Why Are My Cucumber Leaves Curling?

Two-spotted spider mite

Tetranychus urticae is the most significant pest of UK greenhouse cucumbers. Tiny (0.3 to 0.5 mm), yellowish-green mites live and feed on the undersides of leaves; feeding damage produces characteristic pale yellow stippling on the upper leaf surface; leaves curl, dry, and turn bronze or brown in heavy infestations; fine webbing is visible on leaf undersides and between leaves. Thrives in hot, dry greenhouse conditions and multiplies extremely rapidly.

What to do

• Maintain high humidity in the greenhouse by damping down (wetting the floor and staging) in the morning and at midday; introduce the predatory mite Phytoseiulus persimilis (widely available from UK biological control suppliers) at the first signs of spider mite activity before the population becomes very large; P. persimilis is most effective above 16 to 18°C in moderate humidity; ensure the greenhouse environment is suitable for the predator before introducing it; acaricide sprays are available as a backup but rotate between different chemical classes to avoid resistance.

Powdery mildew

Golovinomyces cichoracearum and related species produce the same characteristic white, powdery, talc-like coating on cucumber leaf surfaces as on courgette and squash. Very common in UK greenhouses from July to September and on outdoor ridge cucumbers from August onward in warm UK summers.

What to do

• Ensure adequate ventilation in the greenhouse to prevent the stagnant, warm, dry air conditions that favour mildew; maintain consistent root moisture with regular watering; remove and dispose of the first mildewed lower leaves in general waste; sulphur-based fungicide sprays are approved in the UK for powdery mildew on edible plants; check the edible crop withholding period on the product label; note that some sulphur products can damage cucumbers in high temperatures.

Mosaic virus

Cucumber mosaic virus (CMV) produces very similar dramatically blistered, puckered, severely curled, and distorted leaves with a characteristic mosaic pattern of dark green and yellow-green patches in cucumber as in courgette. Transmitted by aphids. Remove and destroy infected plants immediately.

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; cover young plants with fine insect-proof mesh to exclude aphids during the most vulnerable early growth period; do not save seed from infected plants; choose CMV-resistant varieties where available.

Root rot and overwatering

Overwatering is a common cause of cucumber failure in greenhouse growing; keeping growing bags or pots in a persistently waterlogged state leads to Pythium root rot. Progressive yellowing and wilting that does not recover with watering are the primary symptoms; the base of the stem may show brown, soft discolouration at or near soil level.

What to do

• Water in the morning so the root zone has time to partially dry before the cooler night temperature; allow the compost surface to dry slightly between waterings; ensure growing bags and pots have adequate drainage; do not over-water; remove affected plants that have collapsed from root rot and replace the growing media before replanting; prevent by using good-quality, free-draining compost and consistent but not excessive watering.

Frequently asked questions

Why are my cucumber leaves curling?

Cucumber leaves curl most commonly because of two-spotted spider mite (Tetranychus urticae; most significant pest of UK greenhouse cucumbers; tiny 0.3 to 0.5 mm yellowish-green mites on leaf undersides; pale yellow stippling on upper leaf surface; leaves curl dry and turn bronze or brown in heavy infestations; fine webbing visible on leaf undersides and between leaves; thrives in hot dry greenhouse conditions; multiplies explosively; maintain high humidity by damping down; introduce Phytoseiulus persimilis predatory mite at first signs), powdery mildew (white powdery talc-like coating on leaf surfaces; very common in UK greenhouses from July to September; adequate ventilation; consistent root moisture; remove first mildewed lower leaves in general waste; sulphur-based fungicide approved in UK for powdery mildew on edible plants; note some sulphur products can damage cucumbers in high temperatures), mosaic virus (CMV transmitted by aphids; dramatic mosaic pattern of dark green and yellow-green patches; blistered puckered severely curled and distorted leaves; remove and destroy infected plants immediately; do not compost; control aphids rigorously; fine insect-proof mesh for young plants; CMV-resistant varieties where available), or root rot and overwatering (Pythium root rot in persistently waterlogged compost; progressive yellowing and wilting; brown soft discolouration at stem base; water in morning; allow compost surface to dry slightly between waterings; ensure adequate drainage).

