Why Are My Courgette Leaves Curling?

Powdery mildew

One of the most consistently troublesome diseases of all cucurbits in UK gardens from July to September. White, powdery, talc-like coating develops first on the upper surfaces of the older lower leaves and spreads progressively upward; affected areas curl slightly inward and turn pale and bleached; in severe infections leaves turn entirely white, then yellow, then die. Spreads rapidly in dry, warm, late summer conditions.

What to do

• Give generous spacing (at least 90 to 120 cm between bush courgette plants) for good air circulation; apply a thick mulch (7 to 10 cm) around the base in spring; water deeply at the base of the plants (not overhead) once or twice per week during dry spells from June to September; remove and dispose of the first mildewed lower leaves in general waste (not compost) to slow the spread; sulphur-based fungicide sprays are approved in the UK for powdery mildew on edible plants and are most effective when applied at the first signs in late July or early August.

Mosaic virus

Cucumber mosaic virus (CMV), courgette yellow mosaic virus (ZYMV), and watermelon mosaic virus (WMV) are transmitted by aphids and produce very dramatic, distinctive symptoms: strongly blistered, puckered, severely curled and distorted leaves with a characteristic mosaic pattern of dark green sunken patches and pale yellow-green or yellow raised patches; the entire plant becomes stunted; fruits are distorted, mottled, misshapen, and unmarketable. No cure; remove and destroy infected plants.

What to do

• Remove infected plants immediately and dispose of in general waste; do not compost infected material; control aphids rigorously from the earliest stages of plant growth as aphids are the primary virus vectors; cover young plants with fine insect-proof mesh during the most vulnerable early growth period to exclude aphids; choose CMV-resistant courgette varieties where available; do not save seed from infected plants.

Water stress

Very dramatic midday wilting is often completely normal and physiological: the large leaf surface area loses water faster than roots can replenish it in the hottest part of the day; the plant recovers fully by late afternoon. True drought stress does not fully recover by evening or the following morning and requires deep watering. Waterlogging causes progressive yellowing and root rot from below.

What to do

• Check soil moisture before watering midday-wilted plants: push a finger 5 to 7 cm into the soil near the plant; if moist, midday wilting is physiological and normal; if dry, water deeply at the soil level (not on the leaves) in the early morning or late evening; do not water onto wilted leaves in hot sun; apply a thick mulch to reduce soil moisture loss; ensure containers have large drainage holes to prevent waterlogging.

Aphid infestations

The peach-potato aphid (Myzus persicae) and melon and cotton aphid (Aphis gossypii) attack growing shoot tips and developing young leaves; feeding causes the developing young leaves to curl, pucker, and distort, and the growing point to stunt. Aphids on cucurbits are especially significant as the primary vectors of the mosaic viruses described above; controlling aphids reduces virus transmission risk.

What to do

• Squash or rub off aphid colonies with fingers; knock off with a jet of water; cover young plants with fine insect-proof mesh to exclude aphids during the most vulnerable early establishment period; encourage natural predators (ladybirds, lacewing larvae, hoverfly larvae); apply insecticidal soap spray targeting the colonies directly if needed; check the edible crop withholding period on the product label before harvesting courgettes after spraying.

Frequently asked questions

Why are my courgette leaves curling?

Courgette, squash, and pumpkin leaves curl most commonly because of powdery mildew (one of the most consistently troublesome diseases of all cucurbits in UK gardens from July to September; white powdery talc-like coating on upper surfaces of older lower leaves spreading progressively upward; affected areas curl slightly inward and turn pale and bleached; generous spacing for air circulation; thick mulch; deep watering at base not overhead once or twice per week in dry spells; remove first mildewed lower leaves in general waste; sulphur-based fungicide approved in UK for powdery mildew on edible plants), mosaic virus (CMV ZYMV WMV transmitted by aphids; very dramatic strongly blistered puckered severely curled and distorted leaves with characteristic dark-green-and-yellow mosaic pattern; entire plant stunted; fruits distorted mottled misshapen and unmarketable; no cure; remove and destroy infected plants in general waste; do not compost; control aphids rigorously from earliest stages; fine insect-proof mesh cover for young plants; CMV-resistant varieties where available), water stress (dramatic midday wilting usually physiological and normal; large leaf surface area loses water faster than roots can replenish in hottest part of day; recovers fully by late afternoon; true drought stress does not fully recover by evening; push finger 5 to 7 cm into soil to check moisture before watering), or aphid infestations (peach-potato aphid and melon and cotton aphid attack shoot tips and developing young leaves; squash or knock off; fine insect-proof mesh to exclude aphids; encourage natural predators; insecticidal soap spray targeting colonies).

