Dirty Rotten Fungi

A breakdown of the types of wood decay in claims.

By Ralph E. Moon, Ph.D., CHMM, CIAQP , Jeff Wilemon

Claims involving decayed or rotted wood may seem simple on the surface. However, between accurate terminology, science, and policy coverage language, there is much more to know that will aid the claims professional and investigator in reaching an accurate resolution.

This article explores the biological, physical, and chemical conditions that constitute wood rot and decay. Wood decay is examined because the term is a commonly used criterion to evaluate water damage and interpret insurance policy coverage. A review of scientific publications was conducted to identify a working definition of wood decay and to refine an understanding of this process.

Wood Decay: A Practical Definition

Wood decay was first recognized as a biological process by German pathologist Robert Hartig in 1878. He established that fungi caused wood decay from their digestive activities rather than being a by-product of wood deterioration. Wood decay in both the natural and built (man-made) environment is the result of biological activity by fungi (Photo 1).Photo 1

“Decay” and “rot” are interchangeable terms that refer to the biological decomposition and consumption of wood. “Decomposition” and “degradation” refer to the sequential separation of polymers of wood (i.e., cellulose, hemicellulose and lignin) to simpler molecules. Wood decay is distinguished by three characteristics: (1) physical changes such as color, texture, thickness, and weight loss; (2) the pattern of wood and wood fiber degradation; and (3) the production of chemical changes.

Wood Structure and Composition

A cross-section of a timber log reveals decay in the center of the log (Photo 2). The center section is called the “heartwood” and does not conduct water. The outer water-conducting tissue is lighter in color and is called “sapwood.” Timber cut into planks for the built environment consists of heartwood and sapwood.Photo 2

The conducting tissues of wood (xylem and phloem) are long and tubular (Photo 3). The xylem cells provide the structural strength of the wood. The cell walls are made of complex polysaccharides known as cellulose and hemicellulose and are held together with a material called lignin. These structural polymers are comprised of long chains of sugar molecules that are biochemically digested by fungal enzymes to yield monosaccharides (single sugars), a basic fungal nutrient.Photo 3

Fungal Activity Before Wood Decay

Some fungi (e.g., Aspergillus and Penicillium) consume free sugars and surface residues without penetrating or compromising the structural integrity of the wood and are referred to as “surface mold or staining fungi” (Photo 4). Surface molds and staining fungi usually do not cause wood decay because they do not affect the wood composition or structural integrity. These fungi consume simple sugars and starches on exposed surfaces. Their abundance tends to decline as the available nutrients and moisture are depleted.Photo 4

Kingdom Fungi: The Great Recyclers

Fungi do not create their own energy; they must extract it from dead plant and animal cells. The basic elements of wood decay require fungal hyphae penetration and the release of digestive enzymes. Hyphae are microscopic tubular threads that transport moisture and collect nutrients. Collectively, the hyphal network is called a “mycelium” (Photo 5). During active wood decay, the mycelium extracts nourishment and grows by secreting digestive enzymes that break down organic molecules (e.g., cellulose) and absorb available nutrients through their individual hyphae.Photo 5

Fungal Decay of Wood

The distinguishing feature of wood decay is that it occurs when hyphae physically penetrate the wood cell and initiate the breakdown of lignin or cellulose and other macromolecules. Fungal wood decay exhibits many distinguishing characteristics depending on the type of fungi. Wood decay is categorized into three types: brown rot, white rot, and soft rot.

Brown rot fungi (Poria, Serpula, Fibroporia, Coniophora, and Fomitopsis) produce enzymes that predominantly break down cellulose and hemicellulose, leaving behind proportionately more of the darker colored lignin. These fungi also produce hydrogen peroxide to facilitate digestion and yield a characteristic dark brown appearance and fracturing of the wood (Photo 6). In the advanced decomposition stages, brown rot fungi will cause the wood to be dry and easily crumbled. This feature is often misidentified as “dry rot.” Dry rot is a misused term for a form of brown rot decay that is carried out by some fungal genera (e.g., Meruliporia and Serpula). The dry rot fungi are capable of transporting moisture via rhizomorphs over distances and colonizing formerly dry wood. All forms of brown rot and dry rot fungi require conditions above the fiber saturation point (FSP).Photo 6

White rot fungi (Polyporus, Armillaria, Pleurotus, Ganoderma, and Coriolus) produce enzymes that predominantly break down lignin and, to a lesser extent, cellulose and hemicellulose, resulting in a faded, bleached appearance (Photo 7). The wood has a spongy texture with substantial loss of strength. Both brown and white rot are facilitated by fungi known as basidiomycetes.Photo 7

Soft rot fungi (Chaetomium, Alternaria, and Cladosporium) are not digestively selective and consume cellulose, hemicellulose, and lignin in wood buried in soil or wood that is repeatedly exposed to moisture. Microscopically, soft rot fungi lack the hyphal penetration seen in brown and white rot. This form of deterioration is caused by ascomycetes, a completely different group of fungi representative of long-term wood decay and commonly associated with water-damaged building materials.

Factors Affecting Fungal Growth

Fungal decay is initiated by five basic conditions: the presence of viable wood-decay spores, appropriate moisture content, suitable nutrient supply, available oxygen, and favorable temperature. Wood decay only occurs when substrate moisture content is above the FSP—approximately 30 percent, depending upon the wood species.

