Introduction to Fungi, Third Edition

442 HYMENOASCOMYCETES: HELOTIALES (INOPERCULATE DISCOMYCETES)
ascospores released from apothecia on overwintered
leaves. Lesions become visible to the naked
eye in June or July, some 2 months after infection,
as yellowish spots which eventually turn
black. Sections of the leaf at this stage show
an extensive mycelium filling the cells of the
mesophyll, and especially the cells of the upper
epidermis. Between the epidermal cells, a conidial
state called Melasmia acerina develops.
This consists of flask-shaped cavities (spermogonia)
which give rise to uninucleate curved
club-shaped conidia (spermatia) measuring about
6 1 mm (Figs. 15.10a,b). The spermatia are
exuded from the upper surface of the centre
of the lesion through ostioles in the spermogonial
wall. The spermatia do not germinate, even
on sycamore leaves, and it is believed that they
play a sexual role (Jones, 1925), although this has
not yet been proven. Apothecia begin development
in the portion previously occupied by
spermogonia, and the hymenium is roofed over
by several layers of dark cells formed within the
upper epidermis. The asci complete their development
on the fallen leaves and are ripe about
March to April when sycamore leaves of the
new season unfold. The hymenium is exposed by
means of cracks in the surface layer of the fungal
stroma (Fig. 15.9b) and the asci discharge their
spores, sometimes by puffing. Since the ascospores
are very large, their discharge can be
viewed with a dissecting microscope. Although
the ascospores are only projected to a height of
about 1 mm above the surface of the stroma, they
are carried by air currents to leaves several
metres above the ground. The ascospores are
needle-shaped and have a mucilaginous epispore
which is especially well developed at the upper
end (Fig. 15.10d). This probably helps in attaching
them to leaves. Infection occurs by penetration
of the germ tubes through stomata on the
lower epidermis.
Rhytisma acerinum is absent from densely
populated areas, probably because the germination
of ascospores is inhibited by sulphur
dioxide. Greenhalgh and Bevan (1978) have
suggested that the incidence and frequency of
colonization of sycamore leaves by the tar spot
fungus can be used as an accurate visual index of
air pollution, although other interpretations are
possible, such as the removal of fallen leaves
from municipal parks or the drier microclimate
in city centres (Leith & Fowler, 1988).
15.5 Other representatives of the
Helotiales
Many members of the Helotiales are encountered
during fungus forays because they produce
Fig15.9 Rhytisma acerinum. (a) Leaf of sycamore (Acer pseudoplatanus) with developing tar spot lesions. (b) Tarspot from an
overwinteredleaf showing cracking of the surface to reveal the hymenia of the apothecia.The flat central zones indicate areas where
spermogonia had been formed during the previous summer.