Fig. 14.24Powdery mildew of cereals, caused by Blumeria graminis(Ascomycota). (a) Wheat leaves with typical mildew
symptoms. (b) Spore chains which develop from a bulbous basal cell and mature progressively from the base to the
top. (c) Haustorium with multiple finger-like projections within a host cell, typical of many powdery mildew fungi. Note
the host nucleus in the cell, indicating that the cell is alive despite the presence of a haustorium. (d) Two cleistothecia
(closed, sexual fruiting bodies containing asci) ornamented with dark projections. The cleistothecia commonly develop
on mildewed leaves towards the end of the growing season. They break down to release the ascospores, which serve
for dormant survival.
(a)(c)(b)(d)damage to a range of cereal crops. A single “stategically”
positioned lesion at the base of a cereal leaf can effec-
tively prevent that leaf from being a net exporter of
nutrients. This can affect root growth as well as shoot
growth, because roots are relatively poor competitors
for plant assimilates, and so the effects of this fungus
are exacerbated in dry soil conditions. Yet, interestingly,
the spores of powdery mildew fungi cannot infect
in wet leaf conditions, only in drier conditions. Some
other features of these fungi are shown in the legend
to Fig. 14.24.
Downy mildews (Oomycota)The downy mildew pathogens are haustorial biotrophs
with similar behavior to the rust and powdery mildew
fungi although they require more humid conditions for
infection and sporulation. Examples include Bremia
lactucae on lettuce, Plasmopara viticolaon grape vine,
and Pseudoperonospora humulion hops. However, there
is a gradation of behavior in the Oomycota, because
some Phytophthoraspp. which are closely related to
the downy mildews begin their parasitic phase as