of trees. The types of trees attacked is dependent upon the fungus. Some species are generalist and can attack hundreds of species, while other are restricted to either hardwoods (flowering plants) or conifers (cone bearing plants), and still others may be restricted to a single species of tree.

The degree to which trees will have heart rot varies with species. Many trees are short-lived because they are invaded by wood rotting fungus at an early age. Even among the same species of trees, the location of trees may determine its longevity. For example, apple trees in the Midwestern United States usually live no more than 35 years due to wood rotters, while those in New York may continue to bear fruit for up to 75 years or more. Also, trees planted outside of their natural range are usually more susceptible to heart rot possibly because they are adapted for a particular environment, and once removed from their natural habitat, the trees become more susceptible to heart rot. It has been a common practice to attempt to grow trees with superior wood that is not available, locally. However, these commercial enterprises often meet with failure because of the trees inability to adapt to their new environment.

In 1600 Europe attempted to transplant several species of quality timber trees from the Pacific North West, in North America. Seeds of the various conifers were grown, but these trees seldom grew to be more than 20 years old before succumbing to various diseases and rot, and the effort to grow these North American conifers was a dismal failure. However, there have been exception to this rule. The Monterey Pine, of California, can be found in many parts of Southern California as an ornamental tree because of its rapid growth and branching habit, which makes it a good shade tree. Although, not a good tree for lumber, in California, plantations of this species in Australia, New Zealand and Africa had columnar growth and rapidly grew to be tall trees, ideal for lumber. Why this is the case is not known.

The Decomposition or Rotting of Trees

Trees that are long-lived are usually those species that are most resistant to wood rot. Some examples are the Bristlecone Pines, where some individual trees are known to be well over 6,000 years old, the Sequoia Redwood, in California, Western Red Cedar, Incense Cedar and White Oaks to name a few. These trees are more resistant to heart rot because of the accumulation of antifungal compounds that are produced by the tree and which accumulates in the heartwood. Although resistant to rot, they are not immune. It just takes longer for rot to occur. Generally, those trees that have the darker and denser heartwood are the ones that are most resistant to heartwood rot.

The entry of fungi into the heartwood is due to exposure of the heartwood. This comes about in many species of trees when the lower branches become shaded and die. After falling off, the heartwood is exposed and fungi can gain entry. This may also occur artificially, when lower branches of trees are trimmed. Often these exposed areas are covered by paint or antifungal compounds, but this in no way will prevent fungi from entering in these area and causing heart rot to occur, even though the people that manufacture such chemicals will dispute this. There is not even evidence that such chemicals will even slow down the rot.

Once infection has occurred, it progresses rapidly. Mycelium not only grows inward, but also grows along the length of the tree as well. However, for some reason, new wood that is added after the infection does not become rotted. Thus, over time, when the heart wood has decomposed, the tree is hollow, and is now being supported by the wood that has formed since the infection occurred. Depending on the size of the tree, the mycelium may be growing in the tree for years, decades or even a century before fruiting bodies are produced, indicating that the tree has heart rot. By this time, the mycelium is extensive and the tree cannot be saved. I am often asked if removing the fruitbodies will kill the heart rot fungus. You should know by now that this will in no way harm the fungus anymore than removing apples from an apple tree will damage the apple tree.

Commercial Lumber and Rot

Once trees have been logged and cut into lumber, they are still susceptible to wood decay. As already mentioned, different trees will have different susceptibility to fungal infections. This is also true of wood and the same general rule holds true. The darker colored, heavier wood are generally more resistant to decay than those that are lighter in color and weight.

The main cause of infection is the amount of moisture in wood. Generally, free water must be present in wood before an infection can occur. Normally, wood that has a water content of 25-30% is safe from fungi. Thus, to prevent wood decay, keep wood dry. In preventing moisture from being absorbed into the wood, paint may be used for this purpose, but paint in no way prevents decay. In fact Aureobasidium pullulans is a common species of fungus that is known to grow specifically on paint. Nor can stain act as a preservative in any way. However, even in dry wood, decay may occur. If wood has previously been infected and dried. The drying of the wood will not necessarily kill the fungus, but only slow it down. If the wood somehow becomes moist again, growth of the fungus could begin immediately. This would seem to be a major problem in a tropical to subtropical environment, such as Hawai‘i, but I have not received too many request for help in wood rot problems. However, during the last 2-3 years, I have received calls from homeowners that find the fruiting body of Amylosporus campbelli growing on underground roots, fence post, and in one case even in a home. 

