Introduction to Fungi, Third Edition

560 HOMOBASIDIOMYCETES
monokaryons but not on dikaryons. Basidiocarps
develop from dikaryons as flat, fleshy resupinate
structures (Nuss et al., 1991). They may grow
undetected under floorboards and in roof spaces
for long periods and may reach a diameter of
1 2 m. The lower side is brown and corrugated
into shallow pores supporting the hymenium
(Fig. 19.21f). The folding of the hymenophore is
the result of continuous thickening of the
hymenium. The construction is at first monomitic,
but becomes dimitic with the development
of skeletal hyphae. Sporulation is
continuous and it has been estimated that a
basidiocarp measuring 100 cm 2 can produce
300 million spores h 1 . The immense numbers
of rusty-brown basidiospores may form deposits
visible to the naked eye on cobwebs, shelves, etc.
In the basements of buildings containing fruit
bodies, spore concentrations of around
80 000 m 3 air have been detected (see Hegarty,
1991), raising concern over respiratory allergy
which may already have been referred to in the
report to King James I (see above).
Control of dry rot
The economic consequences of failing to eradicate
and control dry rot in a building can be
severe. Modern methods aim to render the
indoor environment hostile to S. lacrymans by
eliminating all routes by which water could gain
access to timber, accompanied by constantly
high ventilation (Bravery, 1991; Palfreyman &
White, 2003). A more traditional method is the
removal of all infected timber and surrounding
sound wood, and the treatment of any timber
left in place with a recommended and approved
fungicide. Replacement timbers should be
selected for their durability properties and
can be treated with pressure-impregnated fungicides
such as copper/chromium/arsenic preservatives.
Plaster can be treated with zinc
oxychloride. Given the low temperature maximum
of S. lacrymans, it is also sometimes possible
to encase an entire building with a tent and
subject it to thermal treatment (50°C). Since
S. lacrymans is susceptible to attack by Trichoderma
spp., biological control has been proposed but
not yet put into practice (Palfreyman et al., 1995).
The most important control measure, however, is
proper construction to ensure that the moisture
level of the timber remains below the point at
which infection can be initiated.
19.6 Polyporoid clade
Included in this clade are certain members of
the Polyporales (¼ Aphyllophorales), a group
comprising hymenomycetes in which (with a
few exceptions) the hymenium is not borne on
the surface of gills. It included bracket fungi
(polypores), tooth fungi, coralloid fungi and
forms with flattened or crust-like basidiocarps.
However, morphological, anatomical and chemical
studies have indicated that this was not a
natural grouping, and molecular phylogenetic
investigations have amply confirmed this view,
with Hibbett and Thorn (2001) showing that the
aphyllophoroid condition occurs in all eight
clades of Homobasidiomycetes. Most aphyllophoralean
fungi outside the polyporoid clade
are now considered to belong to the russuloid
clade (see Section 19.7). The taxonomic hierarchy
within the polyporoid clade is too tentative to be
adopted at present, and therefore we have
desisted from using family names here.
Economically important wood-rotting bracket
fungi are found in the polyporoid clade. As
described in detail on pp. 519 522, two main
types of wood decay caused by basidiomycetes
have been recognized, namely brown-rot in
which cellulose is destroyed whereas lignin is
left essentially unchanged, and white-rot in
which both lignin and cellulose are attacked.
Both types of rot can be caused by members of
the polyporoid clade, e.g. brown-rot by Piptoporus
betulinus (Fig. 19.23d) and white-rot by Trametes
versicolor (Plate 10a). However, both rots can also
be caused by other groups of basidiomycetes.
The mycelium of members of the polyporoid
clade is often perennial in large tree trunks and
may give rise to a fresh crop of basidiocarps
annually. In some species, e.g. Fomes fomentarius
or Ganoderma applanatum (Figs. 19.23b,c), the
fruit body itself may be perennial and new
layers of hymenial tubes develop annually on
the lower side of the basidiocarp. Typically