Introduction to Fungi, Third Edition

DOTHIDEALES
483
Fig17.20 Wheat leaf blotch symptoms caused by
Mycosphaerella graminicola. (a) Infected leaf showing a necrotic
lesion, in the centre of which pycnidia of Septoria tritici have
formed (arrows). (b) Septate conidia of S. tritici,morethan
10 times longer than wide.
its pseudothecia on overwintered stalks and
leaves of numerous monocotyledons and dicotyledons
in subarctic and subalpine regions, and a
period of cold is required for ascocarp initiation
in culture (Barr, 1958; Corlett, 1991). In contrast,
C. herbarum is ubiquitous and common in
temperate regions on senescent and dead plant
material, and in soil. Conidia of this and other
Cladosporium spp. are the most abundant component
of the fungal air spora (Gregory, 1973), and
they are probably the most frequent contaminant
of foodstuffs, textiles and paintwork.
They also frequently contaminate cultures of
other fungi in the laboratory. The conidia of
C. herbarum and other common moulds such as
Alternaria alternata and Aspergillus fumigatus are
associated with severe asthma (Zureik et al.,
2002). Over 30 antigens causing mould allergy
have been described from C. herbarum, and most
of them are secretory or cytoplasmic glycoproteins,
often representing common enzymes such
as enolase or aldehyde dehydrogenase
(Breitenbach & Simon-Nobbe, 2002).
Colonies of C. herbarum are dull olive green
to black in colour, and appear as a network
of hyphae or a plate-like mass (stroma) of
tightly packed, dark, thick-walled cells (McKemy
& Morgan-Jones, 1991). The conidiophores are
branched or unbranched and conidiogenesis is
holoblastic (Fig. 17.23a). The tip of the conidiophore
bulges out to form the first conidium and
it is presumed that all the wall layers of the apex
are involved (Hashmi et al., 1973). The first
conidium buds to form a further conidium and
this process continues so that a chain of conidia
develops in acropetal succession, the youngest
conidium at the end of the chain. Most conidia
have a scar (hilum) at each end, but occasionally
a conidium may form two daughter conidia
at its tip so that, as further conidial development
proceeds, a branched chain develops (see
Fig. 17.23a). Such branch-point conidia have been
termed ramoconidia (Lat. ramus ¼ branch) and
are marked by having a single scar at the base
and two scars at the apex. The conidia of
C. herbarum have dark (melanized) walls which
are slightly roughened. They may remain unicellular
or develop 1 3 transverse septa. Another
common species of Cladosporium is C. cladosporioides,
which has smooth conidium walls.
Cladosporium fulvum (also called Fulvia fulva
or Mycovellosiella fulva) is probably not closely
related to other Cladosporium spp. No sexual state
has been reported, but other Mycovellosiella
spp., like Cladosporium spp., belong to
Mycosphaerella (Crous et al., 2001). In contrast to
most other plant-pathogenic members of the
Loculoascomycetes, C. fulvum is biotrophic. It is
a pathogen of tomato plants, especially in
greenhouses. Conidia infect their host through
stomata, and hyphae spread in the leaf apoplast
in close contact with mesophyll cells, but without
producing haustoria. Sucrose, the major
plant transport sugar, is hydrolysed and taken
up, being converted to mannitol by the fungus
(Joosten et al., 1990). Conspicuous yellow