Agriculture Reference
In-Depth Information
when the disease is seen early in summer and
avoid shaking this plant material that may release
spores. Water in the morning and apply to the soil
and not the leaves. Plants should not be allowed
to dry out, as the weakened condition of the plants
favours infection. Avoid over-fertilizing the plants
with nitrogen fertilizers as this leads to increased
susceptibility.
Resistance . At the present time, some mid-season
cultivars such as 'Pegasus' and some late cultivars
such as 'Florence' show a good resistance to this
disease.
Chemical controls . Potassium bicarbonate, used as
a spray by organic growers, helps provide resistance
to the disease in the leaf cells. Sulphur may be
applied as a fine dust. It may also be applied in a
spray formulation, containing a fatty-acid ingredient
that improves retention on the leaf. Myclobutanil is
a systemic fungicide that may effectively reduce the
disease.
Figure 19.9 Strawberry powdery mildew on leaves
(source: James Lindsey's Ecology of Commanster)
growth and reduced fruit production. The mildew
can infect strawberry flowers, causing a deep pink
colouration. Affected fruit may appear distorted,
and often look rather dull in colour with the seeds
protruding out from the fruit surface. Infection of
young fruit may cause small shrivelled strawberries.
Life cycle and spread . The fungus produces chains of
asexual spores (conidia) from infected leaf surfaces.
These spores are carried by wind to uninfected leaves.
Above about 15°C, these spores are able to infect leaf
tissue (mainly the upper surface) without there being
a surface layer of water, although humid conditions
and dampness do favour the disease. It can thus
be seen that powdery mildews are well adapted for
infection in summer conditions. Strawberry powdery
mildew becomes more serious towards the end of
summer as night humidity increases. It is also more
serious in plastic-tunnel-grown strawberries, where
temperatures and humidity are high.
Overwintering of the fungus is seen in two ways.
During autumn, tiny (1 mm) black spherical spore
cases (containing many spores) develop on the
infected leaves and survive on dead leaf material. In
mild winters, the fungus may survive within the green
shoots of the dormant plant.
Spread is by means of summer spores. New planting
material may introduce the disease. This species of
powdery mildew ( P. aphanis ) is not harboured by any
other garden species other than hops.
Cultural controls . Weak-looking plants may be
removed in autumn as they will be the most likely
ones to harbour the disease over winter. Organic
matter added to soil helps maintain nutrient balance
and soil moisture, and thus help maintain plant
resistance to the disease.
Avoid overcrowding of plants that speeds up the
disease spread. Remove any infected shoots
Other powdery mildews
Apple mildew ( Podosphaera leucotricha ) survives
the winter as mycelium within the buds, which often
appear small and shrivelled on twigs that have a dried,
silvery appearance. Fruit may develop a russeted
surface. This fungus may affect other related species
such as pears, quinces, medlars and ornamental
Malus . Pruning of silvered twigs, and sprays with
sulphur or myclobutanil may be used when mildew
begins to spread through the tree.
Rose mildew ( Sphaerotheca pannosa , Figure 19.10c),
gooseberry mildew ( S. mors-uvae ) occurring mainly
on the gooseberry fruit (see Figure 19.10b), and
cucumber mildew ( S. fuliginea ) on cucumber and
marrow leaves (Figure 19.10a) are three other
important species.
It is important that gardeners realize that although hot
dry summers may simultaneously lead to outbreaks
of powdery mildew species in all the plant species
mentioned above, cross-infection of the disease
does not occur between strawberries, apple, rose,
gooseberry and cucumber crops.
Rose black spot ( Diplocarpon rosae )
This belongs to the Ascomycota group of fungi.
Damage . This common disease in garden and
greenhouse roses is first seen as dark brown leaf
spots which may be followed by general leaf yellowing
and then leaf-drop (Figure 19.11). The infection of
young shoots has a slow weakening effect on the
whole plant.
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