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conditions are quite dry. Serpula lacrymans is also
notable because it posed an enigma for a long time: it
is very common in poorly ventilated buildings in
Britain and much of Europe, but nobody had ever found
it in a natural environment. Singh et al. (1993) then
discovered that it occurs rarely in the Himalayan
forests of northern India, which seem to be its nat-
ural habitat. Dry rot has been recorded in Europe since
about 1765, before there was any export of timber from
India. So this “rare” fungus seems to have arrived in
Europe as air-borne basidiospores and then flourished
in buildings where the climatic conditions are similar
to those in its natural habitat.
The term “brown rot” refers to the characteristic
color of the decayed wood, because most of the
cellulose and hemicelluloses are degraded, leaving
the lignin more or less intact as a brown, chemically
modified framework. However, the decay is typically
irregular, with some groups of wood cells being heavily
degraded while others are only slightly so. This causes
the wood to crack in a brick-like manner.
The hyphae of brown-rot fungi occur very sparsely
in the wood, often restricted to the lumen of woody
cells, and yet they cause a generalized decay in which
the S2 wall layer is almost completely degraded. This
type of decay cannot be explained by the diffusion of
cellulase enzymes, which are too large to diffuse very
far, and too large even to pass through the pores in
the S3 layer. In fact, the cellulases of brown-rot fungi
have little effect on cellulose in vitro, unlike the cellu-
lases of soft-rot fungi. Instead, the brown-rot fungi
degrade cellulose by an oxidative process, involving
the production of hydrogen peroxide during the

breakdown of hemicelluloses. Being a small molecule,

H 2 O 2 can diffuse through the woody cell walls to

cause a generalized decay. In support of this, the char-

acteristic decay pattern of brown-rot fungi can be

mimicked experimentally by treating wood with H 2 O 2

alone, and at least one of these fungi, Poria placenta,

has been shown to degrade cellulose only if hemicel-

luloses also are present, as substrates for generating

H 2 O 2. This mode of attack is an efficient way of using

the scarce nitrogen resources in wood, because it does

not require the release of large amounts of extracellu-

lar enzymes.

White-rot fungi

White-rot fungi are more numerous than brown-rot

fungi. They include both Ascomycota (Fig. 11.20) and

Basidiomycota (e.g. Armillaria mellea, the boot-lace

fungus – see Fig. 5.14, and Coriolus versicolor, shown

in an unexpected setting, Fig. 11.21). Is nothing

sacred?!!

The white-rot fungi seem to use conventional cellu-

lase enzymes for wood decay, but they are extremely

efficient in their use of nitrogen. For example, the nitro-

gen content of Coriolus versicoloris about 4% when the

fungus is grown on laboratory media of C : N ratio

32 : 1, but only 0.2% when grown on a medium of

C : N 1600 : 1. In nitrogen-poor conditions this fungus

seems preferentially to allocate nitrogen to the pro-

duction of extracellular enzymes and essential cell

components, and it also efficiently recycles the nitro-

gen in its mycelia (Levi & Cowling 1969). White-rot

FUNGAL ECOLOGY: SAPROTROPHS 231

Fig. 11.19Part of a pine stump show-
ing the characteristic brick-like decay by
brown-rot fungi.