Introduction to Fungi, Third Edition

334 HYMENOASCOMYCETES: PYRENOMYCETES
create a weak point in the ascospore wall,
causing the spore to gape wide open as it swells
and permitting the germ tubes to emerge
(Beckett, 1976b; Chapela et al., 1990; Read &
Beckett, 1996). The ascus tip contains a cylindrical
apical apparatus which is amyloid (i.e. it
stains bright blue with iodine). The apical
apparatus is pierced by a narrow pore and is
everted as the ascus explodes (Greenhalgh &
Evans, 1967; Beckett & Crawford, 1973).
12.3.1 Daldinia
There are about 13 species of Daldinia worldwide
(Ju et al., 1997), 5 of which grow in Northern
Europe (Johannesson et al., 2000). They form
concentrically zoned perithecial stromata on
wood. The best known is D. concentrica, with
stromata mostly on ash (Fraxinus excelsior) but
occasionally on other hosts. Daldinia loculata
(D. vernicosa) and D. fissa form stromata on
charred branches of birch (Betula), gorse (Ulex)
and some other hosts following fire.
Daldinia concentrica colonizes the living
branches of ash as a symptomless endophyte
but can continue to grow saprotrophically,
fruiting on dead branches and trunks. Recently
infected wood (calico wood) has a dark speckled
appearance caused by the presence of a darkcoloured
mycelium in the vessels of the spring
wood. Boddy et al. (1985) isolated D. concentrica
from attached branches of ash and regarded it as
a primary colonizer. Only a small number of
individual mycelia were isolated from any one
branch, but often these occupied extensive
volumes of wood. Individual mycelia are
detected by their reactions in culture to other
individuals of the same species. Identical mycelia
intermingle freely, showing no obvious reaction,
but mycelia of different genotype show vegetative
incompatibility when confronted. Such
observations have led to the suggestion that
the colonization of attached branches is by a
process of latent invasion (Boddy & Rayner, 1983)
whereby the fungus might be distributed in the
sap stream of the living tree host as mycelial
fragments or spores. It is likely that D. concentrica
is heterothallic (Sharland & Rayner, 1986) like
D. loculata which grows on fire-scorched birch
branches (Johannesson et al., 2001). In this
fungus, the Nodulisporium conidial stroma develops
in spring beneath the birch bark before the
perithecial stroma emerges. Pyrophilous (fireloving)
insects feed on the conidia and also
disperse them. Conidia of different mating types
and different genotypes may be spread in this
way, and since some perithecial stromata are
known to contain several genetically distinct
perithecia, multiple mating events are possibly
involved.
Fig12.10 (a,b) Xylaria hypoxylon. (a) Ascus.The ascus tip to the
right has been stained with iodine to reveal the apical
apparatus. (b) Conidiophores. (c,d) Daldinia concentrica.
(c) Ascogenous hypha (after Ingold,1954b).The numbers
represent successive asci working backwards from the apex.
(d) Nodulisporium-type conidiophore.