Introduction to Fungi, Third Edition

HYMENOCHAETOID CLADE
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one of a group of early-stage ectomycorrhizal
associates of a variety of trees and also forms
mycorrhiza with Arbutus menziesii, a member of
the Ericaceae (Zak, 1976). According to Molina
and Trappe (1984), T. terrestris represents the most
abundant naturally present mycorrhizal fungus
in bare-root nurseries.
19.9 Hymenochaetoid clade
One feature that distinguishes the five
Homobasidiomycete clades considered in the
previous sections from the remaining three
clades is the structure of the parenthesome, i.e.
the membranous structure overarching the
septal pore (see Fig. 18.10). In the five clades
already described, the typical homobasidiomycete
dolipore with a perforated parenthesome is
found, whereas in the hymenochaetoid, cantharelloid
and gomphoid phalloid clades shown in
Fig. 19.2, the parenthesome is generally imperforate
(Hibbett & Thorn, 2001). Imperforate
parenthesomes are also found in certain
Heterobasidiomycetes, namely Dacrymycetales
(Section 21.3) and Auriculariales (Section 21.4).
This character has been discussed as being of
phylogenetic significance, supporting the eightclade
system shown in Fig. 19.2. Hibbett and
Thorn (2001) have estimated that the hymenochaetoid
clade comprises about 630 spp.
recruited from three families, namely the
entire Hymenochaetaceae and parts of
Corticiaceae and Polyporaceae.
19.9.1 Hymenochaetaceae
Most members of the hymenochaetoid clade are
wood-decomposing fungi, exemplified by the
white-rots Inonotus and Phellinus which often
fruit on old living trees and continue to do so
after the death of the host. In general terms, the
basidiocarps are monomitic and annual in
Inonotus but dimitic, hard and perennial in
Phellinus. Generative hyphae typically lack
clamp connections in both genera. Fruit body
morphology is variable, with resupinate and
bracket-like forms most commonly produced;
the hymenium is usually poroid. There are
transitional forms between the genera Inonotus
and Phellinus, and both have now been split up
into several smaller units (Wagner & Fischer,
2001). Here we shall focus on Phellinus sensu lato.
Phellinus sensu lato (c. 180 spp.)
This genus is widespread and cosmopolitan,
affecting both coniferous and broad-leaved
trees. An interesting example is P. weirii, of
which Hansen and Goheen (2000) have given
a superb account. This species causes laminated
root rot in native coniferous trees in the
West Coast forests of North America, affecting
especially Douglas fir (Pseudotsuga menziesii).
Mycelium is able to penetrate intact roots and
then spreads to adjacent trees by root-to-root
contact. Trees of all ages are affected and
are killed when extensive rot of the root
system renders them unstable to storms, often
several years or decades after initiation of
infection. In this way, large genets of P. weirii
slowly spread through the native forest,
profoundly influencing the host population
dynamics and shaping the forest landscape
en route. Mycelium can also survive for decades
in fallen logs. Phellinus weirii has a bipolar
(unifactorial) mating-system although, according
to Hansen and Goheen (2000), basidiospores
are not an important part of the life cycle which
relies mainly on clonal spread.
Phellinus noxius is the cause of root rot in
numerous tree species in Central America and
the Far East, Taiwan being particularly severely
affected. Disease progression is unusually rapid
for this group of pathogens, with infected trees
sometimes dying within one growing season
(Ann et al., 2002). This pathogen may remain
viable in dead colonized roots for several years if
the soil is relatively dry, and the flooding of
affected areas, where practicable, is an efficient
control method.
There are also several species encountered in
gardens, parks and forests in temperate climates,
colonizing the trunks especially of mature and
declining trees. Basidiocarps may be located
several metres above ground. For example,
P. igniarius (Plate 10f) forms basidiocarps predominantly
on willow (Salix) and apple (Malus) trees,