How do I manage powdery mildew on courgette?

Correct spacing and air circulation: at least 90 to 120 cm between bush courgette plants; more for sprawling winter squash; good air movement around and between the large leaves slows mildew spread. Consistent soil moisture: powdery mildew on cucurbits strongly associated with drought stress at root level; thick mulch (7 to 10 cm) of well-rotted garden compost in spring; deep watering at base of plants (not overhead) during dry spells June to September; once or twice per week to soak root zone is more effective than frequent light surface watering. Removing affected leaves: remove and dispose of first mildewed lower leaves in general waste (not compost); removes local source of spores; slows spread to younger upper leaves. Chemical control: sulphur-based fungicide sprays approved in UK for powdery mildew on edible plants; most effective when applied at first signs in late July or early August; repeat at intervals stated on product label; check edible crop withholding period. Mildew-resistant varieties: some courgette varieties described as having improved mildew resistance; does not guarantee mildew-free plant but may extend productive season; seek current UK trials information from RHS or Which? Gardening for latest ratings.

What is cucumber mosaic virus and can courgettes recover?

CMV is one of the most widely distributed and most damaging plant viruses in the world; belongs to the cucumovirus group; infects a very large number of host plant species beyond cucurbits including many ornamental garden plants. Symptoms: very dramatic and distinctive; first signs typically on young developing leaves near growing tip as light yellow mottling or mosaic pattern; as disease progresses very strongly blistered puckered crinkled severely curled and extremely distorted leaves; characteristic combination of very dark green sunken blistered patches and pale yellow-green or bright yellow raised patches between them creating striking dark green and yellow mosaic effect; entire plant progressively more stunted and distorted; fruits misshapen mottled warty and of very poor quality. Transmission: transmitted by aphids in a non-persistent manner; aphid acquires virus in seconds to minutes of feeding on infected plant and can transmit almost immediately to next plant; controlling aphids on the cucurbit plants themselves less effective at preventing virus transmission than might be expected because virus can be transmitted by aphids that alight briefly and do not establish a colony. Recovery: no cure; once infected the virus spreads systemically through the plant; infected plants cannot recover; remove from kitchen garden immediately and dispose of in general waste; do not compost; use CMV-resistant varieties where available; control aphids rigorously from earliest stages; cover young plants with fine insect-proof mesh to exclude aphids during vulnerable early growth period.

Why does my courgette wilt in the middle of the day?

Normal protective midday wilting: the very large leaves have a very large total surface area from which water is lost by transpiration; in the hottest part of a UK summer day (typically midday to mid-afternoon) the rate of transpiration water loss can temporarily exceed the rate at which roots can take up and transport water; the plant temporarily wilts as leaf water content drops; this is a normal physiological response to high temperature and high transpiration demand; a cucurbit plant showing dramatic midday wilting that recovers fully by late afternoon or evening is experiencing normal physiological wilting and does not need watering at that moment if soil is already moist. True drought stress: contrast with a cucurbit plant that wilts in the morning or evening or does not recover by the following morning or continues to wilt progressively on a cool day; this is experiencing true drought stress requiring immediate deep watering of the root zone. Sudden permanent wilting: in cool wet conditions or in a plant that appeared healthy the previous day may indicate root or crown rot rather than drought; examine the crown and stem base at or just below soil level; brown soft rotted tissue indicates Phytophthora crown and root rot. Checking for drought: push a finger 5 to 7 cm into the soil close to the plant; if moist at this depth midday wilting is physiological and normal; if dry the plant is drought-stressed and needs a deep thorough watering at the base at soil level (not on the leaves) in early morning or late evening.