Common pests and diseases: recognising and handling them

Sap-suckers: aphids, mealybug and scale

By far the most common group. They gather on soft new growth and buds and drain the sap, curling the leaves and weakening growth. Their signature is a sticky film (honeydew) building up on leaves and on whatever is beneath them, usually followed by a black sooty mould growing on top of it. Early checking is simple: turn the leaves over and look at the undersides and the inner branch bases — they hide there, not out in the open.

Spider mite: the dry-heat pest

It thrives specifically in hot, dry conditions, so infestations climb at the height of summer. The mites are too small to see easily, but the effect is not: a fine pale stippling spreading across the leaf, and very fine webbing between leaves in advanced cases. Raising humidity around the plant and hosing the foliage measurably suppresses it.

Powdery mildew: the crowding disease

A white, powdery film on the leaf surface, common on grapes, roses and mint. Its direct cause is high humidity combined with poor airflow, which is why it appears inside crowded plants rather than on their exposed edges. The first treatment is not a spray but a prune: thin the congested growth and water at the root rather than over the leaves.

Root rot: the killer you cannot see

The most dangerous item on this list, because the symptoms show above the soil while the cause sits below it. The plant wilts and yellows even though the soil is damp, the wilting is read as thirst, watering is increased, and the end comes faster. The practical rule: if a plant wilts and the soil around it is wet, stop watering and check the drainage immediately. Prevention is far easier than cure, and it comes down entirely to soil that drains and a pot with a real hole in it.

When to call a specialist

Some situations are not solved with a general-purpose product off the shelf:

• An infestation spreading across several different plant species within days
• Symptoms on a palm, where reaching and treating it requires equipment
• Sudden wilting of a large, established tree
• A problem that has recurred in the same spot despite being treated more than once