wall, to gain access to the thick, cellulose-rich S2 layer.
When the penetration hyphae find a longitudinal
plane of weakness in the S2 layer, they produce
broader T-shaped hyphae which grow along the plane
of weakness and secrete cellulase enzymes. The diffu-
sion of these enzymes creates a characteristic pattern
of decay, seen as rhomboidal cavities within the cell
wall. These persist even when the fungi have died, leav-
ing the characteristic “signature” of a soft-rot fungus.
The soft-rot fungi have little or no effect on lignin,
which remains more or less intact. All the soft-rot
fungi need relatively high nitrogen levels for wood
decay, typically about 1% nitrogen content in the
wood. If this is unavailable in the wood itself, then nitro-
gen can be recruited from the environment, such as
the soil at the bases of fence posts.
The fungi that cause soft rots include several
Ascomycota and mitosporic species, such as
Chaetomiumand Ceratocystisin terrestrial environ-
ments and species of Lulworthia, Halosphaeria, and
Pleosporain marine and estuarine environments.
Brown-rot fungiBrown-rot fungi are predominantly Basidiomycota,
including common species such as Schizophyllum com-
mune, Fomes fomentarius(the “hoof fungus” of Scottish
birch woods; see Fig. 2.24), and the “dry-rot fungus”,
Serpula lacrymans(see Fig. 7.8). Many of the brown-rot
fungi produce bracket-shaped fruitbodies (basidiocarps)
on the trunks of dead trees, but the characteristic fea-
ture of these fungi is that the decaying wood is brown
and shows brick-like cracking – a result of the uneven
pattern of decay, causing the wood to split along lines
of weakness (Fig. 11.19) (Bagley & Richter 2002).
The “dry-rot” fungus, Serpula lacrymans, causes
major damage to structural timbers in the buildings of
Europe. However, the term dry-rot does not indicate a
different type of wood decay; instead, it signifies that
this fungus, once established, can generate sufficient
water from the breakdown of cellulose in the wood
to continue growing even when the environmental230 CHAPTER 11
Fig. 11.18(a) Diagram of the cell wall layers in woody tissue, showing the arrangement of cellulose microfibrils. ML =
middle lamella between adjacent woody cells; P =thin primary wall with loosely and irregularly arranged microfibrils;
S1–S3 =secondary wall layers. (b) Characteristic decay pattern of a soft-rot fungus in the S2 layer. The fungus pene-
trates by narrow hyphae, then forms broader hyphae in planes of weakness in the wall, and these hyphae produce
rhomboidal cavities where the cellulose has been enzymatically degraded.