Introduction to Fungi, Third Edition

568 HOMOBASIDIOMYCETES
Fig19.27 Stereum rugosum.
Section of hymenium.The
thick-walled hyphae with dense
contents are sanguinolentous
hyphae which exude a red fluid
when damaged, causing the
hymenium to ‘bleed’.
grows on pine, spruce, birch and other hosts,
and an ‘F’ group growing on fir trees (Abies
spp.) in Southern Europe. These groups, originally
considered as infra-specific populations
of H. annosum, have since been recognized as
separate species (Niemelä & Korhonen, 1998).
The name H. annosum is retained, but in
a restricted sense for the ‘P’ group. The ‘S’ type
is now named H. parviporum and the ‘F’ type
H. abietinum. The first two groups also occur in
North America.
The fruit bodies are perennial, at first thin
and leathery, later hard and woody (the epithet
annosum means aged). They are formed at the
base of dead trees, stumps or beneath fallen logs
and are readily identified whilst still actively
growing by their orange-brown colour with a
white margin. They are dimitic with skeletals.
Several successive layers of hymenial tubes are
formed on old basidiocarps, and basidiospores
are produced throughout the year. The mating
system is unifactorial (i.e. bipolar) with
multiple alleles, probably over 100 (Korhonen &
Stenlid, 1998). Basidiospores germinate to
form a homokaryotic primary mycelium with
multinucleate segments. Anastomosis with
a compatible homokaryon results in the formation
of a heterokaryotic secondary mycelium
also with multinucleate segments and
clamp connections at some, but not all, septa.
The conidial state (Fig. 18.15) has been assigned
to the anamorph genus Spiniger. Conidiophores
can develop on primary and secondary mycelium
and the conidia may be uninucleate or
multinucleate.
Freshly cut stump surfaces are hot-spots for
infection by H. annosum, and the disease foci in
plantations are often from thinning stumps
which have become infected from air-borne
spores. Insects are also possible vectors of
conidia. Colonization of the roots of the stump
is followed by spread to adjacent healthy trees by
root-to-root contact and possibly by root-to-root
graft because the fungus does not grow freely
in the soil. Spread in this way may result in the
development of genets (clones) many metres in
diameter, in which several adjacent trees are
infected by a secondary mycelium (heterokaryon)
of identical genotype (Swedjemark & Stenlid,
1993). The pathogenic effect of H. annosum
may be correlated with the secretion of fungal
toxins, of which several have been described,