Introduction to Fungi, Third Edition

388 HYMENOASCOMYCETES: PYRENOMYCETES
complex appear to be reproductively isolated; for
instance, the sexual state of C. lindemuthianum
(teleomorph G. cingulata f. sp. phaseoli) has not
been found in nature but can be readily induced
in agar culture by the pairing of compatible
isolates (Roca et al., 2003).
Species of Colletotrichum cause serious diseases
on a wide range of plants. These are often referred
to as anthracnose because of the appearance
of sunken necrotic lesions, as exemplified by
C. lindemuthianum which causes anthracnose of
beans, peas and other legumes (Plate 5i). These
necrotic lesions contain the acervuli. Other
important pathogenic species are C. gloeosporioides
(anthracnose of a range of tropical fruits and
many other plants), C. coffeanum (coffee berry
disease), C. gossypii (boll rot and anthracnose of
cotton), C. musae (post-harvest fruit anthracnose
on banana), C. graminicola (anthracnose of maize
and sorghum), and C. coccodes (anthracnose of
tomato, black dot disease of potato). Many of
these produce phytotoxic substances which are
involved in causing disease symptoms (García-
Pajón & Collado, 2003). Whilst diseases in the
field are important, post-harvest rots probably
cause even greater economic damage, especially
in the tropics. This comes about because
Colletotrichum spp. can cause latent infections
which give rise to disease symptoms only during
fruit ripening in storage. Good descriptions of
anthracnose diseases may be found in two
volumes dedicated to Colletotrichum, which is
one of the most important genera of fungal
plant pathogens worldwide (Bailey & Jeger, 1992;
Prusky et al., 2000). Several Colletotrichum spp. have
been developed with limited success as biocontrol
agents against weeds (Watson et al., 2000).
12.10.1 Infection strategies in Colletotrichum
Most species infect their host from germinating
conidia which emit germ tubes terminating in a
melanized appressorium. The details of appressorium
formation and function are very similar
to those in Magnaporthe grisea (see pp. 378 381),
as far as they are known (Bailey et al., 1992).
One of the fascinations of the genus
Colletotrichum lies in the range of postappressorial
infection strategies which various
species have evolved. These have been well
described by O’Connell et al. (2000) and
Latunde-Dada (2001), and are briefly summarized
below.
Necrotrophic pathogens
Colletotrichum capsici is a necrotrophic pathogen
infecting red peppers. It shows intercellular
growth which commences immediately after
penetration and is accompanied by the secretion
of cell wall-degrading enzymes (Pring et al.,
1995).
Hemibiotrophic pathogens
The pattern of infection by numerous species
(exemplified by C. lindemuthianum) is very similar
to that in Magnaporthe grisea (p. 381). The penetration
peg arising from an appressorium forms
a vesicle which emits swollen primary hyphae
inside the first-colonized epidermal cell and in
adjacent cells (see Figs. 12.45f,g). The primary
hyphae do not breach the plant plasma membrane
but invaginate it, and there is evidence of
a distinct matrix between the fungal hypha and
the host membrane, in analogy to the haustorium
formed by biotrophic fungal pathogens
(O’Connell et al., 1985; Mendgen & Hahn, 2002).
After 1 3 days, the biotrophic phase breaks
down and thinner secondary hyphae are
formed which spread the infection, forming
a necrotrophic lesion. Secondary hyphae differ
fundamentally in surface properties from primary
hyphae and are not surrounded by an
extracellular matrix (Perfect et al., 2001). A
modification of this pattern is observed, e.g. in
C. destructivum on cowpea (Vigna unguiculata),
in which the biotrophic stage is confined to
one epidermal cell containing a multi-lobed
vesicle, from which secondary hyphae initiate
the necrotrophic phase (Latunde-Dada et al.,
1996).
Post-harvest pathogens
This group of Colletotrichum spp. is responsible for
most of the diseases of ripe fruits, especially in
tropical areas. An example is C. musae on banana
fruits. Spores alighting on unripe fruits prior
to harvest may germinate and form appressoria
and even penetration pegs, but there the