Introduction to Fungi, Third Edition

MICROASCALES
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Fig12.38 Scopulariopsis brevicaulis. Conidiophores terminating
in conidiogenous cells (annellides) from which chains of conidia
develop.The stippled areas at the tips of the annellides indicate
the region of growth associated with the development of
successive conidia.
silage, and on keratin-rich substrates. Because
of their ability to degrade keratin and grow at
37°C, some Scopulariopsis spp. can be pathogenic
to humans, occasionally causing deep-seated
mycoses (de Hoog et al., 2000a). Not surprisingly
for such versatile and adaptable fungi, they
quickly develop resistance against commonly
used antifungal drugs (Cuenca-Estrella et al.,
2003). This feature, rather than their pathogenicity
per se, seems to be the main reason why
they can be troublesome in clinical situations.
Several well-known Scopulariopsis states
have only recently been assigned to
heterothallic Microascus species (Abbott & Sigler,
2001). Detailed developmental studies of perithecium
formation in homothallic species have
been carried out by Corlett (1966). The vegetative
hyphae of Microascus consist of uninucleate segments.
Sexual reproduction is initiated when
a coiled ascogonium forms and becomes
ensheathed by hyphae arising from the ascogonial
base as well as the surrounding vegetative
mycelium. At this primordium stage, the outermost
layer already becomes melanized, and it
surrounds the developing pseudoparenchyma.
The primordium enlarges by further growth of
the pseudoparenchyma. The ascogonium is positioned
initially in a cavity in the centre of the
primordium, but soon sterile hyphae grow
inwards from the surrounding pseudoparenchyma,
raising the ascogonium towards the
apex of the perithecium. From the ascogonium,
ascogenous hyphae grow between the sterile,
inwardly radiating hyphae and form asci without
croziers from their tips which have become
binucleate. Simultaneously, at the tip of the
perithecium pseudoparenchymatous hyphae
begin to grow inwards and then upwards,
rupturing the apical wall and extending a
perithecial neck with an ostiole. During maturation
of the perithecium, the sterile hyphae in
the perithecial centre lyse, followed by disintegration
of the ascus walls, so that the mature
perithecium contains an outer melanized
layer surrounding a pseudoparenchyma which
encloses masses of free ascospores. These ooze
out through the ostiole as a tendril.
12.7.2 Ceratocystis and Sphaeronaemella
(Ceratocystidaceae)
The most important genus of this small family
is Ceratocystis (14 species) associated with living
trees. Some species cause vascular wilts, perhaps
the most important pathogen being C. fagacearum,
the cause of oak wilt, which is especially
destructive in the United States. Ceratocystis
fimbriata attacks an exceptionally wide range of
plants, including the European plane (Platanus
acerifolia), Eucalyptus spp., mango, coffee, sweet
potato and rubber (Kile, 1993). However, infections
by Ceratocystis spp. need not be destructive
to the host, and some species produce benign