6 Wood Discoloration

6
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; Table 6.1).
Discolorations in standing trees occur after wounding by wound reactions
ofthetree(Chap.8.2)andbythecolonizationofthestemwoodwithbacteria
and fungi as a result of microorganism-own pigments (e.g., melanin of bluestain
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 Wa˙zny 1992a), whereby
the green algae Chlorhormidium flaccidum (Kützing) Fot. and Chlorococcum
lobatum (Kortschikoff) Fritsch & John caused even slight cell wall erosion
(Krajewski and Wa˙zny 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)
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