Introduction to Fungi, Third Edition

462 LOCULOASCOMYCETES
Fig17.2 Leptosphaeriaacuta on overwinterednettle stem.
(a) Pseudothecia. (b) Phoma anamorph.Bothimages to same
scale.
named Phoma acuta, and culture studies have
confirmed that this stage is the conidial state of
L. acuta (Müller & Tomasevic, 1957). Pseudothecia
of L. acuta ripen in the spring and discharge
ascospores as the new season’s nettle shoots are
elongating. There are no obvious disease symptoms
on infected plants during the summer and
the fungus persists as a symptomless endophyte,
with the pseudothecia and pycnidia developing
during the winter on dead stems of the past
season’s growth.
17.2.2 Leptosphaeria and Phaeosphaeria
An unusually high degree of taxonomic confusion
has arisen in attempts to delimit and define
several important plant-pathogenic species
which clearly belong to the Pleosporales but
show immense variations in their anamorph
and teleomorph features. One example is a group
of coelomycetous species comprising Septoria
with multiseptate conidia more than 10 times
longer than wide, Stagonospora with multiseptate
conidia less than 10 times longer than wide
(Fig. 17.4), and Phoma with aseptate globose
or slightly elongated conidia (Figs. 17.6 and
17.7). The most common sexual states associated
with Septoria and Stagonospora are Leptosphaeria
and Phaeosphaeria (Leuchtmann, 1984). As with
the anamorphs, the morphological features of
these teleomorphs show transitions, but a broad
distinction can be made by Leptosphaeria being
associated mainly with dicotyledons whereas
Phaeosphaeria is pathogenic on monocotyledons
(Cunfer & Ueng, 1999). These two genera have
also been separated by critical DNA sequence
analyses (Câmara et al., 2002).
Among the cereal pathogens belonging here,
Phaeosphaeria (Stagonospora) nodorum has two
formae speciales, one on wheat and the other on
barley, whereas Phaeosphaeria avenaria
(Stagonospora avenae) is a more diverse species
complex (Ueng et al., 1998) but is not as serious a
pathogen in most agricultural situations. These
fungi cause leaf blotch diseases which are very
common in most cereal-growing regions and
often become the major cereal leaf disease in wet
and cool conditions. Epidemics are slowed
during periods of dry weather. Serious infections
can cause heavy crop losses if they start early and
affect the uppermost leaves, which contribute
most photosynthetic product to the developing
grains. Stagonospora nodorum also infects the
heads of wheat, causing glume blotch. Lesions
develop as small brown spots which enlarge into
irregular brownish necrotic areas, giving the leaf
a speckled appearance. Within the necrotic
areas, pycnidia develop beneath the epidermis
which they eventually pierce, releasing a tendril
of cylindrical, three-septate pycnidiospores
(Fig. 17.4d). These can spread the disease to
neighbouring plants by rainsplash. In S. nodorum,
a second pycnidial state containing minute
unicellular conidia has also been discovered
(Harrower, 1976). These infect host leaves via
germ tubes which penetrate the stomata,
whereas penetration from Stagonospora-type conidia
is directly through the cuticle (Karjalainen &
Lounatmaa, 1986). There is no evidence that
either type of conidium plays a sexual (spermatial)
role. Low temperatures (5 10°C) and irradiation
with ultraviolet light favour conidium
production. Both S. avenae and S. nodorum survive
the winter either on living volunteer plants or as
pycnidia and pseudothecia on stubble. Epidemics
can also be started from infected seeds, and from
ascospores which are released from pseudothecia