6 Wood Discoloration

142 7 Wood Rot
Table 7.2. Some common white-rot fungi
Fungus Predominant occurrence
standing tree timber outdoors softwood hardwood
Armillaria mellea × × ×
Donkioporia expansa indoor × ×
Fomes fomentarius × ×
Heterobasidion annosum × ×
Meripilus giganteus × ×
Phellinus pini × ×
Polyporus squamosus × ×
Schizophyllum commune × ×
Stereum sanguinolentum × × ×
Trametes versicolor × ×
7.3
Soft Rot
The term “soft rot” was originally used by Findlay and Savory (1954) to describe
a specific type of wood decay caused by Ascomycetes and Deuteromycetes
which typically produce chains of cavities within the S2 layer of soft- and
hardwoods in terrestrial and aquatic environments (Liese 1955), for example
when the wood-fill (Fig. 7.4a) in cooling towers became destroyed despite
water saturation, and when poles broke, although they were protected against
Basidiomycetes. About 300 species (Seehann et al. 1975) to some 1,600 examples
of ascomycete and deuteromycete fungi (Eaton and Hale 1993) cause soft rot,
e.g., Chaetomium globosum (Takahashi 1978), Humicola spp., Lecythophora
hoffmannii, Monodictys putredinis, Paecilomyces spp., and Thielavia terrestris.
Soft-rot fungi differ from brown-rot and white-rot Basidiomycetes by growing
mainly inside the woody cell wall (Fig. 7.4b). The wood is colonized via
the wood rays. In conifers, the fungi penetrate, starting from the tracheidal
lumina, by means of thin perforation hyphae of less than 0.5µm thickness into
the tertiary wall and re-orientate then as thin hyphae after L-bending in one
direction or after T-branching in both directions along the microfibrils in the
secondary wall (soft rot type 1, Nilsson 1976).
In longitudinal wood sections, hyphal activity is recognizable in the polarized
light by rhombus-shaped cavities in the secondary wall of different size and
arrangement (Levy 1966; Butcher 1975), which may be lined up like a string
of pearls (Fig. 7.4c): The thin hypha stops its growth and the cavity is then
developed around the hypha by the release of enzymes (putatively endoglucanases)
along what is described as the proboscis hypha. Within the cavity,
hyphal thickness increases to about 5µm. From the tip of the cavity, the next
fine hypha starts its growth, which results in the next cavity, and continuous
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