Wood Discoloration

The damage of wood by fungi is essentially caused by the degradation of the cell wall by fungi, which decreases the mechanical wood properties and substantially reduces wood use.

However, wood quality is also influenced by bacterial, algal and fungal discolorations (e.g., Grosser 1985; Zabel and Morrell 1992; Eaton and Hale 1993). Discolorations in the wood of living trees, in round wood, timber and wood in service are long-known problems and are based on different biotic and abiotic causes (Bauch 1984, 1986; Kreber and Byrne 1994; Koch et al. 2002; Koch 2004; ).

Discolorations in standing trees occur after wounding by wound reactions of the tree and by the colonization of the stemwood with bacteria and fungi as a result of microorganism-own pigments (e.g., melanin of blue-stain fungi, Zink and Fengel 1989) or of their metabolism (brown, white, and soft rot in trees, chemical reactions of accessory compounds after pH-change by wetwood bacteria and in the splash-heart of beech trees).

Algae like Chlorococcum sp. and Hormidium sp. soiled and discolored timber surfaces (Ohba and Tsujimoto 1996; also Krajewski and Wainy 1992a), whereby the green algae Chlorhormidium flaccidum (Kutzing) Fot. and Chlorococcum lobatum (Kortschikoff) Fritsch & John caused even slight cell wall erosion (Krajewski and Wainy 1992b).

Table 6.1. Biotic and abiotic wood discolorations (completed after Bauch 1984; Butin 1995)

tree reactions on wounding
microbial discolorations
- staining by algae, molds, blue stain and red
-streaking fungi
- grey stain of poplar wood by Phialophora fastigiata
- pink stain by Arthrographis cuboidea
- black streaking of beech wood by Bispora monilioides
- red spotting of beech wood by Melanomma sanguinarum
- "green rot" by Chlorociboria spp.
- wood rots

physiological reaction of living parenchyma cells ("Ersticken" of beech and oak) biochemical reaction by wood-own enzymes ("Einlauf" of alder) chemical reactions (iron-tannic acid reaction of oak, discoloration of hemlock by zinc) combined reaction (brown discoloration of Ilomba by bacterial pH-increase and subsequent chemical reaction of phenols) The wood-discoloring molds and staining fungi live on nutrients in the parenchyma cells of the sapwood. Conifers and hardwoods, round wood, lum-ber, finished wood and wood products can be colonized. Discoloring fungi do not cause any or only very little cell wall attack. Prioritization of the color damage depends on subsequent wood use.

Several Deuteromycetes and Ascomycetes stain woody substrates. Phialo-phora fastigiata (Hyphomycetes) causes a grey stain of poplar wood. Arthro-graphis cuboides (Hyphomycetes) produces a pink stain in several hardwoods and softwoods, and a naphthalenedione has been isolated from such wood (Golinski et al. 1995).

Red alder wood used in the USA for furniture is stained reddish purple by Ophiostoma piceae if not rapidly processed after harvesting (Morrell 1987). Black streaking of beech logs occurs by Bispora monilioides. Red spotting of beech wood is effected by Melanomma sanguinarum (Doth-ideales).

Paecilomyces variotii produces a yellow discoloration of oak wood during drying through its pH-change, which causes chemical reactions of the hydrolyzable gallotannins (Bauch et al. 1991). So-called green rot is caused by species of the ascomycete Chlorociboria (Helotiales). Chlorociboria aeruginascens and C. aeruginosa discolor rotten and moist hardwood (and conifer) branches and other woody debris in the forest (Jahn 1990). The green wood has often been employed in marquetry and veneering and is a feature of the famous Tunbridge ware (Ellis 1976).

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