strains of Powdery mildew effect all of the cereal types. They
can be summarised as follows:
(Blumeria) graminis f. sp.tritici - Affects Wheat crops
(Blumeria) graminis f. sp. hordei - Affects Barley crops
(Blumeria) graminis f. sp. avenae - Affects Oat crops
(Blumeria) graminis f. sp. secalis - Affects Rye crops
mildew is probably the commonest disease of cereals, occuring
in all areas that the crops are grown. Disease symptoms can
occur on all aerial plant parts, but are most frequently seen
on leaves. Early stages of the disease consist of chlorotic
flecks on the plant tissue. A white, fluffy mildew pustule soon
develops, which produces masses of powdery spores. Older mildew
pustules appear grey or brown. The fungus infects only the outer
epidermal plant layers, meaning that the pustules can be scraped
mild chlorosis can be seen in affected tissues, especially at
the beginning of natural leaf senescence, but the pathogen does
not usually kill the host. Towards the end of the season, brown-black
sexually produced spore cases (cleistothecia) may be found embedded
in mildew pustules.
the UK, the fungus survives the winter mainly as dormant mycelium
in host tissues such as volunteer plants and over-wintering
crops. Cleistothecia are probably unimportant in this respect
although they may enable the fungus to survive in debris in
the absence of the crop for several weeks.
Infections of autumn sown crops during autumn arise from wind
dispersed conidia, although ascospores produced from cleistothecia
in plant debris may also contribute to inoculum. Conidia germinate
over a temperature range of 5 - 30 deg C, but temperatures of
15 - 20 deg C accompanied by a few hours of high relative humidty
(above 90%) are optimal for germination. Free water tends to
inhibit spore germination and some spores will germinate at
only 80 - 85% relative humidity. Under optimal conditions, the
latent period is 7 days, and as a result, mildew epidemics tend
to occur during warm weather with alternating dry and wet periods
accompanied by breezes to disperse spores. Disease development
is inhibited at temperatures over 25 deg C.
mildew tends to be severe in lush, over fertilised, early sown
winter cereal crops. The disease is the second most common disease
of winter wheat, and the most widespread and severe disease
of spring and winter barley.
losses in the field in susceptible crops have been estimated
at up to 25% in the UK.
control of powdery mildew involves the eradication of volunteer
cereals, which can harbour inoculum over winter, together with
disposal of crop debris which may be infected with cleistithecia.
Avoiding very early sowing and excess nitrogen fertiliser applications
will also help to reduce disease.
resistance is important in all cereals. Much of the resistance,
however, is major gene and as such can break down as a result
of a shift in virulence in pathogen populations.
control of powdery mildew, particularly in barley and wheat
is widely practised. Initially, systemic seed treatments containing
azole fungicides may help to reduce disease for the first few
weeks after the crop emergence. Foliar-applied fungicides can
be used on crops at the start of disease epidemics. Approved
morpholine based fungicides are usually used, such as Torch
(spiroxamine), Corbel (fenpropimorph) and Fortress (quinoxyfen).
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