Introduction to Fungi, Third Edition

11
Plectomycetes
11.1 Introduction
The class Plectomycetes originally contained all
ascomycetes which produce their asci within a
cleistothecium, i.e. a ‘closed case’. DNA sequence
comparisons have revealed that this character
was a fairly good one because, with the major
exception of the powdery mildews (Erysiphales;
see Chapter 13) and few scattered examples in
the Pyrenomycetes (Chapter 12) and Helotiales
(Chapter 15), most cleistothecium-forming fungi
and the anamorphs associated with them have
been found to be monophyletic (Berbee & Taylor,
1992a; Geiser & LoBuglio, 2001). As they stand
now, the Plectomycetes can be defined by the
following set of characters (Alexopoulos et al.,
1996; Geiser & LoBuglio, 2001).
(1) A cleistothecium or gymnothecium is
usually present; a cleistothecium proper has a
thick and continuous (pseudoparenchymatous)
wall, whereas in the gymnothecium the wall
consists of an open cage-like construction
of hyphae, the reticuloperidium (Greif &
Currah, 2003). Naked asci are produced only in
rare cases.
(2) Ascogenous hyphae are usually not
conspicuous.
(3) Asci are scattered throughout the cleistothecium,
not produced by a fertile layer
(hymenium).
(4) Asci are mostly globose and thin-walled,
and the ascospores are released passively after
disintegration of the ascus wall, not by active
discharge.
(5) Ascospores are small, unicellular and
usually spherical or ovoid.
(6) Conidia are commonly produced from
phialides (in Eurotiales) or as arthroconidia,
which are typically formed as chains of conidia
alternating with sterile cells. An arthroconidium
becomes released when the neighbouring cells
disintegrate. This rhexolytic secession is typical
of the microconidia of Onygenales and
Ascosphaerales. The alternative is schizolytic
secession in which adjacent cells separate when
the septum joining them splits into two (see
Figs. 8.9, 10.10), but this is not found in the
Plectomycetes. However, terminal thick-walled
chlamydospores and multicellular blastic macroconidia
may be produced by some Plectomycetes.
Plectomycetes are predominantly saprotrophic
fungi associated with the soil. Many have
a capacity to degrade complex biopolymers, e.g.
starch and cellulose, while others degrade
proteins such as keratin which makes up hair,
horn and feathers. If proteolytic fungi can grow
at 37°C, they are potentially pathogenic to
mammals, and some of them are indeed among
the most dangerous fungal pathogens of man.
Many other Plectomycetes produce important
secondary metabolites, e.g. antibiotics and
mycotoxins.
Several taxonomic arrangements have been
proposed for the Plectomycetes (e.g. Kirk et al.,
2001; Eriksson et al., 2003), but we have chosen
that of Geiser and LoBuglio (2001) because of its
clarity. This divides the Plectomycetes into three
orders, the Ascosphaerales, Onygenales, and
Eurotiales (Table 11.1). We will consider