Amylosporus.gif (35077 bytes)
Left Image: Amylosporus campbelli has been found, commonly on fence poles and even in a home. Middle Image: Lanai deck with Pycnoporous sanguineus fruiting bodies. Right Image: Close up of fruiting bodies from middle image.


What Can You Do About Heart rot?

It is likely that people have utilized some means of wood preservation for as long as wood has been used. In the Bible, in the book of Genesis, the story of Noah's ark told of how it was coated with pitch, resin found in trees, both as a sealant and preservative.  Like many other problems that has come about in society, the best control is to take preventive measures:

Because some trees are well known because of their age, size and may even have sentimental values, efforts have been made to save infected trees. Tree surgeons have often been called in to remove the infection area and then patch it up with cement. However, if the infection is not completely removed, which is often the case, in a few years, only the cement will remain where the tree once stood. Thus, there has never been any evidence that any trees that have been infected with heart rot can be saved with any of the methods described above and "tree care" businesses have largely discontinued the practice of hollowing out infected cavities and patching.

Some species of trees, as mentioned above, have naturally occurring anti-fungal compounds and chemical preservatives are generally not necessary. However, these types of wood are more costly than those without such natural protection. However, the latter types of wood can be protected by use of chemical preservatives.

There are a number of chemicals that are used commercially to prevent delay. Pentachlorophenol, a light oil or coal tar creosote is generally used as a wood preservative and is used on telephone poles and railroad ties is commonly used. While it does an excellent job in protecting wood from fungal rot, it cannot be used on wood that will have be in contact with people since it is toxic and has an unpleasant odor as well. 

Telephone poles from http://i01.i.aliimg.com/photo/v0/112450885/Wood_Utility_Poles_and_Railroad_Sleepers.jpg and railroad ties treated with coal tar creosote from http://www.diamondk.com/images/railroadties/369by389/09-1 Railroad Ties A1.JPG. 

An alternative to pentachlorophenol and used in residential construction since the mid 1930's is chromated copper arsenate (CCA), a mixture of chromium, copper and arsenic. The application of this compound leaves a greenish tinge to the wood  that has been the most commonly used preservative for protection against fungi, insects and marine borers. However, concerns of arsenic leaching from the treated wood became a concern and in 2004, the Environmental Protection Agency recommended to discontinue the use of CCA for residential construction, but will continued to be used for other types of applications, e.g. industry and commercial usage.

Wood treated with chromated copper arsenate gives wood a greenish tinge. Commonly used in residential construction until 2004, from http://www.lumbertalk.com/wp-content/small-treated-solid-wood-columns-BIG.jpg.

 

A number of other wood preservatives are also available, including alkaline copper quanternary and copper azole that gives wood an appearance similar to CCA, but is less toxic. Many other preservatives are available, some better than others and less or more harmful to the environment, etc., but examples we have covered will suffice to demonstrate the pros and cons of preservatives. 

 

Deterioration Caused by Fungi

Many of the goods and material that we utilize are plant and animal products. These include wood, wood products and food.They, too, are subject to deterioration by Fungi. The process of deterioration by Fungi does not differ from that of rot, but we may object to it more, on a personal level, because now items that may be "near and dear" to us are being impacted. We call it deterioration in order to make it sound like a more "evil" process because the fungi are now destroying something that is of value to us. However, because of the efforts that we make to protect our commercial goods, the substrate is usually not totally destroyed, but deterioration does occur which will at least lead to loss in quality of the product. Some of the substrates mentioned above are expected to become infected with fungi, but there are many examples of substrates that we would not expect to become infected and damaged by Fungi. 

Wet and Dry Rot In Houses

In areas that are very wet, there is the possibility that the wood on the houses may rot. There are two types of rots that are recognized: wet and dry rot. Unless your house is built from redwood, cedar or some other wood with natural anti-fungal compounds, houses are normally built with treated wood such as CCA or other chemicals utilized to prevent fungal decay.

