Pests and Diseases of Indoor Plants

It is always sad when a favourite plant suddenly looks sick and stops growing. This can happen even to the most expert of growers, but, if caught in time, the problem need not be too serious. Often, particularly with the beginner, it is a result of incorrect treatment. Normally the appearance of a disease is a sign of poor growth conditions and treatment – like too much water, not enough light, too stuffy a room or too draughty a position. Quick action is necessary. First cut out the affected part of the plant and spray with fungicide. Then consider why and how the plant caught the disease. Damage can also be caused by pests. These are sometimes on the plant when it is bought, or are transmitted from plant to plant. They may come in on the air and be encouraged by a hot, dry atmosphere.

Golden rules to remember

Common garden pests, such as slugs, snails and caterpillars, can invade the house and eat plants. Remove them by hand and discard mutilated leaves. Against such pests as worms and small bugs in the soil, use a well diluted mild disinfectant. Never use infected plants for propagation. Burn or destroy all infected material. Never place in a compost bin.

Some common diseases

Crown and stem rot

The centre of the plant turns soft and rotten and the plant collapses. This fungus spreads quickly and is usually fatal. Throw the plant away. If you catch the plant early, remove the affected part and withhold water temporarily. Move the plant to an airy spot and increase warmth.

Damping off

Young cuttings and seedlings collapse at the base through rotting. Remove plants to an airy, cooler position and water with cheshunt compound. Unsterilized compost can often cause damping off, so check the compost before planting.

Grey mould (Botrytis)

A prevalent disease, particularly with soft-leaved plants. It is a grey fluffy mould that covers leaves, stems and buds. Usually caused by too much humidity, bad ventilation and too cool a temperature. Remove all affected parts and move to an airy position. Spray with fungicide.

Leaf spot

Moist brown spots appear on the foliage of larger-leaved plants like dracaenas, cordylines, dieffenbachia and some philodendrons. It can be caused by bacteria or fungi. Sometimes the spots may become larger, to cover and kill the leaf. Remove the infected leaves and burn. Spray the plant with fungicide. Keep the plant dry and do not mist it for a week or two.


A white powdery coating which appears under similar conditions to botrytis. Cut away affected parts and soft leaves and wipe off hard leaves. Spray with fungicide or dust with sulphur. Move to an airy place.


Rust spots form on the leaves. This disease commonly affects pelargoniums. Remove affected leaves and spray with fungicide.

Sooty mould

This often appears in conjunction with aphids, mealy bug and scale. It does not really harm the plant, but it is very unsightly. It can also reduce growth by blocking the leaf spores. Wipe off with a swab dipped in a weak solution of mild antiseptic and water.


This can affect many indoor plants. The leaves become blotchy and mottled with yellow and growth is stunted. It can be brought on by insects in the soil or can be in the plant when bought. There is no known cure and the plant must be destroyed.

Some common pests

Aphids (greenfly)

This is an extremely common, small sap-sucking insect. It is normally green but can also be grey, black or orange. It prefers plants with soft tissue material and goes for young shoots and flower buds.

Found in clusters on the stem and underside of leaves. If left untreated, the plant becomes weak and a sticky deposit is found. To treat, spray regularly every 14 days with an insecticide based on malathion or derris.

Mealy bug

One of the commonest of pests among hardwooded plants, they are easily controlled if caught early, but difficult to get rid off if they have been allowed to establish themselves. These small insects cover themselves with a cottonwool-like web and nestle in leaf nodes and joints. Spraying with malathion will clear minor attacks, but the best method is to paint out each bug with methylated spirits applied with a small brush.

Red spider mite

Another very common pest, particularly with the thicker-leaved plants like ficus, these insects breed very quickly in a hot, dry atmosphere. Regular misting helps to keep this nasty pest at bay. The mites are so tiny as to be invisible to the naked eye, and can be found underneath the leaves where they suck out the sap. The leaves develop yellow blotches and eventually drop off. Webs are also found between the stem and leaves. To treat, spray every seven to ten days with malathion, or derris.


These are small, round, brown insects which look like discs and attach themselves to the underside of leaves, often on the veins. They are immobile and have an outer shell-like casing. They can be gently knocked off a leaf or wiped with a swab dipped in methylated spirits. After cleaning, spray the plant with a malathion insecticide. Sometimes, if a leaf is badly infected, it is best to remove and burn it. If left, this pest will kill the plant.


These are tiny, black insects which suck buds, flowers and leaves leaving a silvery streak. They are often found on pelargoniums, fuschias and begonias, and particularly on the flowers which become disfigured.

To remove thrips, shake the plant over some newspaper. When the thrips fall out, they can be burnt. Spray the plant with malathion or derris insecticide. It is wise to remove affected flowers and leaves.

White fly

White fly are small, white moth-like insects that attack the underside of leaves. They are unsightly, but it is the greenish larvae which do the damage. They suck the sap and deposit a sticky honeydew. Badly infested leaves turn yellow and drop off. The flies spread rapidly from plant to plant, so take care, for they are difficult to get rid of. Spray every three days with a nicotine, malathion or derris insecticide.

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