Pests and diseases begin to appear as the temperature rises towards the end of spring, particularly if the weather is also damp. Keep a watchful eye for early signs of problems so that you can stop them from taking hold.
Powdery mildew
This disease thrives in overcrowded and dry conditions and shows as a grayish white coating on leaves and shoots. Avoid by pruning and spacing plants to admit plenty of air, water plants well in dry weather and remove any affected growth such as white distorted shoots on apples or dying forget-me-not leaves.
Aphids
These start multiplying in mid and late spring. Disperse them with a jet of water from a hose, spray with insecticide or encourage their natural predators.
Caterpillars
Various kinds appear on plants as soon as moths and butterflies are on the wing. Look under leaves for egg clusters, which can be crushed with finger and thumb. Remove caterpillars whenever you notice them.
Slugs and snails
These all-year pests are particularly active in spring, feeding hungrily on young growth and seedlings. Collect them on damp evenings for disposal; trap them under orange or grapefruit skins laid on the ground; use proprietary beer traps; discourage them with barriers of grit or crushed eggshells; encourage frogs, toads and thrushes; or, as a last resort, use slug pellets sparingly, scattered thinly but frequently, especially in damp weather.
Vine weevils
Both adult weevils and their underground grubs are a continuous threat to pot-grown plants and open ground plants such as bergenias and strawberries, but they are particularly active in late spring. Water plants in containers with biological controls based on parasitic nematodes or imidacloprid. Keep the soil regularly cultivated around outdoor plants to expose the grubs to birds.
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Snails will feed on seedlings and young growth.
