Introduction to Fungi, Third Edition

482 LOCULOASCOMYCETES
Fig17.19 Mycosphaerella brassicicola. (a) Section of
a brassica leaf with a subepidermal
pseudothecium containing asci. Note that there is
no hamathecium. (b) Pseudothecium as seen from
above, showing the ostiole. (c) Intact bitunicate
ascus. (d) Stretched ascus resulting from extension
of the endotunica. (e) Ascospores. Scale bar:
40mm (a,b) and10 mm(c e).
hyphomycetous, being produced from pigmented
aerial hyphae. They are long and tapering, as
shown by C. beticola (Fig. 17.21), which is seedborne.
It has long been known that Cercospora
infections are much less severe in shaded
plants as compared to those growing in direct
sunlight. The reason for this lies in the production
of a potent photosensitizing toxin, cercosporin
(Fig. 17.22), by many Cercospora spp. A
fascinating and lucid account of cercosporin has
been written by Daub and Ehrenshaft (2000).
Cercosporin, upon absorption of light energy,
is activated to an energized state in which it
reacts with molecular oxygen, converting it into
various radicals but especially into singlet
oxygen ( 1 O 2 ). This energized form of oxygen
is highly reactive, rapidly destroying organic
molecules and especially membrane lipids.
Cercosporin is a non-selective toxin, affecting
bacteria, plants, fungi and animals unless these
produce protective antioxidants such as carotenoids.
It is not yet entirely clear how Cercospora
spp. protect themselves against their own toxin,
but part of the answer may lie in keeping it in a
non-reactive (reduced) state. Another mechanism
is the synthesis of vitamin B 6 (pyridoxine) and
its derivatives which can act as antioxidants.
Cercosporin has been found only in Cercospora
species (Goodwin et al., 2001), but biosynthetically
related photosensitizers are produced by a
range of plant-pathogenic fungi, especially in the
Loculoascomycetes (e.g. Cladosporium, Alternaria,
Stemphylium spp.).
17.3.4 Cladosporium
The anamorph genus Cladosporium is large,
containing about 60 species. A key to species in
culture collections has been published by Ho
et al. (1999). Several species have Mycosphaerella
teleomorphs, e.g. C. herbarum is the anamorph
of M. tassiana (von Arx, 1950; Barr, 1958),
C. echinulatum is the anamorph of M. dianthicola
and C. humile the anamorph of M. macrospora. The
association of Cladosporium with Mycosphaerella
has been supported by molecular studies (Crous
et al., 2001). However, most species are without
known teleomorphs. Mycosphaerella tassiana forms