Introduction to Fungi, Third Edition

PLEOSPORALES
479
Fig17.17 Venturia inaequalis. (a) Apples showing scab symptoms.
(b) Section through a pseudothecium in an overwintered
apple leaf.
practised approach is disease forecasting,
based on the knowledge that ascospore
discharge occurs within 1 2 h of wetting ripe
pseudothecia, and that infection requires leaf
surface wetness for some 25 h at 6°C or9hat
16 24°C (Smith et al., 1988). Under certain
circumstances, e.g. after a prolonged dry
period, some time will elapse before the pseudothecia
have produced a fresh crop of ascospores
after the onset of rain, and this can be
integrated into forecasting systems (Stensvand
et al., 2005). Protective fungicide sprays have to be
applied as soon as possible after the onset of
conditions conducive to infection, and especially
if a high density of air-borne ascospores has
already been detected by spore traps or other
means (Kollar, 1998). Curative fungicides can be
applied one or a few days after infection.
Numerous fungicides are in use against apple
scab. Protective agents include copper-based
formulations which are registered in some
countries even for organic farming, or the thiol
reactant captan. Important curative fungicides
include strobilurins (respiration inhibitors), and
myclobutanil and imidazoles (demethylation
inhibitors of ergosterol biosynthesis).
A different control strategy is to reduce the
available ascospore inoculum in the spring by
encouraging the decomposition of leaves during
winter. This can be achieved by applying urea to
the leaf litter, or by using a flail mower to shred
the leaves (Sutton et al., 2000). It may also prove
possible to spray leaves before leaf-fall with
spores of fungi antagonistic to Venturia (Carisse
et al., 2000). Yet another approach is the breeding
of resistant cultivars.
17.2.10 Sporormiella
There are about 70 species of Sporormiella (Ahmed
& Cain, 1972). Molecular studies indicate that the
genus has affinities with Pleosporales (Liu et al.,
1999). Most species form pseudothecia on the
dung of herbivores, but some are isolated from
the soil or as endophytes. Characteristic features
are dark, transversely septate ascospores which
may disarticulate into separate part-spores, each
of which is capable of germination, and whose
walls are often marked by a hyaline longitudinal
or oblique germ slit. Sporormiella intermedia is one
of the common species and has thin transparent
pseudothecial walls through which asci can be
seen (Fig. 17.18a). The ascospores of S. intermedia
are four-celled and surrounded by a mucilaginous
envelope. Spore discharge is nocturnal
(Walkey & Harvey, 1966b).
Because of the unmistakable shape of its
ascospores and its association with dung,
Sporormiella has been used in an archaeological