Agriculture Reference
In-Depth Information
Figure 19.7
Cabbage downy mildew symptoms on a
stock leaf. Note that the infection is on the lower leaf
surface
should be avoided, as these fungi increase most
rapidly under these conditions. Copper formulation
(known as 'Cheshunt mixture') may be used as a
drench to slow down the increase of damping off. No
systemic fungicide is currently available to gardeners
for the control of 'damping-off' disease.
Figure 19.6
Damping off on seedlings. Note the
shrivelled appearance of the leaves on infected plants
Downy mildew of cabbage and related
plants (
Perenospora brassicae
)
get older. Although the mature plant is not seriously
affected, it is a common experience that on removal
of the plant and replacement with a young rose plant,
there is a quite rapid decline in its vigour, called 'rose
sickness'.
Life cycle and spread
. Both
Pythium
and
Phytophthora
occur naturally in soils as saprophytes, but under
damp conditions they produce the asexual spores that
cause infection. These spores are spread by water.
Sexual spores (oospores) are produced in infected
roots (mostly in autumn) and may survive several
months of dry or cold soil conditions.
Control
. Prevention control is best achieved against
these diseases by providing a disease-free growing
medium. This may be produced by using fresh
compost, or by partial sterilization of soil with heat.
Seed producers often coat the crop seed with a
protective seed dressing such as thiram to prevent
early infection.
Water tanks with open tops, harbouring rotting leaves,
are a common source of infected water and should
be cleaned out regularly. Sand and capillary matting
on benches in greenhouses should be regularly
washed in hot water. The use of door mats soaked in
a sterilant may prevent foot spread of the organisms
from one greenhouse to another. Waterlogged soils
This fungus is a member of the Zygomycota group
of fungi. Downy mildews are not closely related to
powdery mildews.
Damage
. This serious disease causes a white bloom
mainly on the under-surface of leaves (Figure 19.7)
where the humid microclimate favours infection
and spore production. Cruciferous plants such as
stocks and wallflowers, cabbage and other brassicas,
and occasionally weeds such as shepherd's purse
are attacked by this 'fungus'. The disease is most
damaging at a time when seedlings are germinating,
particularly in spring when the young tissues of the
host plant are susceptible and favourable damp
conditions may combine to kill off a large proportion of
the developing plants.
Life cycle and spread
. Asexual spores (zoospores) are
produced by hyphae present on the lower leaf surface,
mainly in spring and summer, and are spread by
wind currents. Thick-walled sexual spores (oospores)
produced within the leaf tissues fall to the ground with
the death of the leaf and survive the winter. Spring
infections occur when rain splash carries the spores
up from the soil to the lower leaf surface of seedlings
and young plants.
