6 Wood Discoloration

138 7 Wood Rot
Brown-rot fungi colonize the wood via the rays and spread in the longitudinal
tissue through pits and by means of microhyphae. They grow inside
the cell lumina (Fig. 7.1B) and there in close contact with the tertiary wall.
The low-molecular agents and/or the cellulolytic enzymes penetrate through
the relatively resistant tertiary wall (high lignin content) and diffuse into the
secondary wall, where they degrade the carbohydrates completely (Fig. 7.1).
Typically, brown-rot fungi do not cause lysis zones around their hyphae, while
this is characteristic of many white-rot fungi. The hyphae are surrounded by
slimelayers(Table2.1).
In the early stages of decay, the carbohydrates are rapidly depolymerized.
In Serpula lacrymans, the compression strength is decreased by 45% at only
10% mass loss (Liese and Stamer 1934). Hemicellulose degradation runs up
to about 20% mass loss faster than the respiration of the cleaving products.
The relative lignin content increases parallel to carbohydrate degradation, the
absolute lignin content slightly decreases. Due to the rapid cellulose depolymerization,
the dimensional stability particularly decreases. The wood breaks
up into rectangular blocks if it shrinks by drying (Fig. 7.1A), which led to the
former term “destruction rot”. In some older literature, brown rot is falsely
named as “red rot”, which however means the typical white-rot caused by Heterobasidion
annosum.Inadvanceddecay,brown-rottenwoodcanbecrushed
with one’s fingers to a brown powder (“lignin”). “House rot” means decay inside
buildings, mostly by brown-rot fungi, particularly by Serpula lacrymans,
Meruliporia incrassata, Coniophora species, Antrodia species, Donkioporia expansa
(white rot) and Gloeophyllum species. There are further about 60 more
rarely indoor occurring fungi (Table 8.6).
7.2
White Rot
White-rot research has been reviewed by Ericksson et al. (1990) and Messner
et al. (2003). White rot means the degradation of cellulose, hemicelluloses,
and lignin usually by Basidiomycetes and rarely by Ascomycetes, e.g.,
Kretzschmaria deusta and Xylaria hypoxylon. White rot has been classified
by macroscopic characteristics into white-pocket, white-mottled, and whitestringy,
the different types being affected by the fungal species, wood species,
and ecological conditions. From microscopic and ultrastructural investigations,
two main types of white rot have been distinguished (Liese 1970).
In the simultaneous white rot (“corrosion rot”), carbohydrates and lignin
are almost uniformly degraded at the same time and at a similar rate during all
decay stages. Typical fungi with simultaneous white rot are Fomes fomentarius,
Phellinus igniarius, Phellinus robustus,andTrametes versicolor in standing
trees and stored hardwoods (Blanchette 1984a). Wood decayed by F. fomen-
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