Common Plant Pests And Diseases


Pesticides and fungicides should be handled with extra care. Always keep them away from children and household pets. Before using any chemical preparation in the garden, first read the label and follow the manufacturer’s instructions.

Aphids (greenfly and blackfly) The growing points and leaves of many kinds of plants, both ornamental and edible, can be affected. Spray with derris or malathion.

Apple scab Infected fruits and leaves of apples and pears develop black spots. Spray with benomyl or captan from green flower-bud to fruitlet stages.

Big-bud (gall mite) The buds of blackcurrants become swollen and fail to open. Spray bushes with lime sulphur at the flowering stage and again three weeks later. Pick off and burn infested buds in mild attacks, or destroy severely infested bushes completely.

Birds Bullfinches often strip buds off trees and shrubs in winter. If only a few plants are involved, black cotton thread or fine mesh woven among branches acts as a deterrent. Blackbirds and thrushes feed on ripening berries, which need similar protection. Fruit cages, although initially expensive, are the only real solution.

Sparrows create havoc in seed beds by making dust bath hollows and by eating seedlings. Pigeons are particularly fond of brassica seedling leaves. In all these cases, cloches, fine netting or black cotton thread will help minimize damage and are more effective than bird scarers.

Black spot The upper surfaces of rose leaves develop black spots. Spray infected plants with captan and spray again at fortnightly intervals. Rake up and burn infected leaves.

Carrot fly Yellow or white maggots bore holes into carrot and parsnip roots. Treat seed beds before sowing with an application of diazinon or trichlorphon. Caterpillars The larvae of many moths and butterflies damage leaves, buds, shoots and roots of ornamental and edible plants. Hand pick and destroy caterpillars in mild infestations and spray affected plants with fenitrothion or derris.

Club root Members of the cabbage family become stunted, with swollen and foul-smelling roots. Practise crop rotation and avoid growing brassicas on infected soil. Dress seed bed or planting holes with calomel dust before sowing or planting. Remove and burn diseased plants.

Codling moth Whitish grubs tunnel through apple and pear fruits. Spray trees with fenitrothion in mid-June and twice more at three-week intervals. Place bands of sacking round the trunks in late June to trap the larvae; remove and burn the bands during the winter.

Cutworm Root vegetables and flowering plants are attacked by soil-dwelling caterpillars. The pests are active at night and the plants are chewed or severed completely at ground level. Treat soil with an application of diazinon or chlorpyriphos. Earwig The buds, petals and leaves of chrysanthemums, dahlias and other plants are eaten by these shiny brown pests with pincer-like tail ends. Apply gamma HCH (formerly BHC) dusts or sprays to affected plants, treating the surrounding area of soil as well.

Flea beetles Seedlings of turnips and other members of the brassica family, including wallflowers, are attacked; holes, looking like gunshot, appear on the leaves. Use seeds treated with insecticide and dust or spray infested seedlings with derris or gamma HCH (BHC). Grey mould (botrytis) This fungal infection is one of the most troublesome garden problems. Fruit, vegetables, bulbs and herbaceous plants are all vulnerable. A grey, fluffy growth is the main symptom. Remove infected parts if the infection is mild, otherwise destroy badly infected plants. Treat plants with captan, benomyl, thiram or thispanate-methyl systemic fungicide.

Mealy bug Patches of white, woolly covering on the stems of greenhouse plants are symptoms of mealy bug infestation. The insects are concealed underneath the covering. Spray with malathion or dimethoate.

Mildew White powdery patches, on both indoor and hardy plants, are symptoms of mildew. Spray apples with benomyl, thisphanate-methyl systemic fungicide or dinocap at the pink-bud stage, and other crops as soon as the disease is seen. Repeat once or twice weekly.

Onion fly White grubs eat the bulbs of spring-sown onions; onions grown from sets often escape attack. Treat soil near infested onions with calomel dust or chlorpyriphos. Attacks carrots and cabbages too.

Potato blight Brown spots or blotches appear on the leaves and stems of potato and tomato plants. Unless the disease is checked quickly, the crops will be ruined. Apply copper fungicide spray in late June and repeat at least twice.

Red spider mite Both indoor and outdoor plants are vulnerable; the tiny red or yellow mites live on the leaf undersides. Leaves become mottled and bronzed and sometimes fine webs are visible. Spray with derris or malathion.

Scale insects These small, limpet-like pests mainly attack plants grown under glass, although occasionally hardy trees and shrubs are infested. Treat with malathion or petroleum oil sprays. Dabbing the individual scales with cotton wool dipped in methylated spirits is effective in the case of mild infestations.

Slugs and snails Slime trails and irregular-shaped holes in leaves indicate the presence of slugs and snails. Place metaldehyde or methiocarb slug baits near affected plants, particularly freshly transplanted crops.

Thrips These small dark insects are extremely numerous in the summer. Attacks can result in silvering or streaking on leaves and flowers. Spray affected plants with malathion.

Whitefly These tiny, winged, moth-like pests attack indoor and outdoor plants. They feed on the undersides of the leaves and produce a sticky honeydew, which in turn encourages the growth of sooty mould. Spray infested plants with gamma HCH (BHC) or malathion.