How do I control spider mite on cucumbers?

Identifying spider mite: examine undersides of cucumber leaves with magnifying glass (10x sufficient); look for very tiny (0.3 to 0.5 mm) yellowish-green or pale orange-brown mites; clusters of tiny spherical cream-coloured eggs; fine silky webbing on leaf underside and between leaves; pale yellow stippling on upper leaf surface most easily visible above-ground symptom. Prevention: maintaining high humidity is single most effective preventative; two-spotted spider mite thrives in hot dry conditions and multiplies extremely rapidly in an unventilated dry UK greenhouse; damp down (wet the greenhouse floor and staging) in morning and at midday in hot UK summer weather; adequate ventilation prevents extreme heat build-up that favours spider mite. Biological control: predatory mite Phytoseiulus persimilis most effective most widely used and most recommended biological control for two-spotted spider mite in UK greenhouse conditions; feeds specifically and exclusively on Tetranychus urticae; available from UK biological control suppliers by mail order; introduce at first signs of spider mite activity before population becomes very large; most effective above 16 to 18°C in moderate humidity; ensure greenhouse humidity is suitable before introducing. Chemical control: acaricide sprays registered in UK for spider mite on edible plants; T. urticae can develop resistance to chemicals very rapidly especially after repeated use of same product; rotate between different chemical classes with different modes of action; check product label for edible crop withholding periods; stop chemical treatment well before introducing biological control agents.

Why are my cucumber leaves turning yellow?

Yellow stippling on upper leaf surface with fine webbing on undersides: strongly indicates two-spotted spider mite; examine leaf undersides with magnifying glass to confirm. Lower leaves turning yellow while upper leaves remain green: can have several causes: normal senescence (oldest lowest leaves naturally turn yellow and die as plant grows taller and they become shaded; limited to oldest lowest leaves; not a cause for concern), magnesium deficiency (very common in cucumber plants in grow bags or containers; interveinal yellowing of older lower leaves (area between leaf veins turns yellow while veins themselves remain green initially); treat with foliar spray of magnesium sulphate (Epsom salts: 20g per litre of water) or water around plant with diluted Epsom salts solution), overwatering and root rot (progressive yellowing of all leaves starting from base combined with wilting that does not recover with watering; check compost or soil moisture around roots), or nitrogen deficiency (all leaves gradually becoming paler and more yellow rather than typically the lower leaves first; heavy feeders requiring regular liquid feeding with high-nitrogen cucumber or tomato feed). Mosaic patterning of dark green and pale yellow-green or yellow: strongly indicates mosaic virus infection (CMV ZYMV or WMV); pattern is a mosaic not solid yellowing; leaves also typically distorted blistered and curled; remove and destroy infected plants.

How do I grow cucumbers in a UK greenhouse?

Temperature: minimum 16°C for good growth and fruit set; night temperatures below 12 to 13°C cause growth to stop and can damage the plant; in unheated UK greenhouse cannot reliably start until late April to May when night temperatures more consistently above 12°C; heating to minimum 16°C overnight allows earlier start; daytime temperatures in closed greenhouse in summer can reach 35 to 40°C which is too hot; adequate ventilation essential. Sowing and planting: sow seed on edge in individual small pots at 20 to 25°C (typically 5 to 7 days germination); plant into final growing position (large pot 10 to 15 litres or growing bag or greenhouse border) when plant has 2 to 3 true leaves; plant with compost surface at exactly the same level as in pot; do not bury stem as this increases risk of stem base rot. Training and support: climbing plants that must be trained and supported; install vertical wire netting or cane system; train single main stem up vertical support; pinch out growing tip when main stem reaches top of support; allow lateral shoots to develop; pinch out laterals at two leaves beyond a developing fruit. Watering and feeding: consistent regular watering essential; never let compost dry out; equally never keep in persistently waterlogged state; water in morning; begin liquid feeding with high-potassium cucumber or tomato feed from first fruit development; feed at every watering with half-strength solution or at every other watering with full-strength solution.