Introduction to Fungi, Third Edition

EUAGARICS CLADE
551
Control of witches’ broom disease is difficult
but crop losses can be reduced by sanitation,
i.e. the regular removal of brooms and diseased
pods. Other measures involve the use of
Trichoderma isolates as agents of biological
control. Some are antagonistic, but T. stromaticum
is mycoparasitic on mycelium and basidiocarps
(Samuels et al., 2000; Sanogo et al., 2002).
Systemic fungicides are also used. Further,
there is an ongoing search for hosts resistant to
the pathogen which could be used for breeding
purposes. The origin of the cocoa pathogen is
thought to be native (wild) species of Theobroma
growing in the forests, but other C. perniciosa
strains attack members of the Solanaceae, Bixa
spp., lianas (climbers) and woody debris on the
forest floor.
A closely related fungus, C. roreri
(Myceliophthora roreri), causes the damaging
frosty pod rot disease of cacao. Infected pods
are covered by white powdery masses of winddispersed
spores, originally regarded as conidia.
These spores are borne on a dikaryotic mycelium
which can be grown in culture. Dikaryotic
hyphae swell and branch to form sporophore
initials. The nuclei in the dikaryon fuse and septa
are laid down, resulting in chains of diploid cells
which have been interpreted as the equivalent of
probasidia. The diploid cells develop thick walls
to become spores and their nuclei undergo
meiosis, but division may be arrested at a
binucleate state (after the first meiotic division)
or proceed to the formation of nuclear tetrads.
On germination, there are indications that a
four-celled metabasidium may be produced, with
sterigmata which function as infective hyphae.
As in C. perniciosa, this monokaryophase is biotrophic
and can only be grown on living cocoa
tissue. The life cycle of C. roreri is thus considerably
modified, and a recognizable mushroomlike
basidiocarp stage probably does not occur
(Evans et al., 2002, 2003; Griffith et al., 2003).
19.4.10 Mycenaceae
Mycena (about 150 spp.)
This is a large polyphyletic genus of fungi, with
most species clustering in phylogenetic analyses
around the type-species, Mycena galericulata. This
group has been called Mycenaceae by Moncalvo
et al. (2002). Mycena spp. produce rather small,
delicate basidiocarps which have long slender
stipes and conical or bell-shaped caps. Fruit
bodies may emerge singly or in clusters from
wood, leaf litter and other debris such as twigs,
pine cones and bracken petioles in woodland and
pastures. Some species exude latex when the
stipe is broken, e.g. M. sanguinolenta with blood
red latex and M. galopus which produces a
milk white exudate. Much is known about the
autecology of M. galopus, which has a perennial
mycelium when growing in coniferous litter and
also fruits on a wide range of twiggy debris but
does not grow in bulky wood masses, possibly
because of restricted aeration there (Frankland,
1984; Dix & Webster, 1995). It is capable of
growing on most of the constituents of leaf litter
and its ability to break down lignin and cellulose
enables it to function as a typical white-rot decay
fungus. Mycena galopus is a key decomposer of
oak leaves and, within two years of leaf fall,
its mycelium may be present on 80% of fallen
leaves. This species is regarded as a secondary
colonizer, growing on plant material which is
already well-colonized by other fungi. In this
context, it is worth mentioning that many
Mycena spp. produce antifungal metabolites,
including strobilurins, and these may aid in the
displacement of other wood-rotting fungi.
Despite this, moribund fruit bodies of various
Mycena spp., including strobilurin producers,
may be parasitized by the zygomycete Spinellus
fusiger (Plate 3e).
Basidiocarp development in M. stylobates
growing on beech leaves has been described
and well-illustrated by Walther et al. (2001).
An irregular arrangement of interwoven
hyphae within the leaf bursts through to form
an ovoid structure at the surface, composed
mainly of vertically arranged hyphae. Cells at the
margin increase in diameter and enclose the
early stages of the primordium entirely.
Separation of this large-celled wrapping tissue
from internal hyphae results in the formation of
a ring-like groove at the base of the primordium
and a layer of protective hyphae covering a
central bulb. The cells of the outer layer of
vertically arranged hyphae increase in diameter