Serpula lacrymans and Meruliporia incrassata

Dry Rot

Of the two, dry rot is the more serious. In the United States, it is normally caused by two species of polypores, Serpula lacrymans and Meruliporia incrassata. On timber, they are also one of the many species that cause brown rot. The term "dry" here refers to the fungus's ability to conduct moisture into wood that would seem too dry for wood rotters to grow. However, because this particular species can transport water, over a long distance, from a wet area to one that would be thought to be too dry to have a rot problem, by the time it becomes evident considerable damage will have already occurred in your house and often is detected only because of the production of. Its ability to transport water, over a distance, is due to the thick rhizomorphs that are produced. Rhizomorphs are thick root-like growth that are composed of mycelium aggregated into clumps that can not only grow through wood, masonry and bricks as well to carry the infection to other areas of the house.

While these species may be very aggressive in an environment that is optimal for its growth, they may be stopped by altering their environment. Removal of moisture (finding the moisture source may be somewhat tricky), increasing air flow and elevating the temperature to 50ºC will kill the fungus without the use of chemical sprays. Infected wood, however, must be removed and replaced beyond the point where visible sign of wood decomposition is observed. Its inability to thrive in hot environments may be one reason that these species do not occur in Hawai‘i.

Left Image: Serpula lacryman fruiting bodies in house and on furniture, from http://www.mycodb.fr/photos/Serpula_lacrymans_1985_rc_1.jpg. Middle Image: Rhizomorph of Armillaria on tree from http://www.apsnet.org/edcenter/illglossary/Article%20Images/rhizomorph.jpg. Right Image: Meruliporia incrassata from http://www.inspectapedia.com/sickhouse/CrawlMold022DJFs.JPG.

Wet Rot

There are a variety of species that are known to cause wet rot. This condition arises when wood becomes very damp with poor air circulation. While the growth is significantly slower than that of dry rot, wet rot if left untreated, over time, will begin decomposition of wood, changing its weight and color, and weaken its structural integrity. The symptoms of wet rot is variable since a number of species are involved. Wood may be darkened if the wet rot is caused by a brown rot fungus or may be bleached if caused by a white rot fungus. Because the species involved may be variable, wet rot does occur in Hawai‘i.

As in the case of dry rot, decomposed wood must be replaced beyond where decomposition is observed. However, if detected early, application of a 10-20% bleach solution to the infected wood will kill the fungus.

Miscellaneous Problems in Deterioration of Commercial Products

Wood Products

Furniture and other commercial wood products are not made with treated wood and rely on paint, stain and wood sealants to protect wood from fungi. However, these are at best temporary measures. There are a number of species of fungi that can eat away at the paint or any protective layer applied to the surface of wood. Once this "protective layer" is removed, it will abe susceptible to rot. If the surface is superficially cleaned and repainted, the paint will immediately peel away, again, because the cause of the problem, the fungus, was not treated. To eliminate the fungal growth, a 10-20% solution of bleach should be utilized in cleaning the wood surface before repainting.

Glass Lenses

A substrate that few people would guess would be subject to damage by Fungi. Optical lenses of glasses, microscopes and cameras can be etched by some species of Fungi. In this case, glass is not the substrate that the Fungus is attacking, but rather the glue that holds the lens in place. Once the Fungus has established itself and grows on the surface of the lens, acid secreted by Fungus etches the lens surface. A Fungus in the genus Geomyces has been implicated, but the species has not been identified nor is it conclusive that it is the cause of the problem (Wikipedia, 2011).

Camera lens etched by fungal growth. Image from http://farm3.static.flickr.com/2449/3866228053_da9109f8b7.jpg

Kodachrome Slides and Photographs

Photographic and kodachrome slide emulsions are subject to fungal decay when not properly stored. Again, if they are kept in areas with warm temperature and high humidity, there is a strong likelihood that they will be subject to fungal decay.

Left Image: Kodachrome slide with fungal colony, upper right hand corner. Right Image: Colony magnified to show mycelial growth on slide.

Paper and Paper Products

Paper is especially vulnerable to fungi deterioration. In all phases of paper production, the paper or the precursor to the paper product must continuously be treated with various chemicals to prevent fungal growth. Even after the paper is finished, it may still provide a substrate for fungal growth. However, it is not common to see fungal growth on paper since we usually do not store paper in wet, humid conditions. I say not usually, but unfortunately this is not that uncommon at libraries. Libraries with valuable collections are normally air conditioned and the humidity kept low, which is adequate to prevent damage from Fungi, i.e. Hamilton Library here at the University of Hawaii. Unfortunately, Sinclair Library here does not have such facilities and is hot and humid in most parts of the library. area that is not air-conditioned. Those books, especially those that have not been checked out in a long time, often can be seen to have mold growing on them and even if the growth is not obvious, the books often have a very musty odor about them indicating their presence. Remediation is often needed to cleanse Fungi from valuable books.

