Why Are My Marrow Leaves Curling?

Powdery mildew

Powdery mildew (Podosphaera xanthii and related species) is the most prevalent disease of marrow in the UK. It almost always appears by mid to late summer, producing a white, powdery coating on the upper surface of the leaves, then spreading to the undersides. Affected leaves curl, turn yellow and brown between the patches of mildew, and eventually wither and die back. The disease progresses from the oldest leaves upward through the plant. A severely mildewed plant weakens rapidly and the remaining fruits do not develop or ripen normally.

What to do

• Space marrow plants at least one metre apart to allow airflow through the canopy. Do not crowd them with other tall crops.
• Remove and destroy the most severely mildewed leaves promptly. Do not compost them: dispose of them in the council green waste bin or burn them.
• Apply potassium bicarbonate or sulphur fungicide every 7 to 10 days from the first signs of mildew through to the end of the season. Both products have some curative action on established mildew as well as preventive effect.
• Water the soil rather than the foliage. Avoid late-evening watering, which raises local humidity overnight.

Cucumber mosaic virus

Cucumber mosaic virus (CMV) is the most damaging disease of marrow in UK gardens in years with high aphid pressure. The symptoms are a distinctive yellow-green mosaic mottling on the leaves, with irregular areas of bright yellow contrasting with darker green. The affected leaves are often crinkled, distorted, and curled, with a roughened, blistered texture. The growing tips may be distorted. Fruit produced on infected plants may be mottled, misshapen, poorly flavoured, or fail to develop fully. The virus is non-persistent in its aphid vectors, meaning aphids transmit it very rapidly as they probe and taste plants while searching for a host. This makes chemical control of the aphid vectors largely ineffective at preventing transmission.

What to do

• Remove and destroy infected plants promptly to eliminate them as a virus source for aphids to carry to other plants. Do not compost.
• Control aphids on young marrow plants in spring, when the plants are most vulnerable: use fine insect mesh over young plants until they are established, or apply insecticidal soap at the first sign of aphid colonies.
• Remove weeds around the garden, particularly those known to be CMV reservoirs such as chickweed and hairy bittercress.
• Choose CMV-resistant varieties for future planting where available.

Aphids

Aphids, particularly the peach-potato aphid (Myzus persicae) and melon-cotton aphid (Aphis gossypii), colonise marrow shoot tips in spring and early summer, causing the young leaves to curl inward around the feeding colonies. In addition to the direct damage from feeding, aphids transmit cucumber mosaic virus very efficiently, making early aphid control on young marrow plants important for virus prevention as well as direct pest management.

What to do

• Inspect marrow growing tips from transplanting. Remove small colonies by hand. Apply insecticidal soap to the growing tip and underside of young leaves. Encourage natural predators. Cover young plants with fine insect mesh for the first few weeks after transplanting to exclude aphid colonisation.

Drought

Marrow has very large, sappy leaves that lose water rapidly in hot weather. Afternoon wilting and slight leaf curling on hot days is normal and temporary in a healthy plant with adequate soil moisture; the plant recovers overnight. However, persistent wilting or curling that does not recover by morning indicates genuine drought stress. Drought-stressed marrow plants produce fruits with thick, bitter skin and loose, watery flesh, and are more susceptible to powdery mildew.

What to do

• Water marrow deeply and infrequently rather than shallowly every day: a deep watering that wets the root zone to 30 centimetres is more effective than a daily surface wetting. In hot weather, marrow in the open garden may need watering every 2 to 3 days. Apply a thick mulch of well-rotted compost or bark around the plant, leaving a gap at the stem, to retain soil moisture.

Frequently asked questions

Why are my marrow leaves curling?

Marrow leaves curl most commonly because of powdery mildew, cucumber mosaic virus (CMV), or aphids. Powdery mildew (Podosphaera xanthii) produces a white, powdery coating on the upper and lower leaf surfaces and causes the leaves to curl, wither, and die back; it is almost universal on marrow plants in the UK by late summer. Cucumber mosaic virus causes a yellow-green mosaic mottling on the leaves with irregular curling, distortion, and crinkling; it is transmitted by aphids and is common in UK gardens where its aphid vectors are present. Aphids on the growing tips cause the young leaves to curl and pucker directly as well as transmitting CMV. Drought causes the large leaves to curl and wilt rapidly on hot days.

Why do my marrow leaves have white powder on them?

White powder on marrow leaves is powdery mildew (Podosphaera xanthii), a fungal disease that is almost inevitable on UK marrow plants by late summer. Unlike most fungal diseases, powdery mildew does not need wet leaf surfaces to establish: it spreads in dry, warm conditions with high air humidity, which means it is not prevented by avoiding overhead watering. The white, powdery coating appears first on the older leaves and spreads rapidly upward through the plant. Affected leaves curl, wither, and die back prematurely. Severe mildew in August and September can reduce the plant's ability to ripen the last marrows of the season. Management: improve airflow around the plant by spacing plants adequately and removing the most severely affected leaves; apply potassium bicarbonate or sulphur fungicide from the first signs of infection, every 7 to 10 days through the season; remove and destroy plant debris at the end of the season to reduce the spore source for the following year.

What is cucumber mosaic virus on marrow?

Cucumber mosaic virus (CMV) is a very common virus that infects marrow and all cucurbit family members (courgette, cucumber, squash, pumpkin) in UK gardens. It is transmitted by aphids, which pick up the virus from infected plants (including many weeds and ornamentals) and carry it to marrow plants while feeding. The symptoms on marrow are: irregular yellow-green mottling on the leaves (the 'mosaic' pattern), leaf distortion, crinkling, and curling of the younger leaves; the leaf surface often has a roughened, blistered texture. Fruit on infected plants may be mottled, misshapen, or bitter. There is no treatment for CMV once a plant is infected. Control is focused on preventing infection: control aphids on young plants to reduce virus transmission; remove and destroy any infected plants to eliminate them as a virus source; avoid handling healthy plants after touching infected ones; and choose CMV-resistant varieties where available.

Why are my marrow leaves wilting?

Marrow leaves wilt for two main reasons: drought, and vine weevil or cucumber mosaic virus causing root and stem damage. Marrow has very large leaves that lose water rapidly in hot weather: on a hot summer's day, the leaves of a healthy plant often wilt during the afternoon even when the soil is moist, recovering as temperatures drop in the evening. This is normal physiological wilting and does not indicate a problem. However, wilting that does not recover in the evening suggests a more serious issue: check the soil moisture (drought); look at the stem base for soft, rotting tissue (stem rot from overwatering or fungal infection); check roots for vine weevil grubs (cream, C-shaped grubs in the compost); and look for signs of cucumber mosaic virus elsewhere on the plant. Water marrow deeply and consistently rather than shallowly and frequently; a thick mulch of well-rotted compost or bark retains soil moisture and significantly reduces wilting on hot days.