Introduction to Fungi, Third Edition

464 LOCULOASCOMYCETES
Fig17.4 Stagonospora nodorum. (a) Pycnidia seen from above. (b) Pycnidia in section in agar culture. (c) Portion of wall of pycnidium
showing origin of conidia. (d) Conidia. (a,b) to same scale; (c,d) to same scale.
(Pedras & Biesenthal, 1998; Howlett et al., 2001).
In addition, L. maculans is able to detoxify phytoalexins
produced by the host plant (Pedras et al.,
2000). Since P. lingam is not closely related to
most other Phoma spp., it has been re-named
Plenodomus (Reddy et al., 1998).
Diverse anamorphic states are known in
other species of Leptosphaeria and Phaeosphaeria,
and some examples of important pathogens are
summarized in Table 17.1.
17.2.3 Ascochyta and Phoma
Several related Ascochyta-type anamorphs infect
legumes, causing diseases such as blight of
chickpea (A. rabiei), foot rot and blight of peas
(A. pinodes), and leaf- and pod-spot of broad beans
(A. fabae) and peas (A. pisi). The pseudothecial
state, where present, is now called Didymella
(formerly Mycosphaerella). These species usually
overwinter on crop residues, infecting new crops
in spring as ascospores, but they are also seedborne.
During the growing season, the infection
is spread by rainsplash of pycnidiospores. An
excellent review of the biology of Ascochyta,
highlighting the severity of diseases which can
be caused, has been written by Pande et al. (2005)
for A. rabiei. Pycnidia contain hyaline, two-celled
conidia which, according to Brewer and Boerema
(1965), arise by septation from the conidiogenous
cell. A few conidia with no, two or three
septa may be produced occasionally (Fig. 17.5).
Extensive work has been carried out on characterizing
the mating type idiomorphs of
Ascochyta spp. These data and analyses of other
sequences have shown that Ascochyta spp. belong
to the Pleosporales and are not closely related
to Mycosphaerella, which is in the Dothideales
(Barve et al., 2003).
Phoma medicaginis (Fig. 17.6) has also been
shown to be related to Ascochyta spp. (Fatehi et al.,
2003). Unfortunately, little information is available
to circumscribe the very large genus Phoma
which is almost certainly polyphyletic, being
associated with several different teleomorphs.
However, a valuable and much-needed monograph
of this difficult group has been written by
Boerema et al. (2004). The conidial fructification
of Phoma is a pycnidium which usually has only
one ostiole, rarely two (Figs. 17.6a and 17.7b).
Like most pycnidia, its outermost wall layer is
pigmented and consists of cells of distinct shape,
which is a useful feature in species identification.
In the case of Phoma spp., it is called a
textura angularis (Fig. 17.6a). The cavity of the
pycnidium is lined by a hymenium of conidiogenous
cells from which one-celled hyaline
pycnidiospores develop. The absence of filiform