Biology Reference
In-Depth Information
A. SOFT-ROT FUNGI (SRF)
Soft-rot is a very typical decay caused by a large number of Ascomycetes and
Deuteromycetes such as Chaetomium globosum, Ustulina deusta, Alternaria
alternata, Thielavia terrestris and Paelomyces spp. ( Daniel, 1994; Haider and
Trojanowski, 1980; Martinez et al., 2005; Nilsson and Daniel, 1989 ). More
rarely, some basidiomycetes can cause facultative soft-rot decay.
Soft-rot decay involves specialized microhyphae (thickness 0.3-0.4 m m),
which pass through the S2 layer of the secondary wall from the cell lumen
and then reorientate along the cellulose microfibrils by T-branching (two-
directional growth) or L-bending (one-directional growth) ( Buswell, 1991 ).
SRF are able to degrade wood polysaccharides, but weakly affect lignins.
Their limited action results in a soft consistency of wood when placed
in wet environments. In dry environments, wood becomes brown and
crumbly ( Eriksson et al., 1990 ). A generally uniform ontogeny of wood
decay is observed.
There are two types of soft-rot attacks ( Corbett, 1965 ):
(i). Type 1, characterized by the alignment of soft-rot cavities (longitudinal
biconical cylindrical cavities) along cellulose microfibrils. The continuous
enlargement of these cavities can lead to the total destruction of the S2
layer of the secondary wall. The middle lamella and the S3 layer appear to
be easily penetrated by microhyphae. While the former is not degraded by
SRF, the S3 layer is weakly attacked in softwoods ( Daniel, 2003 ).
(ii). Type 2, characterized by the erosion of the secondary wall and a typical
thinning of fibre walls from the cell lumen.
Soft-rot wood decay generally occurs in moist or aquatic conditions, most
frequently on hardwoods (softwoods are only very slightly degraded). It is
especially found on waterlogged woods, archaeological wood and utility
poles ( Martinez et al., 2005 ). SRF often require significant levels of nitrogen
from the environment to cause effective wood decay.
B. BROWN-ROT FUNGI (BRF)
BRF approximately comprise 10% of all wood-decaying fungi and primarily
attack softwoods. Brown-rot is found in coniferous ecosystems ( Fig. 1 A)
where the resulting residues form a large proportion of the humus material.
Geographically, BRF are primarily distributed in the north of the Tropic of
Cancer. They mainly include Basidiomycota (including Fomitopsis palustris,
Gloeophyllum trabeum, Lenzites trabea, Poria cocos, Postia placenta and
Serpula lacrymans, and some species of the genera Daedalea, Piptoporus,
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