Introduction to Fungi, Third Edition

PLEOSPORALES
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Fig17.3 Leptosphaeria acuta.(a)L.S.immature
pseudothecium. Note the young asci (stippled)
elongating between a pre-formed mass of
pseudoparaphyses, and the thin-walled cells
which block the ostiole at this stage but later
dissolve.The centrum subsequently enlarges,
dissolving the pseudoparenchyma surrounding it.
(b) L.S. pycnidium. (c) High-power drawing of cells
lining the pycnidium, showing the origin of the
conidia. (d) Cluster of developing asci from a young
pseudothecium. Note the branching of the
pseudoparaphyses. (e) Stretched bitunicate ascus
showing rupture of the outer wall at its apex.
in the spring and autumn. Good reviews of
S. avenae and S. nodorum have been written by
Eyal (1999) and Cunfer (2000).
An important disease caused by Leptosphaeria
maculans (anamorph Phoma lingam) is blackleg
disease of winter oilseed rape (Brassica napus) and
other brassicas. As the name suggests, the disease
symptoms are seen mainly at the stem bases and
main roots, although the foliage can also be
affected. Infection may occur through leaves,
followed by an endophytic phase lasting several
months, before the blackleg symptoms manifest
themselves. Lesions eventually turn pale and
necrotic, and pycnidia are produced within
them. Where the cortex of infected stems
cracks open, cankers develop. Rain-splashed
pycnidiospores spread the disease during the
growing season. The fungus overwinters on
stubble and infects the new crop by ascospores
released from pseudothecia, although it can also
be seed-borne. The biology of this fungus has been
reviewed by Rouxel and Balesdent (2005).
Leptosphaeria maculans comprises several
morphologically similar species which can be
assigned to two groups, the highly destructive
A group which forms cankers and a less damaging
B group which is confined to infections of
leaves and the pith (Williams & Fitt, 1999).
The B pathotype has recently been named
L. biglobosa (Shoemaker & Brun, 2001). Some of
the disease symptoms are probably caused by
toxins produced by the A pathotype in planta