Cucumbers: Pests & Diseases
Cucumbers usually give the best UK harvests in a greenhouse, polytunnel or very sheltered outdoor site. They grow rapidly in warmth and need constant moisture without saturated roots. Ventilation matters as much as watering, particularly when the crop is dense and fruiting heavily under cover.
Common pests
Glasshouse whitefly is a familiar cucumber pest under cover, leaving honeydew and encouraging sooty mould. Red spider mite develops quickly in dry, hot houses and causes pale speckling, yellowing and webbing. Aphids cluster on soft new growth and can spread cucumber mosaic virus. Slugs and snails may chew seedlings, especially outdoor plants or greenhouse plants close to open doors.
Common diseases
Powdery mildew forms white patches, usually later in the season, and is encouraged by stress and ageing foliage. Downy mildew causes angular yellow patches and greyish growth beneath leaves in humid conditions. Cucumber mosaic virus produces mottled, distorted leaves and misshapen fruit. Grey mould can develop on spent flowers and damaged stems where ventilation is poor. Root problems follow cold, waterlogged compost.
Prevention
Use clean pots and fresh compost, or rotate greenhouse soil and avoid growing cucumbers repeatedly in the same bed. Keep plants well spaced, train stems to improve airflow and remove yellowing leaves promptly. Water evenly at the base, preferably in the morning, and ventilate daily. Select disease-resistant varieties; all-female greenhouse varieties reduce the need for pollination and avoid unwanted seedy fruit.
Non-chemical control
Use physical barriers and evening hand-picking for slugs around young plants. Rub off aphids or rinse small colonies from leaves, and support predatory insects with nearby flowers. In greenhouses, Encarsia can control whitefly and Phytoseiulus can reduce red spider mite. Remove leaves severely affected by mildew and any mouldy flowers or fruit. Dispose of virus-affected plants rather than composting them.
What to look for
Check young plants twice weekly for slug damage, aphids and whitefly beneath leaves. During warm, dry weather, inspect closely for mite stippling and webbing. In humid weather, look for downy mildew and grey mould; from late summer, monitor older leaves for powdery mildew and remove affected material early.
Tips
Keep cucumbers picked regularly and never let compost dry out completely. Shade only during exceptional heat, since plants still need good light. For outdoor types, wait until weather and soil are reliably warm before planting out.
Cucumbers pests & diseases: frequently asked questions
What are the common pests of cucumbers?
Glasshouse whitefly is a familiar cucumber pest under cover, leaving honeydew and encouraging sooty mould.
What diseases affect cucumbers?
Powdery mildew forms white patches, usually later in the season, and is encouraged by stress and ageing foliage.
How do I prevent pests and diseases on cucumbers?
Use clean pots and fresh compost, or rotate greenhouse soil and avoid growing cucumbers repeatedly in the same bed.
General guidance for UK home gardens, focused on prevention and non-chemical control. The RHS or a local garden centre can help with tricky cases. Page updated 18 Sep 2026.