Introduction to Fungi, Third Edition

XYLARIALES
335
Daldinia concentrica forms large (5 10 cm
diameter) hemispherical purplish-brown annual
stromata called ‘cramp balls’ or ‘King Alfred’s
cakes’. They contain ripe asci between May and
October. In cross section (Plate 5a), the stromata
show a concentric zonation of alternating light
and dark bands. The surface of young stromata
may be covered with a pale fawn powdery mass
of conidia. The conidia are dry and ovoid in
shape, developing successively at the tips of
branched conidiophores by the outgrowth of
the wall and, when detached, leave a small scar
(Fig. 12.10d). Conidia of this type have been
named Nodulisporium tulasnei. Perithecia develop
in the outer layers of the stroma. The perithecial
wall is lined by ascogenous hyphae which are
unusual in that there is often a considerable
distance separating successive asci (Ingold,
1954b) (Fig. 12.10c). The stroma of Daldinia
apparently functions as a water reservoir and
detached stromata will continue to discharge
ascospores for about 3 weeks even if placed in a
desiccator (Ingold, 1946). Spore discharge is
nocturnal and the rhythm of spore discharge is
maintained for several days if detached stromata
are kept in continuous dark. In continuous light,
periodic spore discharge ceases after about three
days but is restored on return to alternating light
and dark (Ingold & Cox, 1955). The output
of spores from a single stroma of average size
is about 10 million a night. The ability of
perithecial stromata to store water enables the
fungus to continue sporulating on dry branches.
Mycelial growth is also possible at lower water
potentials ( 10 MPa) than by competing fungi
(Boddy et al., 1985).
12.3.2 Xylaria
There are over 100 species of Xylaria, most of
which are lignicolous, but some are endophytic
and others grow on fallen fruits (Whalley, 1985,
1987). One of the best known is Xylaria hypoxylon,
the candle-snuff fungus. Stromata are common
on stumps and fallen branches of deciduous
trees. As in most Xylariaceae growing on wood,
the boundaries of individual mycelia within
infected tissues are visible as conspicuous black
demarcation lines. The stromata are branched
and cylindrical or flattened. At the upper end,
the stroma is covered by a white powdery mass
of conidia (Figs. 12.10b, 12.11a). Perithecia
develop later at the base of the stroma and are
visible externally as swellings at the surface
(Fig. 12.11b). The apical apparatus of the ascus is
visible even in immature asci after staining in
iodine as a bright blue cylindrical collar pierced
by a narrow pore (Fig. 12.10a). Xylaria polymorpha
(‘dead men’s fingers’) fruits in late summer and
autumn at the base of old tree stumps. The
stromata are swollen, finger-like and clustered
(Plate 5b). The surface is at first covered by an
inconspicuous conidial layer, but eventually
perithecia develop beneath the surface of the
whole stroma, and are not restricted to the basal
region as in X. hypoxylon. Both species are active
wood-rotting fungi causing decay of the whiterot
type. Other common species with a more
restricted host range are X. longipes on fallen
branches of sycamore (Acer pseudoplatanus) and
X. carpophila on fallen cupules of beech (Fagus
sylvatica).
12.3.3 Hypoxylon
This is a large genus of over 120 species (Whalley
& Greenhalgh, 1973; Ju & Rogers, 1996) forming
stromata which are often hemispherical or
sometimes flattened on the surface of wood
and bark. Some show a preference for a particular
host. Common species are H. fragiforme
almost confined to branches and trunks of
Fagus (Fig. 12.12a), H. multiforme on Betula
(Fig. 12.12b), and H. rubiginosum which forms
flat stromata on decorticated wood of Fraxinus.
The young stromata of all these species
bear a conidial felt of the Nodulisporium or
Geniculosporium type (Fig. 12.13; Chesters &
Greenhalgh, 1964). Most species show nocturnal
spore discharge (Walkey & Harvey, 1966b).
Freshly cut lengths of healthy beech branches
incubated under water-saturated conditions
show no evidence of the presence of H. fragiforme,
but if similar sections are incubated under
conditions in which the branches are allowed
to dry, characteristic patches of stained or
discoloured wood become apparent within
21 days (Chapela & Boddy, 1988a,b). Isolations
from such areas produce several genetically