Spalting

Spalting is any form of wood coloration caused by fungi. Although primarily found in dead trees, it can also occur under stressed tree conditions or even in living trees. There are four distinct types of spalting, however only three of them are actually considered pleasing.

Types

Spalting is divided into three main types: pigmentation, white rot and zone lines. Spalted wood may exhibit one or all of these types in varying degrees.

Pigmentation

Also known as sapstain, or in its most common form, bluestain, this type of spalting occurs when the darkly-pigmented fungal hyphae grow in the sapwood parenchyma of a tree [Corbett, N.H. (1975). Micro-morphological studies on the degradation of lignifiedcell walls by Ascomycetes and Fungi Imperfecti. J. Inst. Wood Sci. 14, 18-29.] Rayner, A.D.M., and Boddy, L. (1988). Fungal Decomposition of Wood. Its biologyand Ecology. John Wiley and Sons: New York.] . A visible color change can be seen if enough hyphae are concentrated in an area Zabel, R.A., and Morrell, J.J. (1992). Wood Microbiology. Decay and Its Prevention.Harcourt Brace Jovanovich, Academic Press, INC: New York] . These pigmentation fungi often colonize wood via the rays, but are not considered decay fungi due to their non-destructive use of easily available wood carbohydrates [Dowding, P. (1970). Colonization of freshly bared pine sapwood surfaces by stainingfungi. Trans. Br. Mycol. Soc., 55, 399-412. ] Rayner, A.D.M., and Todd, N.K. (1982). Population structure in wood-decomposingBasidiomycetes. Cambridge University Press: New York. ] . The most common groups of pigmentation fungi are the imperfect fungi and the Ascomycetes . Mold fungi, such as "Trichoderma" spp., are not considered to be spalting fungi, as their hyphae do not colonize the wood internally.

While pigmentation fungi do not degrade the wood cell wall, this type of decay can lead to a reduction in toughness (amount of energy absorbed before breaking), and increased permeability Eaton, R.A., and Hale, M.D.C. (1993). Wood. Decay, pests, and protection. Chapman & Hall: New York.] . Pigmentation can occur on both hardwood and softwood, unlike other types of spalting which are more host specific.

White Rot

The mottled white pockets and bleaching effect seen in spalted wood is due to white rot fungi. Primarily found on hardwoods, these fungi 'bleach' by consuming lignin, which is the slightly pigmented area of a wood cell wall [Liese, W. (1970). Ultrastructural aspects of woody-tissue disintegration. Annu. Rev.Phytopathol. 8, 231-257.] . Some white rotting can also be caused by an effect similar to pigmentation, in which the white hyphae of a fungus, such as "Trametes versicolor" (Fr.) Pil., is so concentrated in an area that a visual effect is created [Blanchette, R.A. (1984). Screening wood decayed by white rot fungi for preferential lignin degradation. Appl. Environ. Microbiol. 48 (3), 647-653.] .

Both strength and weight loss occur with white rot decay, causing the 'punky' area often referred to by woodworkers. Brown rots, the 'unpleasing' type of spalting, effect these same properties, but at a much faster rate . Both types of rot, if left uncontrolled, will turn wood useless.

Zone Lines

Dark dotting, winding lines and thin streaks of red, brown and black are known as zone lines. This type of spalting does not occur due to any specific type of fungus, but is instead an interaction zone in which different fungi have erected barriers to protect their resources . The lines are often clumps of hard, dark mycelium, referred to as pseudosclerotial plate formation [Cease, K.R., Blanchette, R.A., and Highley, T.L. (1989). Interactions betweenScytalidium species and brown- or white-rot Basidiomycetes in birch wood. Wood Sci. Technol. 23, 151-161] .

Zone lines themselves do not damage the wood, however the fungi responsible for creating them often do.

Conditions

Conditions required for spalting are the same as the conditions required for fungal growth: fixed nitrogen, micronutrients, water, warm temperatures and oxygen [Manion, P.D. (1991). Tree Disease Concepts. 2nd ed. Prentice-Hall: New Jersey] .

Water:

Wood must be saturated to a 20% moisture content or higher for fungal colonization to occur. However wood placed underwater lacks sufficient oxygen, and colonization cannot occur .

