Introduction to Fungi, Third Edition

566 HOMOBASIDIOMYCETES
and their potential biotechnological applications.
There is a conidial state called
Sporotrichum pulverulentum (Burdsall, 1981). Little
is known about the ecological role of P. chrysosporium
because this species is rarely found in
nature.
19.7 Russuloid clade
The russuloid clade is probably the most confusing
group in the eight-clade system of Hibbett
and Thorn (2001), containing the complete range
of hymenophore types shown in Table 19.1 and
Fig. 19.1. The phylogeny within this clade has
been examined in detail by Larsson and Larsson
(2003), who found that the main character uniting
all members is the presence of gloeocystidia,
i.e. cystidia filled with light-refractile (lipidrich)
contents (Fig. 19.29b). In Stereum, these are
modified as laticiferous hyphae (Fig. 19.27). Other
characters commonly used to identify russuloid
fungi are diagnostic only at lower taxonomic
ranks, e.g. the ornamented walls of basidiospores
and their starch-positive (amyloid) staining
with Melzer’s iodine. We shall study representatives
of agaricoid basidiocarps (Russulaceae),
polyporoid fruit bodies (Heterobasidion), the
spine-bearing Auriscalpium and corticioid forms
(Stereaceae). The family arrangement is still
tentative, and we give family names only where
these have a fair chance of survival in future
classification schemes.
19.7.1 Russulaceae
There are two important genera, Lactarius and
Russula, and they share many features such
as being ectomycorrhizal species, and having
a characteristically brittle texture to the gills
and general fruit bodies due to the presence
of sphaerocysts embedded in the tissue (see
Fig. 19.4). Basidiospores are typically amyloid
and ornamented with warts or ridges.
Russula (c. 750 spp.)
The genus Russula presents a similar taxonomic
nightmare to Cortinarius, and it is unlikely that
this genus is monophyletic. Species are widely
distributed and ectomycorrhizal. The basidiocarps
(Fig. 19.26a) are moderate to large, often
with a brightly coloured upper cap surface
(white, yellow, green, red, purple or black). The
gills are straight, arranged in a crowded but
regular pattern, and vary from white to strawcoloured.
The spores are ornamented with a
network of branched ridges or plates. Some
common species are R. ochroleuca (edible),
R. fellea (bitter to the taste and inedible) and the
sickener, R. emetica (poisonous). Russula ochroleuca
fruits under coniferous and deciduous trees,
R. fellea under beech (Fagus sylvatica) and R. emetica
under pines.
Lactarius (c. 400 spp.)
The common name for Lactarius is milk cap,
referring to the milky juice which exudes when
the flesh of the cap is broken. The juice varies in
colour from white to yellow, orange or violet
and may change after exposure to the air. For
example, the juice of L. deliciosus (saffron milk
cap; Plate 10c) is carrot-coloured at first, but
turns bright green upon prolonged exposure to
air. The juice also varies in taste, being mild to
faintly bitter in L. quietus, hot and acrid in
L. pyrogalus, or first mild and then acrid in L.
rufus. The juice is contained within broad
laticiferous hyphae (see Fig. 19.4). As in Russula,
the flesh also contains clusters of sphaerocysts.
The gills are generally decurrent and the spores
are ornamented like those of Russula. Some
species of Lactarius have a narrow range of
mycorrhizal partners, e.g. L. torminosus (woolly
milk cap) and L. turpis (ugly milk cap) are
associated with birch (Betula), and L. deliciosus
and L. deterrimus with Pinus and Picea. There
are several species with edible basidiocarps,
but some are poisonous or their edibility is
unknown. Fruit bodies of L. deliciosus are especially
valued, and it is possible to produce them
in plantations of artificially inoculated
host trees.
19.7.2 Bondarzewiaceae
Heterobasidion, a seemingly typical polyporoid
wood-degrading bracket fungus, has been
placed in the russuloid clade in several phylogenetic
analyses. Some authors have assigned it to