Winter Squash: Pests & Diseases
Winter squash need warmth, space and a long growing season to produce hard-skinned fruits for storage. In much of the UK they are started under cover, then planted outside after frosts. Healthy plants usually crop well, but seedlings and foliage need regular attention.
Common pests
Slugs and snails can remove young winter-squash seedlings overnight, particularly in damp beds. Aphids gather on shoot tips and leaf undersides, causing curled growth and spreading viruses. Glasshouse-grown plants may also attract red spider mite or glasshouse whitefly. Mice can occasionally nibble sown seed or ripening fruit, while wasps may exploit damaged fruits late in the season.
Common diseases
Powdery mildew is the usual late-summer problem, making leaves look dusted with white powder and reducing vigour. Cucumber mosaic virus causes mottled, distorted foliage and misshapen fruit, often following aphid feeding. Grey mould can rot fruits where they rest on wet soil. Cool, wet autumn weather may prevent skins hardening and fruits ripening fully, reducing their keeping quality.
Prevention
Grow squash in a sunny, sheltered position with fertile, freely draining soil and enough room for air movement. Rotate cucurbits between beds where possible, clear away old plant debris and water at soil level. Choose varieties suited to UK summers and start plants under cover for an earlier crop. Keep fruit off damp ground on a tile or straw, and avoid handling vines when foliage is wet.
Non-chemical control
Check seedlings at dusk and remove slugs and snails by hand; use physical barriers where practical. Cover young plants with mesh or fleece until established, lifting it for flowering so pollinators can reach blooms. Rub or hose off small aphid colonies, and encourage hoverflies, lacewings and ladybirds with nearby flowers. Remove virus-suspect plants promptly and discard badly mildewed leaves away from the compost heap.
What to look for
From planting onwards, inspect seedlings several times weekly for slug damage and aphids. In midsummer, look beneath leaves for colonies, mites and whitefly, and check foliage for powdery patches. From late summer, examine fruits for soft spots, colour development and a hardening skin, especially after prolonged rain.
Tips
Harvest before frost, leaving a short stem attached. Cure sound fruits somewhere warm, dry and airy for around 10 days, then store them frost-free. Use any fruit with damaged skin first, as it is unlikely to keep as well as fully ripened squash.
Winter Squash pests & diseases: frequently asked questions
What are the common pests of winter squash?
Slugs and snails can remove young winter-squash seedlings overnight, particularly in damp beds.
What diseases affect winter squash?
Powdery mildew is the usual late-summer problem, making leaves look dusted with white powder and reducing vigour.
How do I prevent pests and diseases on winter squash?
Grow squash in a sunny, sheltered position with fertile, freely draining soil and enough room for air movement.
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.