Introduction to Fungi, Third Edition

400 HYMENOASCOMYCETES: ERYSIPHALES
dispersal (Hammett & Manners, 1971, 1974). This
ties in with the duration of conidium production
(about 3 h for each conidium) and the fact that
usually only the terminal, most mature spore
becomes detached from the conidial chain. Thus,
the observed diurnal rhythm of spore abundance
in the air, with a peak usually in early
afternoon, can probably be explained by factors
affecting release, rather than formation, of
spores (Hammett & Manners, 1971, 1974). Both
the formation and release of spores are strongly
inhibited by rain (Hirst, 1953).
The conidia of B. graminis can travel considerable
distances with the prevailing wind. For
instance, Hermansen et al. (1978) have demonstrated
the migration of spores from Northeastern
England and Scotland to Denmark, a
journey which would have taken approximately
48 h. In an extensive survey undertaken across the
whole of Europe, Limpert et al. (1999) found that
B. graminis is a nomadic species, with the
prevailing westerly winds driving waves of populations
eastwards at a rate of about 100 km
year 1 . Since such populations encounter hosts
with different spectra of resistance genes, they
face a selective pressure to adapt, and this may
explain why the complexity of virulence alleles in
B. graminis populations was found to increase
from west to east by about one virulence factor
every 1000 km. Ultra-long distance (intercontinental)
dispersal of viable conidia, as found e.g.
for urediniospores of rust fungi (see p. 632) or
teliospores of smut fungi (p. 639), has not been
reported for powdery mildews.
Whereas conidium formation and release by
B. graminis are favoured by dry windy conditions,
infection proceeds best at 98 100% relative
humidity but is inhibited by free water. The optimum
infection temperature is about 15 20°C.
Thus, the conditions in Northwestern Europe are
ideal for B. graminis in terms of rapidly changing
weather conditions (Smith et al., 1988). Crop
losses of up to 40% have been reported, with
barley the most seriously affected cereal. The
timing of infection is important, early infections
reducing the number of ears and later infections
reducing the size of the grain. Crop damage is
due to a decrease in photosynthesis in infected
leaves, meaning that they are unable to
Fig13.6 Blumeria graminis. (a) Conidial pustules on wheat.
(b) Dark spherical chasmothecia nestling in a felt of surface
hyphae on a wheat leaf sheath.
export carbohydrates to the developing grains.
Heavily infected leaves may even act as a sink
for photosynthetic product (sucrose) from other
leaves.
The chasmothecia of B. graminis are dark
brown and globose, and nestle in a dense mass
of mycelium formed on the basal leaves and
leaf-sheaths of cereals (Fig. 13.6b). In contrast to
the chasmothecia of most Erysiphales, those of
B. graminis do not bear any conspicuous thickwalled
appendages (Fig. 13.7a). Each chasmothecium
has a wall made up of several layers of cells,
surrounding a number of asci. At maturity,
it cracks open by swelling of the contents, and
the asci discharge their spores. In Europe,
chasmothecia are formed in late summer and
it is unclear what their exact role in the disease
cycle is. Ascospores of B. graminis formed in the
current season are capable of infection, and
dried chasmothecia can survive under herbarium
conditions for up to 13 years (Moseman &
Powers, 1957). However, it is generally assumed