Introduction to Fungi, Third Edition

RUSSULOID CLADE
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predominantly benzofuran derivatives (Asiegbu
et al., 1998).
Control of Heterobasidion annosum
Earlier methods of control were aimed at
preventing stump infection by treatment with
biocides such as creosote, sodium nitrite, urea,
and boron-containing chemicals. The last two are
still in use (Pratt et al., 1998; Asiegbu et al., 2005).
An interesting alternative treatment, using
biological control, is to inoculate freshly cut
stumps with the conidia of the saprotrophic
basidiomycete Phlebiopsis gigantea (¼ Peniophora
gigantea; polyporoid clade) which competes
with the parasite in the stumps and prevents
its colonization, killing the cells of Heterobasidion
by hyphal interference (Rishbeth, 1952, 1961;
Ikediugwu et al., 1970). Conidial suspensions of
P. gigantea are available commercially for use in
biological control or, in conjunction with urea,
in integrated control of stump infection in
Picea plantations (Vasiliauskas et al., 2004, 2005).
Trichoderma harzianum is another effective
biological control agent (Holdenrieder & Greig,
1998).
19.7.3 Stereaceae
In this family the fruit body is flattened,
appressed or resupinate, with a smooth (untextured)
hymenium on the lower surface.
Construction may be monomitic or dimitic
with skeletals. Basidiospores are commonly
smooth-walled and amyloid. Most members are
lignicolous and saprotrophic but some are
pathogens of trees. Hibbett and Thorn (2001)
have placed the family in the russuloid
clade, and Larsson and Larsson (2003) found
that there are several groups of Stereum-like fungi
in the russuloid clade. Forms with Stereum-like
basidiocarps also occur in several other clades of
basidiomycetes (Yoon et al., 2003).
Stereum sensu stricto
Basidiocarps of Stereum are common on decaying
stumps and on attached and fallen tree branches.
Stereum hirsutum (Figs. 19.26c,d) forms clusters
of yellowish, fan-shaped leathery brackets with a
hairy upper surface and a smooth hymenial
surface on various angiospermous woody hosts,
especially oak. It is a saprotrophic species
and important as a cause of decay of sapwood
of oak logs after felling. Stereum gausapatum is
another common fungus on oak, which can grow
parasitically on living trees, causing long narrow
decay columns (‘pipe rot’) of the heartwood.
When the hymenium is bruised it exudes a
red latex. The same phenomenon is found in
S. rugosum, common on coppice poles of hazel
(Corylus avellana) and trunks of alder (Alnus
glutinosa), and in S. sanguinolentum, a wound
parasite or saprotroph on conifers. In most
Stereum spp. there are specialized laticiferous
(sanguinolentous) hyphae which extend through
the flesh into the hymenium (Clémençon, 2004).
These hyphae (Fig. 19.27) are thick-walled and are
interpreted as modified skeletals homologous to
the gloeocystidia of other members of the
russuloid clade.
Mating in Stereum
The mating behaviour of Stereum spp. presents
some unusual features. Germinating basidiospores
form a multinucleate primary mycelium,
often with whorls (verticils) of clamp connections
at the septa, a condition described as
holocoenocytic (Boidin, 1971). This feature led
to the erroneous conclusion that S. hirsutum
is homothallic, but in fact it shows unifactorial
(i.e. bipolar) multi-allelic heterothallism (Coates
et al., 1981). The mating type factor has been
termed the ‘C’ factor. Compatible homokaryotic
primary mycelia conjugate to form a heterokaryotic
secondary mycelium which also produces
whorled clamps. However, secondary mycelia can
be distinguished by their yellowish pigmentation
and often leathery surface, in contrast to the
more delicate white primary mycelium. Some
species of Stereum (S. hirsutum, S. sanguinolentum)
include outcrossing and non-outcrossing populations
(Ainsworth, 1987), whereas in others (e.g.
S. gausapatum and S. rugosum) only outcrossing
populations have been detected. In S. hirsutum,
a feature of non-outcrossing homokaryons is
their inability to accept non-self donor nuclei
whilst they themselves can transfer nuclei to
outcrossing strains (Ainsworth & Rayner, 1989;
Ainsworth et al., 1990). In the short term,