Introduction to Fungi, Third Edition

298 PLECTOMYCETES
they are xerophilic, i.e. capable of growing at
a water potential (a W ) at or below 0.85. Thus, they
are major food spoilage organisms, growing on
stored cereals, spices, nuts, bread, dried and cured
ham, pickles, jams and preserves (Lacey, 1994;
Filtenborg et al., 2002). Colonization of food and
feedstuff can result in its contamination by
serious mycotoxins (see pp. 304 306).
Colonies typically spread slowly but quickly
assume a greenish-blue pigmentation due to
abundant conidium formation. The pigmentation
of mature conidia is at least partly due to melanin
(see Fig. 12.46; Plate 4d). The conidial state is more
commonly observed than the teleomorph, and
indeed many species have lost their capacity
of sexual reproduction altogether (Geiser et al.,
1996). Loss of sexual reproduction seems to have
occurred independently on several occasions
within the Eurotiales. When present, the ascocarps
range from gymnothecial structures with
loose mesh-like reticuloperidia (e.g. Talaromyces)
to the hard sclerotium-like fructifications of
certain species of Eupenicillium. The conidial
states are generally phialidic. Microscopically,
the conidiophores of Aspergillus and Penicillium
can be categorized according to the arrangement
of phialides on the conidiophore (Fig. 11.11). The
phylogenetic value of these structures may be
limited, but they are very useful for species
identification. In Aspergillus (Fig. 11.11a), the conidiophore
tip is swollen into a hemispherical or
club-shaped structure, the vesicle. Phialides may
be formed directly at the vesicle surface in which
case the conidiophore is said to be uniseriate
(Figs. 11.11a, 11.16a; Raper & Fennell, 1965).
Alternatively, a palisade of sterile cells (metulae)
is formed by the vesicle, and the tips of the
metulae give rise to the phialides (biseriate
conidiophores; Fig. 11.17a). No vesicle is produced
in Penicillium (Figs. 11.11b and 11.18), and instead
the conidiophore tip either gives rise to phialides
directly in the monoverticillate arrangement
(Pitt, 1979), or it produces one series of metulae
(biverticillate) or further branching layers. In
terverticillate penicilli, the conidiophore tip
produces one or several rami, each of which
develops several metulae which in turn produce
several phialides each. In quaterverticillate
Fig11.11 Examples of conidiophores of three important anamorphic genera of theTrichocomaceae. (a) Aspergillus penicillioides.
(b) Penicillium notatum (¼ P. chrysogenum), the original penicillin-producing strain isolated by Sir Alexander Fleming in1928.
(c) Paecilomyces marquandii. All images to same scale.