Introduction to Fungi, Third Edition

CONIDIUM PRODUCTION IN ASCOMYCETES
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where the hyphae making up the synnema are
intricately interwoven (see Kirk et al., 2001).
In many ascomycetes the conidiophores develop
on or in a stroma (Gr. stroma ¼ bed, cushion),
an aggregation of pseudoparenchymatous cells.
A good example of a conidial stroma is seen in
the wood-rotting candle-snuff fungus, Xylaria
hypoxylon (see Fig. 12.11a). Here, powdery white
conidia develop at the tips of the branches of
the conidial stroma and, later, asci develop in
flask-shaped perithecia at the base of the old
stroma. The term sporodochium (Gr. spora ¼ a
seed; doche ¼ a receptacle) is used for the
cushion-like conidiomata bearing a layer of
short conidiophores. An example is the conidial
(Tubercularia) state of Nectria cinnabarina (see
Fig. 12.20c).
Another type of conidioma is the acervulus
(Lat. acervulus ¼ a little heap), a saucer-shaped
fructification which may develop inside the
tissues of a host plant or may be superficial.
Subepidermal acervuli develop from a pseudoparenchymatous
stroma, and as the acervulus
matures the overlying epidermis of the host
becomes ruptured to expose conidia formed
from conidiogenous cells lining the base of the
saucer. The conidia are held together in slime
and are chiefly dispersed by rain splash. A good
example of an acervular fungus is Colletotrichum
(see Fig. 12.51). The teleomorphs of Colletotrichum,
where known, are species of Glomerella, many of
which are serious plant pathogens. In many
ascomycetes and their allies, the conidia are
borne inside flask-shaped conidiomata termed
pycnidia (Gr. diminutive of pyknos ¼ dense,
packed, concentrated). Traditionally, fungi with
pycnidial and acervular states have been grouped
together in the artificial taxon coelomycetes
(Sutton, 1980), in contrast to hyphomycetes in
which the conidiogenous cells are exposed on
single conidiophores or in synnemata, coremia
or sporodochia (see above). Pycnidia may be
superficial or embedded in host tissue. The opening
of the pycnidium is generally by means of
a circular ostiole. Conidia formed from conidiogenous
cells lining the inner wall of the
pycnidium are held together in slimy masses
which ooze out through the ostiole, sometimes
as spore tendrils. They are generally dispersed
by splash or in water films. In some cases the
pycnidia, instead of producing conidia with an
asexual function, produce spermatia which are
involved in fertilization. Examples of fungi with
pycnidial anamorphs are Leptosphaeria acuta with
a Phoma anamorph (Fig. 17.3), and Phaeosphaeria
nodorum (anamorph Stagonospora nodorum; see
Fig. 17.4).
8.5 Conidium production
in ascomycetes
There are several steps in the production and
release of conidia, namely (1) conidiogenesis,
i.e. conidial initiation; (2) maturation; (3) delimitation;
(4) secession, i.e. separation from
the conidiogenous cell; (5) proliferation of the
conidiogenous cell or conidiophore to form
further conidia. Many of the current ideas on
conidiogenesis stem from a seminal paper by
Hughes (1953) based on light microscopy
studies of conidial development in a range of
hyphomycetes. Hughes classified the development
of conidia in a limited number of ways.
His ideas were extended by other workers, and
advances were also made possible by the use
of electron microscopy and time-lapse cinephotomicrography
(Cole & Samson, 1979).
An excellent review of these aspects of
conidiogenesis has been written by Cole (1986).
The descriptions which follow are based on the
account by de Hoog et al. (2000a). Conidiogenesis
occurs in two ways which appear to be distinct
at first glance: blastic and thallic (see Fig. 8.5).
In reality, when surveying conidium formation
and release in a range of fungi, there is a
continuum of development of which these two
concepts represent extremes (Minter et al., 1982,
1983a,b; Minter, 1984).
8.5.1 Blastic conidiogenesis
The conidium develops by the blowing-out of
the wall of a cell, usually from the tip of a hypha,
sometimes laterally as in Aureobasidium (conidial