6 Wood Discoloration

140 7 Wood Rot
In the successive (sequential) white rot, e.g., by Heterobasidion annosum
(root rot in spruce), Xylobolus frustulatus (“Rebhuhnfäule” in standing and
felled oaks: Otjen and Blanchette 1984, 1985), or in the Chilean “palo podrido”
(Fig. 7.2c), lignin and hemicelluloses degradation run faster at least in
early stages of attack, so that first cellulose relatively enriches. Further fungi
showing successive white rot are e.g., Ceriporiopsis subvermispora, Dichomitus
squalens, Inonotus dryophilus, andMerulius tremellosus. Frequently, e.g.,
by Phellinus pini (Liese 1970) in the heartwood of living conifers as well as
by Bjerkandera adusta and some other fungi (Blanchette 1984a; Otjen et al.
1987), there are small, elongated cavities within a wood tissue, where the
lignin and also the hemicelluloses are “selectively” (preferentially) degraded
(“selective white rot”, “selective delignification”, preferential white rot). The
greatest part of the cellulose remains. These decayed regions are surrounded
by tissue that appears sound (white pocket rot, honeycomb rot; Fig. 7.2d).
With advancing decay, the wood becomes fibrous in texture by the decay of
the more lignified middle lamella/primary wall area. Some Ganoderma species
caused within a wood tissue as well white pocket rot as simultaneous rot, or,
depending on the wood species, white pocket rot in birch and oak and simultaneous
rot in poplar (Blanchette 1984a; Dill and Kraepelin 1986; Otjen and
Blanchette 1986).
The terms “selective white rot” and “selective delignification” have been
propagated in the period of biopulping research (Chap. 9.3) as these terms
promise more experimental success than would do names like successive white
rot. As in most cases of “selective white rot” and particularly in late stages
of attack, cellulose is also degraded to some extent, the term “preferential
delignification” should be used.
Many white-rot fungi, e.g., Heterobasidion annosum (Hartig 1874), Fomes
fomentarius, Ganoderma species, and Trametes versicolor cause black spots of
manganese dioxide deposits in the attacked wood (Blanchette 1984b; Erickson
et al. 1990; Daniel and Bergman 1997). Manganese deposits may occur in
connection with lignin degradation by manganese peroxidase. Physisporinus
vitreus, isolated from cooling-tower wood (Schmidt et al. 1996) exhibited these
manganese deposits predominantly in the slime layer and in the inner S2
beneath a hypha shown by TEM/EDX spectra (Fig. 7.3B).
White-rot fungi attack predominantly hardwoods, either as pioneer organisms
or later in the context of a succession. As conifers are the main timbers
used in the northern hemisphere for constructions, white-rot fungi occur there
rarely in buildings. In Table 7.2, some important white-rot fungi are specified.
In all white rot types, the wood strength properties are reduced to a lesser
extent than in brown-rotten wood, since at the same mass loss, lesser cellulose
is consumed, and it does not come to cracking or cubical rot. In a very late
stage of attack, a wood mass loss of 97% has been measured.
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