Summer Squash: Pests & Diseases
Summer squash includes tender, quick-growing squash harvested young, much like courgettes. It suits a sunny border, raised bed or large container and responds well to rich soil and steady water. Fast growth means problems can escalate quickly, particularly after cool, wet weather or a sluggy night.
Common pests
Slugs and snails are the chief pests of seedlings, often removing leaves or stems overnight. Aphids settle on tender growth and may spread cucumber mosaic virus. Whitefly and red spider mite can occur where plants are grown under cover. Vine weevil larvae may weaken roots in containers. Pollinating insects are beneficial and their absence can cause malformed or aborted fruit.
Common diseases
Powdery mildew is widespread later in summer, producing white patches that slowly cover older leaves. Cucumber mosaic virus causes mottling, curling and distorted fruit after aphids transmit it. Grey mould can attack spent flowers and damaged fruit in humid, stagnant conditions. In cool, wet spells, poorly pollinated young fruits may turn yellow and rot at the blossom end, which is not usually an infectious disease.
Prevention
Grow plants in full sun with at least 90 cm between them, and rotate them away from previous cucurbit beds. Plant only once the soil is warm, then use fleece as early protection against cold and molluscs, removing it at flowering. Mulch to maintain even moisture and water at the base. Remove old leaves and crop debris, and choose resistant varieties when available.
Non-chemical control
Hand-pick slugs and snails after dark, reduce damp refuges and use physical barriers around young plants. Wipe off or rinse small aphid colonies, while attracting ladybirds, hoverflies and lacewings with nearby flowers. Remove individual leaves badly affected by mildew and dispose of mouldy flowers or fruit. Hand-pollinate in the morning during persistently cool or rainy weather if bees are scarce.
What to look for
Check seedlings daily for grazed leaves and stems. From flowering, inspect shoot tips and leaf undersides weekly for aphids, then look over older leaves for powdery mildew from mid-summer. After wet weather, check developing fruits for grey mould, end rot, yellowing or poor pollination.
Tips
Pick summer squash small and often, before skins toughen, to encourage more flowers and fruit. Keep soil evenly moist throughout hot spells. Avoid walking on spreading stems and harvest with a short piece of stem attached to reduce bruising.
Summer Squash pests & diseases: frequently asked questions
What are the common pests of summer squash?
Slugs and snails are the chief pests of seedlings, often removing leaves or stems overnight.
What diseases affect summer squash?
Powdery mildew is widespread later in summer, producing white patches that slowly cover older leaves.
How do I prevent pests and diseases on summer squash?
Grow plants in full sun with at least 90 cm between them, and rotate them away from previous cucurbit beds.
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.