Introduction to Fungi, Third Edition

PLEOSPORALES
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used (Ellis, 1971b). Electron microscopy studies
(Carroll & Carroll, 1971) have shown that
conidial development is blastic, involving the
whole of the wall at the apex of the conidiogenous
cell. The cytoplasmic connection between the
conidiogenous cell and the conidium is narrow,
and is surrounded by two layers of thickened
wall material. Following the detachment of the
first-formed conidium, the conidiophore may
grow out through the detachment scar to form
a second conidium, a process described as
percurrent conidiogenesis. The conidia of
P. herbarum are formed more readily in cultures
illuminated by near-UV light (Leach, 1968),
whereas daylight and low temperature stimulate
pseudothecial development (Leach, 1971). The
fungus is homothallic. According to Meredith
(1965) the conidia are violently jolted from the
tip of the conidiophores.
17.2.5 Lewia
The genus Lewia was named in honour of L. E.
Wehmeyer by Simmons (1986) for Pleospora-like
fungi with Alternaria anamorphs. Six ascocarpic
species have been recognized, fruiting on grasses
(including cereals) and on dicotyledonous hosts
(including Brassica and Pastinaca) (Kwasna &
Kosiak, 2003). The separation of Lewia from
Pleospora is supported by molecular evidence
(Pryor & Gilbertson, 2000).
Lewia infectoria (¼ Pleospora infectoria) forms
black, shining, subepidermal pseudothecia on
overwintered grass and cereal culms. It has
golden-brown muriform ascospores with up to
five transverse septa. The central cells of the
ascospores also contain one or rarely two longitudinal
septa (Fig. 17.10a). In culture, this
fungus forms branching chains of obclavate,
brown-coloured (melanized), muriform, beaked
spores (dictyospores or dictyoconidia) and new
spores are formed at the tip of the chain
(Fig. 17.10c). A darkly pigmented thickened
annulus is visible at the base of the conidium
and at the apex of the conidiophore surrounding
the point of spore separation and, if the spore
has occupied an intercalary position on the spore
chain, there is also an annulus at the opposite
end. Chain branching occurs where a conidium
produces more than one spore. Conidia of this
type have been classified in the anamorph genus
Alternaria, and are poroconidia. The conidial state
of L. infectoria is A. infectoria.
17.2.6 Alternaria
About 50 species of Alternaria are known which
have not been connected to a teleomorph
(Neergaard, 1945; Joly, 1964; Simmons, 1986;
Kirk et al., 2001). The taxonomy of Alternaria is
difficult. Simmons (1992) has given a key to 10
species-groups and Ellis (1971a, 1976) has
described and figured some common species.
Despite the absence of formal evidence for sexual
reproduction in many species of Alternaria,
Berbee et al. (2003) have shown that three species
of Alternaria not known to have sexual states,
A. brassicae, A. brassicicola and A. tenuissima, have
mating type gene sequences. In any one isolate of
these species, only one mating type idiomorph
was found, but in other isolates of the same
species the opposite idiomorph was detected.
This suggests that these currently asexual species
were derived from sexually reproducing
ancestors.
The fine structure of conidial development
from a pre-existing conidium in A. brassicae has
been studied by Campbell (1968). The mature
spore has a two-layered wall, the outer of which
is melanized. A pore develops in the outer wall,
probably by enzymatic activity, and the inner
wall layer expands through the pore to become
the primary wall of the new conidium. Later, this
in turn becomes two-layered. Transverse and
longitudinal septa develop within the spore,
but these are incomplete; a pore in each
septum allows cytoplasmic continuity between
adjacent cells and flow of cytoplasm through the
spore to provide material for the formation of
new spores at the tip of the chain.
The shape of the conidium in Alternaria
affects its aerodynamic properties. Several
species have conidia with long beaks, e.g.
A. solani and A. brassicae (Fig. 17.11). It has been
suggested that the long beaks increase the
chance of wind-dispersal as compared to species
with smaller, non-beaked conidia (Chou & Wu,
2002). Long beaks also increase the drag on the