Why Are My Courgette Leaves Curling?

Powdery mildew

Powdery mildew (Podosphaera xanthii) is by far the most common disease of courgettes in UK gardens and is eventually almost inevitable on mature plants by mid-summer. It appears as white, powdery patches on the upper surface of the leaves, spreading to cover the entire leaf surface as the infection progresses. Heavily infected leaves curl downward, yellow, and eventually die. Powdery mildew is a dry-weather disease, favoured by warm days and cool nights with high humidity but without rain washing the spores away; it is most severe from July onward.

What to do

• Remove and destroy the most severely affected lower leaves early in the infection to reduce the spore reservoir available to infect the upper leaves.
• Spray with a solution of 1 teaspoon of bicarbonate of soda in 1 litre of water, with a small drop of washing-up liquid as a spreader. Apply to all leaf surfaces weekly. This works best as a preventive measure before infection is severe.
• Diluted milk at a 1:9 ratio (milk to water) is equally effective and provides some protein-based protection against new spore germination. Apply weekly to all leaf surfaces.
• Ensure adequate spacing between plants (at least 90 centimetres) and water at the base to keep the foliage as dry as possible. Powdery mildew requires free moisture on the leaf surface to germinate.

Cucumber mosaic virus

Cucumber mosaic virus (CMV) causes some of the most dramatic leaf distortion seen on courgettes in UK gardens. The leaves develop a mosaic pattern of pale and dark green, pucker along the midrib and veins, and curl inward at the margins. The plant's overall growth is stunted, and the fruit of infected plants is typically distorted, small, and mottled or warty in appearance. CMV is transmitted by aphids, which can pick up the virus from an infected plant and transmit it to a healthy one in as little as a few seconds of feeding.

What to do

• Remove and destroy infected plants promptly. CMV is systemic within the plant (present in all cells) and cannot be cured; once a plant shows definite mosaic symptoms, it will not recover and is a source of virus for aphids to carry to adjacent healthy plants.
• Control aphid populations on surrounding plants to reduce transmission risk. Because CMV is transmitted non-persistently (very rapidly, before contact insecticides can act), aphid control alone does not fully prevent CMV spread in gardens with high aphid pressure.
• Reflective mulches (aluminium foil or reflective plastic) laid around the base of courgette plants confuse flying aphids and significantly reduce their landing rate on the crop.

Drought and heat stress

Courgettes have enormous leaves with very high transpiration rates, and they wilt dramatically when water-stressed. On hot summer days, even well-watered courgette plants may show some midday wilting and leaf curl that recovers as temperatures drop in the evening. This physiological wilt is not a cause for concern if the plant is otherwise healthy and recovers overnight. True drought stress, in which the leaves remain wilted or curled through the following morning, requires prompt watering as it directly reduces fruit set and fruit size.

What to do

• Water courgettes deeply and consistently during warm, dry periods. A thorough soaking every 5 to 7 days in dry conditions is better than light daily watering; deep watering encourages roots to extend downward rather than remaining in the dry surface zone.
• Apply a thick mulch of compost, straw, or wood chips around the base of the plant to reduce evaporation from the soil surface. This can halve the amount of watering needed in dry summers.
• Plant courgettes in fertile, moisture-retentive soil or containers with a high proportion of compost or peat-free growing medium. Plants in light, sandy soil require more frequent watering than those in heavier soils.

Aphids

Peach-potato aphid (Myzus persicae) and melon-cotton aphid (Aphis gossypii) both colonise courgettes in the UK, clustering on the underside of leaves and at the growing tips. Heavy infestations cause leaf curl and distortion around the colonies, and, more seriously, both species are efficient vectors of cucumber mosaic virus. Even a small aphid infestation warrants prompt attention on courgettes because of the CMV risk.

What to do

• Inspect the underside of courgette leaves weekly from June onward for aphid colonies. Colonies are often first established at the growing tip or on the youngest leaves, so these are the priority areas to check.
• Squash small colonies by hand or remove them with a strong jet of water. For larger colonies, apply insecticidal soap spray to the underside of the leaves, covering all aphid-infested surfaces.
• Plant flowering plants near the courgettes to attract aphid predators: phacelia, calendula, and poached-egg plant (Limnanthes douglasii) are all effective at attracting hoverflies and other natural aphid enemies.

Overwatering

Overwatering courgettes causes yellowing, wilting, and eventual collapse of the leaves in a way that can be confused with drought. The difference is that overwatered plants wilt even when the soil is wet, and the leaves often feel soft rather than crispy. Roots deprived of oxygen by waterlogged soil cannot function, so the plant cannot take up water even though it is surrounded by it. Courgettes in poorly drained, heavy clay soils or in containers with insufficient drainage are most at risk.

What to do

• Allow the soil to dry slightly between waterings: courgettes prefer consistently moist but never waterlogged conditions. Check the soil moisture at 5 to 10 centimetres depth before watering; if it is still damp, delay watering by another day or two.
• Improve drainage in heavy clay soils by incorporating grit and organic matter before planting. Raise beds or plant in ridges to ensure good drainage in wet seasons.

Frequently asked questions

Why are my courgette leaves curling?

Courgette leaves curl most often from powdery mildew, cucumber mosaic virus, or drought. Powdery mildew (Podosphaera xanthii) produces a white powdery coating on the leaf surface, and affected leaves may curl downward and yellow. Cucumber mosaic virus (CMV) causes leaves to curl, pucker, and develop a mosaic pattern of light and dark green; it is spread by aphids and there is no cure. Drought causes the large leaves to wilt and curl inward dramatically during hot periods, recovering once watered. Identifying which is present is the first step before deciding whether to intervene.

How do I manage powdery mildew on courgettes?

Powdery mildew is difficult to eliminate once established on courgettes but can be slowed. Remove and destroy the most severely affected leaves to reduce spore spread. Improve airflow by ensuring plants have adequate spacing: at least 90 centimetres between plants. Water at the base rather than overhead, and water in the morning so foliage dries before evening. Organic sprays of bicarbonate of soda (1 teaspoon per litre of water with a drop of washing-up liquid as a spreader) or diluted milk (1 part milk to 9 parts water) can suppress powdery mildew when applied weekly before the infection becomes severe. Copper-based fungicides are also approved for UK home garden use. Powdery mildew rarely kills courgette plants outright but it shortens their productive life.

What is cucumber mosaic virus on courgettes?

Cucumber mosaic virus (CMV) is a common viral disease of courgettes, cucumbers, squash, and many other plants in the UK. It is spread primarily by aphids, which pick up the virus from infected plants and transmit it to healthy ones within seconds of feeding. Infected courgette plants develop a patchy mosaic of light and dark green on the leaves, combined with puckering, distortion, and curling of the leaf edges. The fruit of infected plants is also often distorted and mottled. There is no treatment for CMV: infected plants should be removed and destroyed. Controlling aphids reduces spread but may not prevent infection if aphid pressure is high, as the virus can be transmitted before insecticides take effect.

Why are my courgette leaves turning yellow?

Courgette leaves yellow most often from powdery mildew, overwatering, or natural senescence of the older lower leaves. Older leaves at the base of the plant naturally yellow and die back as the plant matures and puts its energy into fruit production; this is normal. Overwatering causes yellowing of multiple leaves simultaneously, often combined with wilting rather than crisp leaf margins. Magnesium deficiency, which is common in sandy or acidic UK soils, causes interveinal yellowing (the veins stay green while the leaf tissue between them turns yellow) on older leaves and can be corrected by applying Epsom salts (magnesium sulphate) as a liquid feed.