Introduction to Fungi, Third Edition

DOTHIDEALES
481
Table 17.3. Some anamorph genera with Mycosphaerella teleomorphs.
Mycosphaerella Anamorph Diseases caused
M. graminicola Septoria tritici Leaf blotch of wheat
M. brassicicola Asteromella brassicae Ring spot of brassicas
M. tassiana Cladosporium herbarum
M. berkeleyi Passalora personata Groundnut defoliation
(unknown) Cercospora beticola Leaf spotof sugar beet
M. fijiensis Paracercospora fijiensis Leaf spot of banana
M. musicola Pseudocercospora musae Sigatoka disease of banana
Crous et al. (2000, 2001) and Goodwin and
Zismann (2001) listed and discussed the bewildering
diversity of anamorph genera (about 23)
connected with Mycosphaerella teleomorphs, and
performed phylogenetic analyses on representatives
of most of them. The list includes pycnidial
forms such as Septoria and Asteromella, but also
numerous hyphomycetous form-genera such as
Cercospora, Pseudocercospora and Cladosporium
(Table 17.3). Despite this range of anamorphs,
molecular evidence has somewhat surprisingly
indicated that the genus Mycosphaerella is monophyletic
(Crous et al., 2000, 2001; Goodwin &
Zismann, 2001). Pseudothecia of Mycosphaerella
are globose and small, rarely more than 100 mm
in diameter. Because they show relatively little
variation, they are difficult to identify to species
level. Pseudothecia develop subepidermally,
usually on leaves. The asci develop in a basal
fascicle. The ascospores are hyaline with a single
transverse septum (Fig. 17.19). In these features
Mycosphaerella is similar to Venturia, although
these two genera are not closely related.
17.3.2 Mycosphaerella graminicola
(anamorph Septoria tritici)
The leaf blotch disease of wheat caused by
M. graminicola (Fig. 17.20) is very similar to that
caused by the wheat strain of Phaeosphaeria
nodorum (see p. 17.2.2), and the two diseases
often co-occur on wheat crops and are controlled
in the same way, especially by the application of
fungicides. The most important fungicides are
strobilurin-type compounds and ergosterol
biosynthesis inhibitors. The ascospores of
M. graminicola, produced from pseudothecia
initially on overwintering stubble and later
from infected leaves, are the main source of
inoculum, and conidia are thought to be of lesser
importance as propagules of the disease (Eyal,
1999). In consequence, the genetic diversity of
M. graminicola in the field is often very high,
with one square metre of infected wheat shown
to contain about 70 genetically different strains
(Zhan et al., 2001). Infection by germinating
ascospores and conidia of M. graminicola is
almost always through stomata (Duncan &
Howard, 2000), in contrast to Phaeosphaeria
nodorum where it occurs directly through the
cuticle. Following penetration, intercellular
colonization of the surrounding leaf tissue by
hyphae of M. graminicola occurs, but the onset
of symptom development is delayed. Recent
reviews of M. graminicola have been written by
Eyal (1999) and Palmer and Skinner (2002).
17.3.3 Cercospora
This very large form-genus (41000 species)
contains numerous important plant pathogens
associated with a wide range of host plants (Farr
et al., 1989). Examples are C. beticola causing leaf
spot of sugar beet, C. zea-maydis (grey leaf spot of
corn), and C. coffeicola (brown eyespot of coffee).
It is difficult to estimate the real number of
species; Johnson and Valleau (1949) isolated
Cercospora from 28 host plants in 16 families,
and all seemed to belong to the same species.
Further, the dimensions of conidia and conidiophores
can vary in response to changes in
humidity. Where known, the teleomorphs of
Cercospora spp. belong to Mycosphaerella (Goodwin
et al., 2001). The conidia of Cercospora are