Temperature:

The majority of fungi prefer warm temperatures between 10 and 40°C , with rapid growth occurring between 20 and 32°C. [Humphrey, C.J., and Siggers, P.V. (1933). Temperature relations of wood-destroying fungi. J. Agr. Resource. Econ. 47, 997-1008.]

Oxygen:

Fungi do not require much oxygen, but conditions such as waterlogging will inhibit growth [Fogarty, W.M., and Ward, O.P. (1973). Growth and enzyme production by Bacillus subtilius and Flavobacterium pectinovorum in Picea sitchensis. Wood Sci. Technol. 7, 261-270.] [Van der Kamp, B.J., Gokhale, A.A., and Smith, R.S. (1979). Decay resistance owing to near-anaerobic conditions in black cottonwood wetwood. Can. J. For. Res. 9(1), 39-44.] .

Time:

Different fungi require different amounts of time to colonize wood. Research conducted on some common spalting fungi found that "Trametes versicolor", when paired with "Bjerkandera adusta", took 8 weeks to spalt 1.5" cubes of Acer saccharum Robinson, S.C., Richter, D.L. and Laks, P.E. (2007). Colonization of sugar maple by spalting fungi. Forest Prod. J. 57(4), 24-32.] . Colonization continued to progress after this time period, but the structural integrity of the wood was compromised. The same study also found that "Polyporus brumalis", when paired with "Trametes versicolor", required 10 weeks to spalt the same size cubes.

Commonly Spalted Woods

The Ohio DNR found that pale hardwoods had the best ability to spalt [Ohio Department of Natural Resources. (2005). Spalted Wood. Division home page: Forest Industries.] . Some common trees in this category include maple ("Acer" spp.), birch ("Betula" spp.) and beech ("Fagus" spp.).

Common Spalting Fungi

One of the more tricky aspects to spalting is that some fungi cannot colonize wood alone; they require other fungi to have gone before them to create more favorable conditions. Fungi progress in waves of primary and secondary colonizers , where primary colonizers initially capture and control resources, change the pH of the wood and its structure, and then must defend against secondary colonizers that then have the ability to colonize the substrate [Holmer, L., Renvall, P., and Stenlid, J. (1997). Selective replacement between species of wood-rotting Basidiomycetes, a laboratory study. Mycol. Res. 6, 714-720.] .

"Ceratocystis spp". (Ascomycetes) contains the most common blue stain fungi [Croan, S.C. (2000). Evaluation of white-rot fungal growth on Southern Yellow Pinewood chips pretreated with blue-stain fungi. The International ResearchGroup on Wood Preservation: U.S. Department of Agriculture, Forest Service. 31st Meeting.] . Trametes versicolor, (Basidiomycetes) is found all over the world and is a quick and efficient white rot of hardwoods . "Xylaria polymorpha" (Pers. ex Mer.) Grev. (Ascomycetes) has been known to bleach wood, but is unique in that it is one of the few fungi that will erect zone lines without any antagonism from other fungi [Campbell, A.H. (1933). Zone lines in plant tissues. I. The black lines formed byXylaria polymorpha (Pers.) Grev. in hardwoods. Ann. Appl. Biol. 21, 1-22.] .

palting Research

Initial lab work was conducted on spalting in the 1980s at Brigham Young University. A method for improving machinability in spalted wood using methyl methacrylate was developed in 1982 [Christensen, K.W. (1982). Improving the Working Properties of Spalted Woods Through Impregnation with Methyl Methacrylate. Thesis: Brigham Young University.] , and several white rot fungi responsible for zone line formation were identified in 1987 [Phillips, L.W. (1987). The Nature of Spalted Wood: Analysis of Zone Line Formation Between Six White Rot Fungi. Thesis: Brigham Young University.] . Current research at Michigan Technological University has identified specific time periods at which certain spalting fungi will interact, and how long it takes for said fungi to render the wood useless . Researchers from this university also developed a test for evaluating the machinability of spalted wood using a universal test machine [Robinson, S.C., Laks, P.E., Richter, D.L. and Pickens, J. B. (2007). Evaluating loss of machinability in spalted sugar maple. Forest Prod. J. 57(4), 33-37.] .

References