Agriculture Reference
In-Depth Information
(a)
(b)
(c)
Figure 19.10a
(a) Cucumber powdery mildew on marrow leaves; (b) gooseberry powdery mildew infects the
fruit; (c) rose powdery mildew
ingredient such as myclobutanil that reaches the
internal leaf tissues and kills the fungus. Several
sprays may be needed in each summer season.
Grey mould (
Botrytis cinerea
)
This
is a fungus classified in the Deuteromycota group
of fungi.
Damage
. Grey mould is most commonly recognized
by the fluffy, light grey fungal mass which follows its
infection. In lettuce, the whole plant rots off at the
base. The plant goes yellow and dies. In tomatoes,
infection in damaged side shoots and light yellow
spots (ghost spots) on the unripe and ripe fruit are
found. Infected strawberry fruit may be covered by
the fungus (Figure 19.12). In many flower crops (e.g.
chrysanthemums), infected petals show purple spots
which, in very damp conditions, lead to a mummified
flower head. This disease may affect many crops.
Life cycle and spread
. Grey mould normally requires
wounded tissue for infection, which explains its
importance in crops which are de-leafed, such as
tomatoes, or disbudded such as chrysanthemums.
Damp conditions are essential for its infection and
spore production. The millions of spores are spread
by wind to the next wounded surface. Black sclerotia
(see p. 252), about 2 mm across, produced in badly
infected plants, often act as the overwintering stage
of the disease after falling to the ground, and are
particularly infective in unsterilized soils on young
seedlings and delicate plants such as lettuce.
Control
. Preventive control may involve partial soil
sterilization. Strict attention to greenhouse humidity
control (particularly overnight) such as correct
ventilation reduces the dew formation that is so
important in the fungus infection. Cutting out of
infected tissue is possible in sturdy stems such as
19
Figure 19.11
Rose black spot. Note the leaf-yellowing
symptom that accompanies the black spot
Life cycle and spread
. Asexual spores (produced within
spore cases (see p. 253) embedded in the leaves) are
released in wet and mainly warm weather conditions,
and are then spread a few metres by rain drops or
irrigation water before beginning the cycle of infection
again. No overwintering sexual stage is seen in Britain
and Ireland, and it is probable that asexual spores
surviving in autumn-produced wood or in fallen leaves
begin the infection process the following spring.
Control
. Removal of fallen leaves is a very important
aspect of control. Resistance is not common in rose
cultivars. Gardeners can use a systemic fungicide