Left Image: Book with spotty colonies of Fungi on pages from 
http://privatelibrary.typepad.com/.a/6a01156f7ea6f7970b011572095e96970b-pi.

Middle Image: Book with massive fungal growth and Right Image: Same book after careful removal of Fungi, from http://universityofglasgowlibrary.files.wordpress.com/2011/08/p1060801.jpg.

Art work are also subjected to molds and must be carefully monitored, with respect to their temperature and humidity, as well as light. Fungi are able to utilize the oils as well as the canvas as substrates for their growth. As in the case of valuable books, remediation can be carried out to restore the paintings.

Before and after images of painting in which remediation has been carried out, from http://www.vonhawk.com/images/vh15.jpg

Clothing

Much of our clothing is subject to fungal decay since they are usually made of textile of cotton or jute. Such decay is more likely to happen in warm humid conditions, such as here in Hawai‘i. Treatment of clothing with copper napthenate can is carried out if clothing is to be subjected to such conditions. One of the major problems that the military first came across here in the tropics, during World War II, was fungal growth on clothing and gear, such as tents, sleeping bags, etc. The fungus most isolated from clothing was Chaetomium, a cellulose, decomposing fungus, in the phylum Ascomycota. In regular clothing, musty odors, discoloration, loss of water resistance, decrease in strength and flexibility of fibers and spotting. Wool is less of a problem since fewer fungi attack this material. Fungal growth on textile products may occur even in the marine environment. The genus Zalerion is a serious problem on ropes and twines used in seawater. Leather in shoes is also very susceptible to fungal decay under moist and humid conditions.

Left Image: Spotty fungal growth on pants from http://www.moldbacteria.com/images/mold_on_clothes.jpg. Middle Image: Sleeve of shirt with light brown fungal stain from http://i.ehow.com/images/a05/4a/5o/mildew-stains-out-clothes-200X200.jpg. Right Image: Shoe with green fungal growth from http://www.tristaterestores.com/blog/?p=71.


Jet Fuel and Kerosene

A danger that is more of a problem in the tropics is fungal contamination of jet and diesel fuel and kerosene. Hormoconis resinae is the microorganisms that makes up most of the biomass in the contamination, along with approximately 30 other species. Although normally a soil inhabiting fungus, its air-borne spores are able to find their way into storage chambers where the fuel is stored. When contamination occurs in planes, the mycelium produced by the fungi will cause the fuel to deteriorate, block the fuel pipes and filters and could possibly even short circuit electrical equipment.

Left Image: Aviation fuel clean and contaminated with H. resinae. from 
http://www.fueltest.com.au/media/273_cleananddirty.jpg

Right Image: "Diesel sludge" contaminated with H. resinae from http://www.fueltest.com.au/media/1529_Hand_fills_of_the_stuff.jpg

Fungi in Outer Space

The problems that we feared the most as we attempt to place permanent satellites in orbit around our planet is that a meteorite will strike it and cause untold damage. However, there is already a major problem that has to with fungal deterioration. For the thirteen years that the space station Mir was in orbit for 15 years and there was constant problems with deterioration of vital equipment. These problems have often been in the news media and the specific causes were never mentioned until recently when it was revealed that deterioration of equipment caused by mutated fungi that had been breeding in the space station was the cause. The same problems are not plaguing the International Space Station.

The contamination of fungi in both space station occurred with the restocking of supplies and other cargo that were being delivered. On the journey to the space stations, with the shuttle crafts from Nasa, as well as other ships from other countries, Fungi were brought along and began growing in the space stations. Much of the damage occurred with electrical equipment that were shorted out by fungi decaying various electrical components.

Star Bulletin article, 10/5/2000, relating story about mutant fungi destroying equipment on space station Mir. 

 

Left and Middle Images, Mir Space Station and International Space Station, Courtesy of NASA. Right Image: Picture from International Space Station showing fungal growth from Vesper, et al. (2008)

While it appears that just about all commercial products that we own are subject to decay, that's not quite true. There has not yet been a metal decaying species of Fungi that has been discovered. 

