Introduction to Fungi, Third Edition

ERYSIPHE
403
Fig13.9 Summer shoot of oak (Quercus)infectedbyoak
powdery mildew, Microsphaera (now Erysiphe) alphitoides.
Contrast the stunted appearance of the infected foliage with
the healthy basal leaves formed in spring.
common during hot summers or in hot climatic
zones.
Uncinula
Members of the genus Uncinula (now Erysiphe sect.
Uncinula) also produce several asci in each
chasmothecium but are distinguished by their
chasmothecial appendages which are uncinate,
i.e. they have recurved tips. Strikingly similar
appendages are found in the unrelated genus
Sawadaea (see Fig. 13.10b). Undoubtedly the bestknown
species is U. necator, the cause of powdery
mildew of vines. Chasmothecia are quite rare,
and when the fungus first appeared in Europe
in a glasshouse in Kent in 1845 it was known
only in its anamorphic state and given the name
Oidium tuckeri, after Mr Tucker, the gardener
who discovered it. An accidental introduction
from North America, U. necator quickly spread
throughout the major vine-growing regions of
Europe, causing such severe damage especially in
France that the entire wine industry was
threatened. By 1854, the French wine production
had fallen from 54 million to 10 million hectolitres.
Fortunately, Mr Tucker had also discovered
that a mixture of sulphur dust and lime
provided good control of the disease, and this
simple fungicide is still used today (Smith et al.,
1988). An excellent account of the arrival of vine
powdery mildew (and many other fungal
diseases) has been given by Large (1940).
Because of the extremely high value of the
vine crop, extensive epidemiological data have
been compiled and are used for disease forecasting
models (Jarvis et al., 2002). Conidia are,
of course, the major inoculum, and they are
dispersed by wind movements and also by the
air currents generated by high-pressure spraying
equipment (Willocquet & Clerjeau, 1998).
Infections are mainly found on the upper
(adaxial) surface of leaves, in contrast to the
grape downy mildew (Plasmopara viticola; see
p. 120) which is more common on the underside
(abaxial surface). The grapes themselves are
also readily infected by U. necator; if they are
young, they will be aborted altogether whereas
older grapes suffer skin damage, and moulds
such as Botrytis cinerea can easily infect through
these cracks. Uncinula necator overwinters as
mycelium in dormant buds (Rugner et al., 2002),
although chasmothecia formed in autumn can
survive the winter on twigs and give rise to
sizeable quantities of ascospores capable of
causing new infections in spring (Jailloux et al.,
1998, 1999).
13.4.3 Species previously attributed to
Erysiphe
Species of Golovinomyces (formerly Erysiphe cichoracearum
sensu lato) are not closely related to the
Erysiphe as circumscribed above. They have an
anamorph (Oidium subgenus Reticuloidium; Cook
et al., 1997) that has catenate conidia with slightly
roughened conidial surfaces as seen under the
scanning electron microscope (see Fig. 13.13c).
Braun (1987) has placed the forms affecting
Asteraceae (including lettuce) in G. cichoracearum
and most of the plurivorous forms in the morphologically
very similar G. orontii. Important crops