Introduction to Fungi, Third Edition

480 LOCULOASCOMYCETES
Fig17.18 Sporormiella intermedia. (a) Pseudothecium with asci visible through the transparent wall. (b) Ripe unextended ascus
showing the double wall. (c) Elongating ascus showing rupture of the outer wall (ectotunica) and extension of the inner (endotunica).
(d) Ascospore separated into its four component cells. (e) Intact ascospore.
context as an indicator of changes in vegetation
and land management. Thus, Burney et al. (2003)
have shown for the island of Madagascar that
Sporormiella was very common before the settlement
of humans which occurred after ad 200
and then declined in abundance along with the
extinction of several groups of large herbivores.
After ad 1100, spore densities of Sporormiella
in sediments showed an increase, coinciding
with the introduction of grazing domesticated
livestock.
17.3 Dothideales
This is a large group of ascomycetes containing
an enormous variety of conidial forms, both
hyphomycetous and pycnidial. Where present,
the teleomorph consists of dark-celled pseudothecial
ascomata, usually developing as
locules within an ascostroma. The asci are
bitunicate (fissitunicate). In contrast to the
Pleosporales, inter-ascal tissue (hamathecium
with pseudoparaphyses) is generally lacking. A
relationship between the Dothideales and the
Pleosporales has been suggested on the basis of
molecular data (Silva-Hanlin & Hanlin, 1999;
Lumbsch & Lindemuth, 2001). Many members
are saprotrophic on dead plant material, but
some grow as endophytes and some are plant
pathogens. The single ascocarpic example which
we shall study is Mycosphaerella.
17.3.1 Mycosphaerella
Mycosphaerella is one of the largest genera of
ascomycetes, containing over 2000 described
species (Corlett, 1991). However, many of the
names are based mainly on the association of a
Mycosphaerella with a particular host plant. Given
the lack of critical inoculation experiments to
clarify their host range, mating experiments or
DNA sequence comparisons with similar forms
on other plants, it is likely that many names
are synonyms. Some species are plurivorous,
growing on a broad range of monocotyledonous
and dicotyledonous hosts. Many species of
Mycosphaerella cause diseases of economic significance,
and some of them are listed in Table 17.3.
Most of these diseases involve the necrosis of
host plant tissue, and the toxins produced by
the pathogens are commonly associated with
the disease symptoms (e.g. Cercospora beticola; see
p. 481).