Introduction to Fungi, Third Edition

PLEOSPORALES
461
Pseudoparaphyses arise near the upper end of
the cavity and grow downwards. Their tips soon
intertwine and push between the other cells of
the stroma so that free ends are seldom found.
They may thus be distinguished from the true
paraphyses of other fungi (e.g. Sordaria) which are
formed from hyphae attached at the base of the
cavity, extend upwards and are free at their
upper ends. They may also be distinguished from
apical paraphyses which are attached above,
arising from a clearly defined meristem near
the apex of a perithecium, and form a welldefined
palisade of hyphae free at their lower
ends (see the Nectria type of development, p. 337).
In the Pleospora type of development, asci arise
amongst the pseudoparaphyses at the base of the
cavity and grow up between them. The ostiole
develops lysigenously, i.e. by breakdown of preexisting
cells. Development of this general type
has been described in P. herbarum (Wehmeyer,
1955; Corlett, 1973), Leptosphaeria (Dodge, 1937),
Sporormiella (Arnold, 1928) and other fungi (see
Luttrell, 1951, 1973). A more recent discussion
of pseudoparaphyses development in the Pleosporales
and its taxonomic implications has been
written by Liew et al. (2000).
17.2.1 Leptosphaeria
Leptosphaeria species fruit on moribund leaves
and stems of herbaceous plants. There are
probably some 100 species, many growing on a
wide range of hosts, but others are confined to
one host plant. Although most are saprotrophic
or only weakly pathogenic, some are troublesome
pathogens, e.g. L. coniothyrium, the cause of
cane blight of raspberry, and L. maculans which
causes blackleg of oilseed rape and other
brassicas. Characteristic features are the fusoid,
yellow or pale brown ascospores with two or
more transverse septa. Anamorphic states are
pycnidial (see Table 17.1).
Leptosphaeria acuta fruits in abundance in
spring at the base of overwintered, decorticated
stems of stinging nettles (Urtica dioica). The black
shining pseudothecia are somewhat conical and
flattened at the base (Fig. 17.2a). Bitunicate asci
elongate within a pre-existing group of branching
pseudoparaphyses, and close examination of
the direction of growth and branching indicates
that the pseudoparaphyses may be ascending
and descending. The ostiole of the perithecium
is formed lysigenously by breakdown of a preexisting
mass of thin-walled cells (Fig. 17.3a).
The bitunicate structure of mature asci is
difficult to discern because, as the ascus expands,
the inner wall protrudes through a thin area
in the outer wall at the ascus tip (Fig. 17.3e)
and then the inner wall extends. Thus the
ascus tip in expanded asci is single-walled.
The ascospores have about 11 transverse septa
and are discharged successively at intervals of
about 5 s.
Associated with the thick-walled conical
pseudothecia on the nettle stem are thinnerwalled,
slightly smaller, globose pycnidia with
cylindrical necks (Figs. 17.2b and 17.3b). The
cavity of the pycnidia is lined by small spherical
cells which give rise to numerous rod-shaped
conidia (Fig. 17.3c). These are dispersed by rain
splash or in water films and are capable of
germination, which suggests that they do not
function as spermatia. Such pycnidia have been
Table 17.1. Anamorphic states of some species of Leptosphaeria and Phaeosphaeria.
Teleomorph Anamorph Disease
L. bicolor Stagonospora sp. Sugarcane leaf scorch
L. coniothyrium Coniothyrium fuckelii Raspberry cane blight
L. maculans Phoma lingam Blackleg of oilseed rape and other brassicas
P. avenaria Stagonospora avenae Oatleaf blotch
P. microscopica Phaeoseptoria festucae Leaf spot of fescue and other grasses
P. nodorum Stagonospora nodorum Glume and leaf blotch of wheat and barley