growers of Streptocarpus
and other fine houseplants
Diseases and Pests
are easily grown and usually suffer little from diseases and
pests, but one should be on the look-out and take the necessary
action at the first sign of any trouble.
garden chemicals are introduced, so others are withdrawn. Because
of the frequent changes it is best, once a problem is identified,
to visit a garden centre and find a current control for that
particular disease or pest.
growing conditions for plants and a lack of stress will keep
pest and disease problems to a minimum, and if they do arise,
the spread of the problem will be slower. Good hygiene is of
paramount importance. Compost must be sterile so that problems
are not introduced from this source. Always use a reputable
brand. Sterilise all previously used pots and propagation trays.
remove any plant debris so that fungi do not have a chance to
grow and spread their spores. Weeds must not be allowed to become
established as they may harbour pests and diseases which act
as a reservoir, allowing the infection of your cultivated plants.
best control of fungal diseases is preventative: get the growing
excessive sunlight which can damage the surface of the leaf, allowing
the infection to start
ensure there is adequate ventilation
plants do not like to be kept cold and damp
are left standing in water, the roots cannot function because
of lack of air in the compost; rot may start at the base of the
plant, and the leaves wilt. (Do not confuse this with the plant
wilting because it is too dry.) If the compost is wet and the
plant wilts, let the compost dry out completely before giving
it any more water. The plant may then recover.
one or two leaves wilt, rot may be occurring at their bases; if
they are given a pull they may come clean away, leaving the rest
of the plant healthy
any old and decaying material completely. Good hygiene eliminates
the source of spores that would infect other plants
occurs if leaves are left wet and cold, especially in winter.
Grey fluffy growth may show, the leaf will rot away at the
end or holes may develop in it. Break off any affected parts
of the leaf. Do not allow any dead plant parts, such as the
petals, to lie on the leaf as these will act as a source of
infection. In a greenhouse, improve ventilation if the disease
has appeared, and some additional heating may be required
mildew can occur on the leaves, the flowers or the flower
stalks. The disease shows as whitish spots which gradually
expand to form larger circles. Again, good air movement
will help to prevent this disease. Powdery mildew is not
usually a problem on streptocarpus, but will move on to
them from more susceptible plants, such as saintpaulias.
For some reason, purple-flowered streptocarpus seem more
prone to the disease than other colours.
many pot plants, streptocarpus can be infected with these
pests. Aphids that infest streptocarpus are small soft-bodied
insects either green or orange in colour. They travel
long distances on air currents, so that they seem to appear
on the plants spontaneously. Once they arrive new generations
are produced very rapidly. They are more likely to occur
on plants that are stressed by frequently being allowed
to become over-dry and wilted. The aphids may occur at
the centre of the plant or on the flowers, the buds and
their stalks. Often the most obvious symptom is the mass
of tiny discarded white skins stuck onto the plant. These
must not be confused with whitefly. If aphids occur on
young plants, the leaves may be curled and distorted.
individuals are all females that produce live young at
a rate of three to six a day. These reach maturity within
a week and themselves start producing offspring. Thus
colonies rapidly build up. Some individuals will walk
or fly to adjoining plants and so the process will begin
feed on the plant allowing a lot of the sweet sap that
they imbibe to pass straight through their bodies; it
is called honeydew. This forms a sticky layer on the leaves
below. This sugary layer is food for unsightly black sooty
mould which will grow on the leaves. Once the aphids are
controlled, the sooty mould disappears as it runs out
use biological control (see below) or use a spray with
a propietary insecticide whether as an aerosol or mixed
pests may be easily missed as they are small, white, waxy,
flattened insects that fix themselves alongside the veins
on the underside of the leaves, where they suck the sap.
Large colonies have the appearance of a white woolly mass.
vine weevil is wingless, about 3/8th inch (9mm) long,
black with yellow speckling and a pointed head. It is
common both outdoors and under glass. The adult chews
notches in leaves. It is nocturnal, spending its days
concealed under pots or in plant debris. Its eggs are
laid near the base of a plant. The grubs that emerge are
white with brown heads and are legless, up to 3/10th inch
(8mm) long. They look like plump white maggots and chew
into the base of the plant causing wilting and collapse.
small black flies are about 1/10th inch (2mm) in length.
They run about on the surface of the compost, and when disturbed
fly off. The slender larvae are white with black heads;
they may feed on root hairs and kill young plants and reduce
the vigour of older plants. They live more frequently on
peat and decaying matter and normally do not cause a problem.
are rarely found on streptocarpus. Whiteflies are more likely
to develop and feed on other plants and merely rest on the
streptocarpus. They are small white moth-like insects that
will fly into the air with a characteristic jerky flight
if disturbed. Aphid (greenfly) cast
skins are sometimes mistaken for whitefly.
mites (cyclamen mites)
mites are too small to be seen with the naked eye. The symptoms
are distorted leaves that are often brown and rusty-looking
at the base. The flowers are also distorted, hairy-looking,
with a darker colour than usual and blotchy. There is no
cure for these mites available to the amateur, although
use of an insecticidal soap may control them to some extent.
It is best to isolate any infested plants and, if necessary,
destroy them before the mites spread onto other plants.
Since the mites cannot fly, they can only move slowly from
plant to plant, although they can stick on clothing and
be moved about that way. Many pot plants beside streptocarpus
are susceptible to these pests.
are elongate insects 1/10th inch (2mm) long that will
cause small pale blotches to occur on the flowers, and
pollen to be shed from the anthers. They are very small,
but can be seen as little torpedo-shaped yellowish/brown
creatures if a flower is shaken onto a sheet of paper.
Thrips are becoming very widespread.
the edges of the leaves of flowers are being eaten away
and slime trails are visible, then the problem is caused
by slugs. Normally these are not much of a nuisance except
when conditions are suitable, such as under greenhouse
benching where it is damp and shady. They are encouraged
by rotting vegetation. Look at the affected plant, for
the slug may well be found under a leaf, in the pot or
under the base of the pot. It can be easily be removed
a small hole is found at the base of the flowers, suspect
bumble bees. Since these insects want to get at the nectar
at the base of the flower, they will try to insert their
proboscis down the corolla tube. They then find they cannot
reach the nectar so they resort to puncturing a hole at
the base of the corolla tube. There is little that can
be done to prevent this damage except by somehow excluding
these insects. This is fairly superficial, and of course
is unlikely to occur to plants growing in a house.
use biological control of pests on our nursery and have found
the method very successful. It must be noted however that biological
methods alone may only control pests but not eradicate them. Biological
control can be done on an amateur scale in a glasshouse or conservatory.
For more information have a look at Defenders
Natural Biological Pest Control
Rex and Gareth Dibley.
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