Introduction to Fungi, Third Edition

564 HOMOBASIDIOMYCETES
of peroxidase activity which suggests limited
lignin breakdown. The fungus is bipolar with
about 30 alleles at the mating type locus.
Multiple infections of single birch trunks are
comparatively rare, but where they do occur,
transverse sections of trunks with multiple
infections show a characteristic pattern of
black ‘zone lines’ marking the interface between
antagonistic dikaryons (Adams et al., 1981). The
dikaryons retain their integrity and persist over
several years. A fresh crop of basidiocarps is
produced annually on standing trees or on fallen
trunks. The birch polypore was formerly known
as the razor strop fungus in the days when the
basidiocarps were used to hone ‘cut throat’
razors.
Laetiporus
The poroid Laetiporus and Phaeolus, together with
Sparassis which produces lobed, cauliflower-like
fruit bodies (Fig. 19.23g), form one of the few
well-resolved branches within the polyporoid
clade (Wang et al., 2004). Biological features
uniting these genera are that they cause wood
decay of the brown-rot type often in living
trees, and that their mating system is bipolar.
The former species L. sulphureus sensu lato has now
been subdivided into several new taxa which
show a certain degree of host specificity (Burdsall
& Banik, 2001). Infection is typically through
wounds of living trees, causing an intense
brown-rot in the heartwood of standing trees.
The mycelium is clearly long-lived, with fresh
crops of basidiocarps (Plate 10b) produced
annually for several years in the living tree,
and later from the dead trunk. Laetiporus is
considered to be one of the main causes for the
hollowing of old oak trees in parks. Fruit trees
(especially apple) are also affected. In the Alps,
broad-leaved trees are attacked at altitudes
below 3000 ft whereas coniferous species are
infected higher up in the mountains. In addition
to basidiospores, L. sulphureus also produces a
chlamydosporic conidial state called Sporotrichum
(see Fig. 18.16).
Laetiporus sulphureus sensu lato has traditionally
been considered an edible species, as its
common name ‘chicken of the woods’ indicates.
However, consumption has been associated with
gastrointestinal upsets ( Jordan, 1995).
Polyporus
Polyporus squamosus (Fig. 19.23e) is a wound
parasite of deciduous trees such as elm (Ulmus),
beech (Fagus) and sycamore (Acer pseudoplatanus),
producing an intensive white-rot. The mycelium
persists on dead trunks, stumps and logs, and
forms successive annual crops of basidiocarps
during the early summer. The large, fan-shaped
fruit bodies are creamy yellow, with brown
scales. They are edible. Their texture is distinctly
fleshy due to a dimitic structure with binding
hyphae.
Ganoderma
There are more than 250 species of Ganoderma.
They are white-rot fungi causing root and
stem rots of hardwood and softwood hosts.
A distinguishing feature is that the spore appears
double-walled, with a dark-coloured inner layer
bearing an ornamentation which pierces the
hyaline outer one, so that the spore appears to
have a spiny surface (Fig. 19.25). Mims and
Seabury (1989) have interpreted the basidiospore
wall of Ganoderma lucidum as comprising three
Fig19.25 Ganoderma applanatum. Basidiospore.The truncate
portion is the apex of the spore. Spiny extensions of the
darker inner spore wall penetrate the hyaline outer wall.