Symptoms and diagnosis of unhealthy plants

If your plant looks unhealthy find the appropriate symptom below, and diag-nose the cause of illness from the possibilities given. A sticky distorted look to the leaves, white webbing or mould probably means pest or disease has attacked.

Plants growing slowly

If a plant is growing very slowly or not at all during the summer- – most plants of course would not be expected to grow in winter – it is probably either under-nourished or overwatered. If you are watering and feeding it correctly then it could need re-potting.

Sudden dropping of buds, flowers or leaves

Usually this is caused by the plant having had too much water or a bad shock – a sudden drop or rise in temperature, a cold draught, extra light, or a change in its position.


If leaves start wilting it could be that the plant is too hot or that there has been a sudden rise in the temperature. Alternatively it could be waterlogged because of over-watering, or simply dying of thirst. It should be easy enough to isolate which factor is responsible.

Yellowing leaves

It is normal for the occasional lower leaf to turn yellow and then fall off but if several do so a cold draught or over-watering is usually responsible. Alternatively if the leaves of lime-hating plants such as Begonias or Azaleas turn yellow but remain healthy this is usually a protest against the inclusion of lime in the potting compost or a sign that hard water (something else that they hate) has been used to water them.

Variegated leaves turning all one colour

If variegated leaves start to lose their stripes and markings it is only because the plant is not getting enough light. Move it to the window and the leaf colours will return.

Pests and diseases

Attack by pests or disease should be rare if your plants are well-tended and well-fed. But if they are attacked, prompt action can save them.

The common pests Aphids (Greenfly) appear on the underside of leaves or on young shoots during spring and summer. The leaves look distorted and may fall off, and the whole plant has a sticky look. Cure by the sparing use of an insecticide safe for indoor use, or spray with weak soap flake solution once weekly, paying particular attention to the undersides of leaves.

Scale Insects are tiny waxy shells. They cling to the stems and leaves of the shrubby plants and weaken them. Cure by rubbing off the insects with a match-stick tipped with cotton wool which has been dipped in methylated spirit.

Red Spider Mites attack only under hot dry conditions. They damage the underside of leaves and sometimes leave a white webbing behind them. The tops of the leaves look grey and brittle. Cure by using a safe insecticide. Or try spraying the plant with water – red spiders hate water, and it will help the plant to breathe.

Mealy-Bugs are small, white and furry-looking. They appear in summer on the undersides of leaves and the joints of stems. Cure by treating as for scale insects.

Thrips are tiny black flying insects which attack leaves and flowers leaving white dots and streaks in their wake. Cure by the sparing use of an insecticide which is safe for indoor use.


In all cases cut out the diseased area immediately and move the diseased plant away from other healthy ones.

Rot usually attacks the roots and stems of plants and is often fatal. Cure by watering sparingly and keeping the plant in a reasonably warm room.

Mildew appears in summer in the form of white mould on the plant’s stem and leaves. Cure by moving the plant to a well-ventilated spot. Check for possible overwatering as this can cause mildew.

Tiny round holes sometimes appear in a plant’s leaves but apparently do not affect the health of the plant itself. Unfortunately there is no known remedy for this disease and, in fact, little is known about what causes it.

Causes of death

If a plant does die it is helpful to know, if possible, what caused its death so that you can avoid the same thing happening again. Causes of death are varied (and include attacks by the pests and diseases listed above) but the five most common ones are given below.

Drowning. Many plants, particularly in winter, die because they are drowned. Remember a wilted look could be due to overwatering as well as to thirst so before you water a wilted plant check whether the leaves are yellowish – too much water – or brown and shrivelled-looking – drought.

Draughts. While many plants like fresh air most dislike draughts, particularly cold ones. Avoid putting a plant in a direct line between door and window, for when both are open an unpleasant crosscurrent will hit it. Make sure, too, that if you leave a plant on a window-sill throughout the winter there are no cracks in the window frame which will allow in icy draughts.

Lack of Humidity. Central heating, gas fires, and most other forms of artificial heat give a hot dry atmosphere. Plants then die through lack of humidity.

Lack of Light. Most plants, including the shade-lovers such as Ferns, will eventually die if they are kept in deep shade all the time. Plant symptoms of lack of light are no flowers or poor flowers on flowering plants, a weak stem, small pale leaves, and a leggy look to the plant as it strives to find the light.

Humans are perhaps the most dangerous pest of all, insisting on touching and stroking leaves and flowers. Cure by discouraging them or, as a last resort, putting plants above their reach!

Drought. Obviously, no plant can survive without water, and remember that during the summer growing season a plant will need more water than it did during the winter.

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