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Streptocarpus: Diseases and Pests  

Streptocarpus 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.

As new garden chemicals are introduced, so others are withdrawn.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.

Good 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.

Always 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.

DISEASES

The best control of fungal diseases is preventative: get the growing conditions right.

  • avoid excessive sunlight which can damage the surface of the leaf, allowing the infection to start
  • always ensure there is adequate ventilation
  • remember plants do not like to be kept cold and damp
  • never overwater
  • If plants 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.
  • if just 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
  • remove any old and decaying material completely. Good hygiene eliminates the source of spores that would infect other plants

Botrytis (Grey mould)

Botrytis 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 in winter,

Powdery mildew

Powdery 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.

 

 

PESTS

Aphids (greenfly)

Like 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.

The 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 again.

Aphids 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 of honeydew.

Either use biological control (see below) or use a spray with a propietary insecticide whether as an aerosol or mixed with water.

Mealybug

These 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 weevils

The 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.

Sciarid flies

These 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.

Whiteflies

These 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.

 

Tarsonemid mites (cyclamen mites)

These 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.

Thrips

Thrips 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.

 

 

 

Slugs

If 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 and destroyed.

Bumble Bees

If 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.

 

BIOLOGICAL CONTROL

We largely 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

Text © Rex and Gareth Dibley.

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