Literature Cited

Vesper, S.J., W. Wong, C.M. Kuo and D.L. Pierson. 2008. Mold species in dust from the International Space Station identified and quantified by mold-specific quantitative PCR. Research in Microbiology. 159: 432-435.

Wikipedia contributor, Geomyces. Wikipedia, The Free Encyclopedia; 2011 Sept. 25, 04:32 [cited 2011 Sept. 25]. available from: http://en.wikipedia.org/wiki/Geomyces.

Most of the information gathered for this page was from the following:

Christensen, C.M. 1965. The Molds and Man. University of Minnesota Press, St. Paul.

Wikipedia contributor, Dry Rot. Wikipedia, The Free Encyclopedia; 2011 Sept. 25, 04:32 [cited 2011 Sept. 25]. available from: http://en.wikipedia.org/wiki/Dry_Rot.

Wikipedia contributor, Wood Preservation. Wikipedia, The Free Encyclopedia; 2011 Sept. 25, 04:32 [cited 2011 Sept. 25]. available from: http://en.wikipedia.org/wiki/Wood_preservation.

 

Glossary

Brown Rots: Rot caused by fungi that digest mostly the cellulose from the wood, but not the lignin. Because lignin is brown, the wood that remains is usually a dark brown.

Carnivore: Flesh eating animal. Consumer of herbivores.

Catalyst: Substance that accelerates the rate of a chemical reaction.

Cellulose: A carbohydrate, main constituent of cell walls of plants.

Consumers: Referring to organisms in the food pyramids that cannot produce their own food and are either directly or indirectly dependent upon plants for food.

Decomposer: An organism, often a bacterium or fungus, that feeds on and breaks down dead plant or animal matter, thus making inorganic nutrients available to plants.

Detritus
: Disintegrated or eroded organic material.

Detritivores: Detritus eating animals.

Digestive Enzymes: Used here to refer to proteins that are involved in digestion of specific organic compounds into simpler compounds that can be absorbed through the fungal cell wall.

Food Pyramid: A succession of organisms that constitutes a continuation of food energy from one organism to another, e.g., producers (plants) to herbivores to carnivores, as each consumes a lower member and in turn is preyed upon by a higher member.

Heartrot: Decomposition or decay of heartwood.

Heartwood: The older, inactive central wood of a tree or woody plant, usually darker and harder than the younger sapwood.

Herbivore: An animal that derives in nutrition from plants.

Lignin: Non-carbohydrate component of cell wall that gives wood its elastic quality. Along with cellulose hardens and strengthens the cell walls of plants.

Microbivores: Microorganisms that consume decomposers.

Mineralization: The process by which complex organic compounds are decomposed to their simplest, inorganic form.

Pectin: The "glue" that holds plant cells together.

Producers: Reference to plants in the food pyramid since they are the primary source of all food.

Sapotroph: An organism that derives its food from non-living and decaying organic material.

Sapwood: Newly formed outer layer of wood that is usually lighter than the heartwood.

White Rot: Rots caused by fungi that digest mostly the lignin from the wood and not the cellulose. Because the cellulose is white, the wood that remains appears to be bleached.

Wood Decomposing Fungi: Those fungi that decompose wood.

Questions to Think About

  1. Although detritivores do not decompose organic material, they do help in accelerating the rate at which waste material may be decomposed. Explain their role in accelerating decomposition.
  2. In the decomposition of a leaf, explain why it is not possible for one species of fungus to entirely decompose the leaf?
  3. After decomposition of a substrate, such as a leaf, has occurred and all of complex compounds have been reduced to simple minerals that can now be utilized by the plant, the fungus mycelium, itself, is also made up of complex compound. What group of organisms decomposes the fungus mycelium? What is the benefit to the plants of having the fungus mycelium, in the soil, slowly decomposing?
  4. If there is a polypore growing out of a tree, in your backyard, will removing the fruiting body of the polypore prevent the fungus from further rotting your tree? Explain your answer.
  5. What is the biggest problem with using chemical preservatives on wood/timber that we have described?
  6. Commercial products can show signs of mold growth when humidity is high and temperatures are warm. If money is no object and you were given the task of preventing fungal growth from growing on the books in Sinclair Library, what would you do?
  7. What can you do to keep your clothing, books and electrical equipment, e.g. stereo system and television, safe from fungal deterioration?