Wood rot fungi (WRF) decay occurs over a wide temperature range from 10 degrees C (50 degrees F) to 35 degrees C (95 degrees F). WRF are aerobic organisms, meaning they require oxygen to metabolize organic compounds such as wood to derive nutrients and micronutrients. Wood does not decay when saturated or submerged in water because of low oxygen availability.

Stages of Wood Decay

Regardless of the fungal species, the decay process represents a sequence of enzymatic and microbial events in the presence of excess moisture. Depending on the technical reference, researchers with the U.S. Department of Agriculture Forest Products Laboratory describe three to five progressive stages of wood decay under varied and localized conditions. The three most prominent are described here.

Early or incipient stages of wood decay are characterized by slight to moderate discoloration, such as a bleached or darkening of the grain, and sometimes impart a blotched or mottled appearance. The surface may exhibit a sheen and appear persistently wet with the advancement of the mycelium. Dimensional changes are minimal. Strength loss varies widely depending on the species of fungi present and the type of wood, with reductions of six percent to 50 percent when mass is depleted by one percent to 10 percent.

Mid-stage wood decay causes more apparent lightening or darkening of the wood depending on the type of fungi present. The mycelium may become visible, and there will be deep hyphal penetration below the surface and into the wood fibers, resulting in increasing friability and structural degradation. Over time, the darkening and cubic fracturing of brown rot and the spongy faded appearance of white rot become increasingly apparent. Loss of strength is substantial, with reductions in strength of 20 percent to 80 percent when mass is depleted by five percent to 10 percent.

Late stages of wood decay produce obvious color and structural changes with the characteristics of brown and white rot being readily observed and fully distinguishable because of their selective nutrient removal. The wood will exhibit dimensional changes, profound loss of strength, and potential failure. Multiple fungal species may be present, and fruiting bodies may develop.

Summary Observations

  • Wood decay is a long-term process requiring months to years of persistent moisture exposure to occur.
  • Wood decay requires five conditions: the presence of viable wood-decay spores, appropriate moisture content, suitable nutrient supply, available oxygen, and favorable temperature.
  • Wood decay is a biological process where its progress is dictated by the fungi that consume the wood and the environmental conditions present.
  • Wood decay does not occur until the wood materials reach the fiber saturation point.
  • Wood decay begins when fungal hyphae penetrate a wood cell and digestive enzymes are released.
  • Staining fungi and surface mold generally do not cause wood decay because they consume simple sugars and starch on the surface of the wood.
  • Some staining fungi can cause soft rot; this occurs with extended duration.
  • Advanced stages of wood decay generally are recognized as dark discolorations and the loss of wood mass and weight.

The authors would like to thank Dr. Chin Yang, Nicholas Albergo, P.E., DEE, John Marquardt, P.E., Alice Varney, P.E., Robert Braun, P.E., Jeremy Beagle, Michael Bass, and Don Nehrig for their peer-review comments.


  • Anagnost, S. “Wood in the Built Environment – Conditions for Mold and Decay.” In Sampling and Analysis of Indoor Microorganisms. Yang, Chin S. and Patricia Heinsohn (Eds.). John Wiley & Sons, Inc, Hoboken, NJ. 2007.
  • Carll, C. G. and Highley, T. L. “Decay of Wood and Wood-Based Products Above Ground in Buildings.” Journal of Testing and Evaluation (JTEVA), Vol. 27, No. 2. March 1999.
  • Carll, C. G. and Wiedenhoeft, A. “Moisture-related Properties of Wood and the Effects of Moisture on Wood and Wood Products,” Chapter 4 in Moisture Control in Buildings: The Key Factor in Mold Prevention, 2nd ed. Heinz R. Trechsel and Mark T. Bomberg, eds. ASTM International. West Conshohocken, PA. 2009.
  • Clausen, C. A., “Biodeterioration of Wood.” Chapter 14, Wood Handbook – Wood as an Engineering Material. Forest Products Laboratory, United States Department of Agriculture Forest Service. Madison, WI. April, 2010.
  • Clausen, C. A., L. Haughton and C. Murphy, “Evaluating Wood-Based Composites for Incipient Fungal Decay with the Immunodiagnostic Wood Decay Test,” United States Department of Agriculture, Forest Service. General Technical Report FPL-GTR-142 December, 2003.
  • De Groot, R. C., Wood Decay Ecosystem in Residential Construction, Forest Service, U.S. Department of Agriculture, Trees and Forests for Human Settlements, Proceedings, XVIth IUFRO World Congress, p. 334-352, 1976. 
  • Levy, J.F. and D.E. Eveleigh, “The Natural History of the Degradation of Wood [and Discussion],” Philosophical Transactions A, Published 30 April 1987

Ralph E. Moon, Ph.D., CHMM, CIAQP, is with GHD, a worldwide consulting firm. He has been a CLM Fellow since 2013 and can be reached at www.ghd.com.

Jeff Wilemon, CIEC, is with GHD, a worldwide consulting firm. He has been a CLM Fellow since 2013 and can be reached at www.ghd.com.

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