Introduction to Fungi, Third Edition

364 HYMENOASCOMYCETES: PYRENOMYCETES
draws heavily on the summary in Boucias and
Pendland (1998). Water is required for germination,
which is followed by attachment to the host
cuticle by means of an appressorium.
Appressorium development is stimulated by
contact with a hard surface and can take place
on glass or polystyrene, so long as complex
nitrogenous substances (e.g. yeast extract or
peptone) are present. In nature it is presumed
that such compounds are derived from the
cuticle. The wall of the appressorium is
surrounded by a coat of mucilage which tightly
attaches the appressorium to the host integument.
An infection peg from the appressorium
penetrates the surface layer of the integument,
the epicuticle, but when it reaches the procuticle
(the layer beneath it), the infection peg expands
to form a penetration plate which grows out
parallel to the surface of the integument. From
the penetration plate, penetration hyphal bodies
develop and, from these, vertical penetration
hyphae grow through the innermost layer, the
procuticle, to the hypodermis and the body
cavity. From the vertical penetration hyphae,
hyphal bodies in turn develop which become
dispersed in the haemolymph. The hyphal bodies
come to rest in the fat bodies of the insect and
then give rise to mycelium, by which time,
48 72 h post infection, the host is dead. Death
is probably brought about by the action of
secondary metabolites which function as toxins.
These include a series of depsipeptides
(destruxins A E), a hydrophobin, cytochalasins
and alkaloids.
Metarhizium anisopliae var. acridum (formerly
known as M. flavoviride) has been used in the
biological control of grasshoppers and locusts
by suspending conidia in oil for low volume
application. As for B. bassiana, however, the
commercial viability of M. anisopliae as a mainstream
insecticide remains to be established
(Hajek et al., 2001).
Tolypocladium inflatum
This is the anamorph of Cordyceps subsessilis which
fruits on the larvae of scarabeid beetles (Hodge
et al., 1996). It is the commercial source of the
immuno-suppressant cyclosporin A, which has
become a crucial drug in the treatment of the
rejection reaction after organ transplantation
(Dreyfuss et al., 1976; Borel, 1986). Other secondary
metabolites are the efrapeptins, compounds
with anti-fungal and insecticidal properties
(Krasnoff & Gupta, 1992). Tolypocladium inflatum
grows in soil. In culture it first forms Acremoniumlike
conidia on single slender phialides, but
later copious hyaline conidia are produced
in slime from clusters of phialides with a
globose base and a long, narrow tapering neck
(Fig. 12.35c).
12.6 Ophiostomatales
This group contains about 6 genera (110 species)
of perithecial ascomycetes which are mainly
saprotrophic or parasitic on woody hosts. The
perithecia are non-stromatic, generally longnecked
and solitary. We shall consider only
Ophiostoma.
12.6.1 Ophiostoma
There are about 100 species of Ophiostoma (Grylls
& Seifert, 1993; Seifert et al., 1993), largely
confined to wood and bark and associated with
bark-boring beetles which disperse their ascospores
and conidia. Some cause fatal diseases
of trees, notably Dutch elm disease (see below).
Others, e.g. O. piceae (Fig. 12.36), cause blue-stain
(¼ sap-stain) of conifer wood (Seifert 1993;
Gibbs, 1993). The anamorphic fungus Sporothrix
schenkii, which is closely related to Ophiostoma
(Berbee & Taylor, 1992b), is a human pathogen
(Summerbell et al., 1993).
The perithecia of Ophiostoma are black in
colour with a bulbous base and a long cylindrical
neck, the ostiole of which is surmounted by
a ring of stiff tapering hairs which hold the
hyaline unicellular ascospores in a mucilaginous
blob. The asci have thin evanescent walls and
the ascospores are released into the body of the
perithecium and move up the narrow tube inside
the neck. The perithecia closely resemble those
of Ceratocystis and the two genera are sometimes
synonymized (see de Hoog & Scheffer, 1984).
However, molecular and morphological comparisons
have indicated that, despite their