Plant Diseases And Insect Pests


To guard against plant pests and diseases, do not use unsterilized garden soil indoors. Microbes which cause root and stem rot, together with insects which attack roots, are to be found in it. Soil can be cleaned by hanging in a cotton bag over boiling water in a closed container. It is much simpler, however, to buy sterilized compost from your nurseryman. ‘Baby Bio’ Potting Compost for example is reliable, widely available and not very expensive, or there are many other proprietary brands.

As well as insect pests, there are many plant diseases, but fortunately comparatively few of them affect those growing indoors. The two which you should guard against are:

1. Rot

This is usually associated with over-watering, especially in winter. Root-rot will result in the loss of the plant.

To control it

Always use sterilized soil, never unsterilized soil. Keep plant warm and don’t over-water, especially in winter. Spray with ‘Benlate’ or similar proprietary fungicide.

2. Mildew (Botrytis)

White or grey mould on stems and leaves of plants. More frequent in Summer, and often the result of a stagnant humid atmosphere in which the spores of the disease thrive.

To control it

Improve growing conditions and ensure good ventilation. Spray with ‘Benlate’ or other proprietary fungicide.

Preventative action

When it is necessary to remove a leaf, stem or flower which will leave a moist open wound, spray the cut gently with flowers of sulphur (obtainable from any chemist). This is an old, proven remedy which will inhibit fungal disease from becoming established in the wound.


Blackfly, Greenflygreenfly

Breed in numerous groups on new shoots and under leaves, which become distorted and the plant is weakened. Some leaves may fall and the affected parts of the plant are covered with sticky honey dew exuded by the insects. The presence of ants always invites closer investigation. They farm aphids in much the same way as a dairy farmer does cows, in order to harvest the honey dew, and will transfer the aphids to other plants to breed new ‘herds’.

Red Spider Mites

Minute pests which attack the undersides of leaves, on which a white webbing is sometimes visible. The upper surface of the leaves becomes dry and brittle, and the plant is weakened. These insects multiply rapidly and often remain unseen until large groups are present. A magnifying glass aids early discovery; try a straight clear glass filled with water. Through it you should see the culprit even though its image will be distorted. They like hot dry conditions and are very persistent.

To control

Spray with liquid derris and repeat at an interval of two days.

White Flieswhite-flies

Tiny moth-like flies which suck sap. Foliage becomes mottled and growth is weakened. Infested areas become covered in sticky honey dew.

To control

Spray with ‘Rio Sprayday’ or other proprietary brand of greenfly killer. Repeat after three days.

Scale Insects

Small immobile waxy shells found clinging to stems and leaves of shrubby indoor plants. Growth is weakened and sticky honey dew is exuded over the plant.

To control

Use a matchstick tipped with cotton wool soaked in methylated spirit to rub the pests from the plant.


Tiny, black, winged insects which attack leaves and flowers, leaving white dots and streaks. Growth of plant is stunted. This is a fairly uncommon pest.

To control

Spray with Malathion or Topgard dust.

Mealy Bugs

Small pests with a white, cottony covering; found on the undersides of leaves and on the joints of stems during summer.

To control

Use a matchstick tipped with cotton wool soaked in methylated spirits to dab pests off plants.

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