Common Pests and diseases


Here at Gardening Direct, we have created a series of articles to help you identify and treat common pests and diseases in your garden.




Colonies of small round-bodied insects suck the sap from leaves and distort plant growth. Aphids are mostly green or black in colour and mostly wingless. They are most likely to appear in the spring or early summer in the open, but throughout the year under glass or indoors. Use a recommended insecticide and spray only affected areas. Cover plants with netting or criss-cross black cotton about 12cms above the plants, a cheap, easy and harmless protection.

Red Spider Mite

A fine light mottling of the leaves and later a bronze discolouration. It mainly affects greenhouse plants, but also fuchsias, impatiens, and primulas. Tiny insects appearing red in colour that will distort plant growth. They prefer hot, dry conditions. Use a recommended insecticide and spray only affected areas.

Slugs and snails

Irregularly shaped holes eaten in bulbs, roots, stems, leaves and flowers. Slime trails reveal their active presence. They usually feed at night and hide in the day. They are most likely to appear in warm and humid weather in the spring and autumn. Use recommended slug pellets and apply after planting.


Related to aphids but white, winged and small. They are found on the underside of leaves where they feed on the sap and a black sooty mould forms will. Use a recommended insecticide and spray only affected areas.

Common diseases

Grey Mould (Botrytis) A fungi that infect damaged areas and causes them to rot with a grey mould covering the infected area. The fungi are favoured by damp, overcrowded conditions with poor ventilation. Pickoff dying flowers and buds. Use a recommended fungicide and spray only affected areas.

Fungal infections

Most fungal infections are caused by a damp environment. Just a small amount of air movement will help keep the top growth dry. So try to keep a small vent open in your glasshouse or conservatory. You can also grow them on, on a windowsill, leaving the window open just a crack, as security allows. Another trick is to water plants in the mornings allowing them to dry out in the day. Plants wet at night are more prone to disease.


Powdery mildew which is caused by a fungus develops as powdery white mildew which appears on leaves shoots and flowers. Downy mildew forms a whitish furry covering on the underside of the leaves. Both look unsightly and will weaken the plant. Remove affected leaves immediately to prevent spread. Avoid overwatering plug plants as mildews favour damp conditions. Use a recommended insecticide and spray only affected areas.


A fungus which particularly affects damping off in young plants and causes roots to die back in mature plants which will then inhibit growth. Good hygiene is essential when growing on mini plugs to prevent infection. Good air circulation will help and avoid overwatering as roots will be weak. Use a recommended spray or drench the compost before planting.


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