Introduction to Fungi, Third Edition

356 HYMENOASCOMYCETES: PYRENOMYCETES
E. baconii has been given to the fungus on Agrostis
stolonifera, and E. clarkii to that on Holcus lanatus
(White, 1993). The form on Festuca rubra and F.
valesiaca is E. festucae (Leuchtmann et al., 1994).
The uppermost leaf sheath of flowering tillers
becomes surrounded by a white mass of mycelium
2 cm or more in length, and at the surface
small unicellular phialoconidia are produced
(Fig. 12.29b). These conidia function as spermatia
(see below). Later, the conidial stroma becomes
thicker and turns orange in colour as perithecia
are formed (Plate 5f). The perithecia produce
numerous asci, each with a well-defined apical
cap, and containing eight long narrow ascospores
which may break up within the ascus to
form part-spores (Fig. 12.29d). The mycelium is
for the most part intercellular, unbranched and
mainly located in the pith, although intracellular
penetration of the vascular bundles is found
in the region of the inflorescence primordium.
Perithecial stromata are formed only on tillers
containing inflorescence primordia, and by
manipulating incubation conditions it has been
shown that the formation of stromata is correlated
directly with the presence of an inflorescence
primordium rather than with external
conditions (Kirby, 1961).
Epichloe typhina is heterothallic with a unifactorial
(bipolar) mating system (White & Bultman,
1987). Throughout its range Epichloe is attacked
by a parasitic fly, Botanophila phrenione (Phorbia
phrenione), which feeds on conidia, conidiophores
and hyphae. The relationship is a symbiotic one.
Before laying eggs, female flies feed on conidia
and hyphae from conidial stromata. Possibly they
are attracted by the white colour and distinctive
smell of the stroma (Leuchtmann, 2003). The
conidia remain viable after passing through the
gut of the flies. After laying an egg in a fresh
conidial stroma an ovipositing female shows an
unusual but characteristic pattern of behaviour,
walking in a linear or spiral path around the
conidial stroma whilst dragging its abdomen and
depositing a trail of faecal material with viable
conidia on the receptive hyphae of its surface,
so spermatizing them. This track is later marked
by the development of perithecia (Bultman
et al., 1998).
Perithecial ontogeny has been studied by
White (1997). The perithecial primordium develops
as a cavity lined by inwardly directed
branched hyphae. In E. typhina, at the base of the
cavity a mound of ascogenous tissue appears.
This is made up of ascogenous hyphae with
croziers and with lateral paraphyses, but the
paraphyses do not persist as the asci mature. The
perithecial ostioles protrude above the surface of
the stroma and are lined by curved periphyses
(Fig. 12.29c). The apical apparatus of the ascus
consists of a thickened ring pierced by a narrow
canal continuous with the cytoplasm of the ascus
(Figs. 12.29d, 12.30a) and through the canal the
ascospores are discharged singly, one after
another.
Ascospores may segment into part-spores
within the ascus or after discharge. It has been
claimed that they never germinate directly (i.e.
by germ tube), but only by the production of
conidia from narrow tapering phialides (Figs.
12.30b,c; Bacon & Hinton, 1988). However, our
own observations on ascospores of E. typhina
from D. glomerata show that direct germination
may occur (Fig. 12.30d). Primary conidia may
germinate directly or by forming secondary
conidia, a process described as microcyclic conidiation
(Bacon & Hinton, 1991). Tertiary conidia
may also develop from secondary conidia.
Attempts to infect grasses from ascospores
have generally been unsuccessful, so it is likely
that infection is by conidia. After allowing
ascospores to be discharged close to emerging
inflorescences of uninfected Lolium perenne
plants, about 12% of the seeds gave rise to
infected progeny, but whether infection was
directly from ascospores or from primary or
secondary conidia was not determined (Chung &
Schardl, 1997a). Experimentally, it has not been
possible to infect developing seeds of Dactylis,
and the only effective method of infection is by
application of ascospores or conidia to the cut
ends of green stubble (Western & Cavett, 1959).
If this is the natural route of infection in other
grasses, it may explain the greater incidence of
the disease in Agrostis in heavily grazed pastures
(Bradshaw, 1959).
The time of release of ascospores coincides
with the emergence of larvae of the parasitic fly