Introduction to Fungi, Third Edition

478 LOCULOASCOMYCETES
is commonly found in the field, whereas
Pyrenophora-type pseudothecia are uncommon.
Species belonging to this group are pathogens
of cereals and grasses, and some of them cause
significant diseases in agricultural situations.
The disease symptoms are similar to those
caused by other members of the Pleosporales,
and phytotoxic substances are produced by
several members of the genus.
Pyrenophora tritici-repentis (anamorph Drechslera
tritici-repentis) occurs on a range of grasses,
including, as the name suggests, Agropyron
(formerly Triticum) repens and wheat (Triticum
aestivum). The disease caused is known as yellow
leaf spot or tan spot of wheat (De Wolf et al.,
1998). Pyrenophora tritici-repentis is spread as seedborne
infections but also overwinters on infected
stubble, which is the most important source of
inoculum, giving rise to pseudothecia and
conidia in spring. Pseudothecia can be identified
by their large size and by the presence of dark
setae around the pseudothecial neck (Fig. 17.16a).
The ascospores are transversely septate, with a
longitudinal septum also present in one of the
central cells. The conidia of Drechslera triticirepentis
are very large and have a variable
number of distosepta (Fig. 17.16b). They are
sometimes produced on the pseudothecial
setae, or they arise directly from stubble or
from necrotic leaf lesions. Phytotoxins are
involved in causing disease symptoms. Most
unusually, they consist of at least two extracellular
proteins synthesized by ribosomes
(Wolpert et al., 2002). They are a critical factor
in determining the host specificity of infections.
Pyrenophora teres is common wherever barley
is grown, and is the major barley pathogen,
especially in humid regions. This species exists in
two forms which are distinguished by their
symptoms, P. teres f. teres causing net blotch on
barley leaves and P. teres f. maculata causing
brown leaf spots. These two forms can hybridize
in the laboratory and also in the field (Campbell
& Crous, 2003). In addition to ascospores and
Drechslera-type macroconidia similar to those of
P. tritici-repentis, a pycnidial state producing
unicellular conidia is also apparently associated
with P. teres, although its role in the disease
cycle is uncertain (Smith et al., 1988). The
epidemiology of the disease is similar to P.
tritici-repentis, as is the involvement of phytotoxins
in causing leaf necrosis. Leaf chlorosis and
necrosis can be reproduced by phytotoxins
purified from cultures of P. teres. However,
biochemically the phytotoxins involved are
rather different, the most potent of them being
the aspartic acid derivative aspergillomarasmine
A (Weiergang et al., 2002).
17.2.9 Venturia
The genus Venturia contains some 50 species
which cause scabs, i.e. limited lesions with a
scurfy appearance, on the leaves and fruits of
various trees. The genus is an unusual member
of the Pleosporales in producing asci with oneseptate
ascospores, but Silva-Hanlin and Hanlin
(1999) have confirmed its position within this
order. The most important species is V. inaequalis
which parasitizes apple (Malus spp.) and hosts
related to it. This fungus is cosmopolitan and
extremely common on apple fruits and leaves if
fungicide treatments are not carried out
(Fig. 17.17a). In many regions, scab is the most
serious apple disease. The fungus overwinters on
fallen leaves which, in spring, give rise to
pseudothecia (Fig. 17.17b) releasing ascospores
during periods of wetness. The ascospores
require surface wetness in order to infect apple
leaves. Infection is mediated by appressoria, but
the invasion is limited to the space between the
cuticle and the epidermis; the latter is not
pierced, and haustoria are not formed. In this
way, the fungus persists for several weeks.
Conidia are eventually produced from such
subcuticular stromata, and they spread the
disease during the growing season. Invasion of
host tissue takes place only on dead leaves in the
autumn when V. inaequalis switches to a saprotrophic
growth phase and produces pseudothecial
initials. The biology of Venturia, which
has been summarized by MacHardy et al. (2001),
is thus very unusual for members of the
Pleosporales.
From the above summary of the infection
biology of Venturia it is apparent that the key to
apple scab management lies in controlling the
ascospore inoculum in spring. One commonly