Why Are My Runner Bean Leaves Curling?

Blackfly

Blackfly (Aphis fabae, also known as black bean aphid) is the most common and damaging pest of runner beans in UK gardens. Colonies establish first at the growing tips, where the sap is richest and the tissue most tender, and spread rapidly in warm weather to cover the tips, leaf stalks, and underside of young leaves. Heavily infested growing tips become blackened, wilted, and distorted with aphid mass, and the surrounding leaves curl around the colonies. Blackfly also excretes honeydew, which coats the foliage and encourages sooty mould growth.

What to do

• Pinch out the growing tip of each runner bean plant when it reaches the top of its support. This removes the favoured feeding site and the most sappy new growth, making the plant less attractive to newly arriving blackfly and easier to inspect for new colonies.
• Squash small colonies by hand as soon as they appear. The stems and tips are easy to crush between fingers for quick physical control of early infestations.
• Apply insecticidal soap spray to larger colonies, covering the growing tip, leaf stalks, and underside of young leaves thoroughly. Repeat every 5 to 7 days.
• Avoid overfertilising with high-nitrogen fertilisers: lush, nitrogen-rich growth is more attractive to blackfly than harder growth on a well-balanced feed.

Drought

Runner beans are highly sensitive to drought, particularly during flowering and pod set. Even brief periods of water stress during these critical stages cause the flowers to drop without setting pods, dramatically reducing the harvest. The large leaves wilt and curl in hot conditions, and prolonged drought also reduces plant vigour and makes the remaining pods tough and stringy. Runner beans have high water requirements compared with most other vegetables and need consistent moisture throughout the growing season.

What to do

• Water runner beans deeply and consistently during dry periods, prioritising irrigation when the plants are in flower. A thorough soaking every 5 to 7 days in dry conditions is more effective than frequent light watering.
• Apply a generous mulch of compost or straw around the base of the plants, extending out to the drip line. This conserves soil moisture dramatically and reduces the frequency of watering needed during summer.
• Runner beans benefit from a well-prepared planting trench enriched with compost or well-rotted manure before planting: moisture-retentive, organic-matter-rich soil supports the plant through dry periods far better than thin or sandy soil.

Halo blight

Halo blight (Pseudomonas savastanoi pv. phaseolicola) is a bacterial disease of beans that causes small, water-soaked spots on the leaves surrounded by a distinctive pale yellow or cream halo, giving the disease its name. In cool, wet conditions, the disease spreads rapidly and the leaves may curl, yellow, and die. The pods of infected plants also develop water-soaked spots. Halo blight is seed-borne: it enters new gardens primarily through infected bought seed, particularly saved seed from an infected crop.

What to do

• Remove and destroy infected plants and leaves promptly to reduce the spread of the bacteria via rain splash and contaminated hands or tools.
• Never save seed from a crop that showed halo blight symptoms: halo blight is seed-borne and will be carried into the following year's crop.
• Buy seed from reputable UK suppliers: commercial seed is tested for halo blight and certified clean.
• Avoid overhead watering and working among wet plants: halo blight bacteria spread in water droplets and through contact with contaminated wet surfaces.

Red spider mite

Two-spotted spider mite (Tetranychus urticae) is primarily a problem on runner beans grown under cover (in greenhouses or polytunnels) or during very hot, dry summers in UK gardens. The tiny mites feed on the underside of leaves, causing a fine, pale stippling on the upper surface and a characteristic bronze or yellowish discolouration. In heavy infestations, fine webbing appears on the underside of leaves and between leaf and stem. Leaves may curl and brown at the margins in severe attacks.

What to do

• Increase humidity around the plants by misting the underside of leaves with water: spider mites thrive in hot, dry conditions and are suppressed by high humidity.
• Introduce the predatory mite Phytoseiulus persimilis as a biological control in greenhouse crops: this is highly effective against spider mite and is available from UK biological control suppliers.
• Apply insecticidal soap spray to the underside of affected leaves, ensuring thorough coverage where the mites are concentrated.

Slugs and snails

Slugs and snails attack runner bean seedlings and young plants, making irregular holes in the leaves that cause the remaining tissue to curl and distort as it grows. Seedlings can be killed outright by slug attack. Older, established runner bean plants are less vulnerable but can still suffer significant leaf damage in wet conditions when slug activity is high.

What to do

• Protect young runner bean plants at transplanting with physical barriers: copper rings, collars, or a ring of sharp grit around each plant base. Nematode slug control (Phasmarhabditis hermaphrodita) applied to the soil around young plants provides effective protection in moist conditions.
• Check under pots, boards, and debris near the bean frame at night and remove slugs by hand. Beer traps sunk to soil level near the plants provide additional control.

Frequently asked questions

Why are my runner bean leaves curling?

Runner bean leaves curl most often from blackfly infestation, drought, or halo blight. Blackfly (Aphis fabae) colonises the growing tips and the underside of young runner bean leaves, causing the leaves to curl and distort around the colonies. In heavy infestations the growing tip is completely smothered in black aphids and wilts. Drought causes the large leaves to wilt and curl inward; runner beans are particularly susceptible to drought stress during flowering and pod set, which causes flower drop and reduces yield. Halo blight (Pseudomonas savastanoi pv. phaseolicola) causes water-soaked spots on the leaves surrounded by a pale yellow halo, with overall yellowing and curling in severe infections.

How do I control blackfly on runner beans?

Blackfly (Aphis fabae) is the most common pest of runner beans in UK gardens. It colonises the growing tips first and spreads rapidly in warm weather. The most important cultural measure is to pinch out the growing tip of runner bean plants when they reach the top of their support structure: this removes the soft, sappy growth that blackfly prefers and discourages colonisation. For established infestations, squash colonies by hand or spray with insecticidal soap, ensuring thorough coverage of the growing tip and the underside of young leaves. Natural predators including ladybirds, lacewings, and parasitic wasps build up during summer and often control populations naturally by mid-summer without any intervention.

Why are my runner beans not producing pods?

Runner beans commonly fail to set pods in hot, dry weather because the pollen becomes non-viable at high temperatures and the flowers drop without fertilisation. This 'blossom drop' is most common in July and August during hot spells. Runner beans also need pollinators, primarily bumblebees, to transfer pollen between flowers. In gardens with low pollinator activity, flowers may remain unfertilised even in ideal weather conditions. Solutions include: misting the flowers in the morning during hot, dry spells to cool them and improve pollen viability; ensuring the soil is consistently moist; and planting flowers attractive to bumblebees near the runner bean frame.

When should I sow runner beans in the UK?

Runner beans are sown in the UK from late April to late May under cover (in pots on a windowsill or in a greenhouse), or directly outside from late May to June after the risk of frost has passed. Runner beans are frost-tender and a late frost can kill young plants, so do not plant outside before the last frost date for your area. In most of the UK, late May is safe for outdoor sowing or transplanting. Runner beans need a warm soil to germinate well: soil temperature below 12 degrees Celsius causes seed to rot rather than germinate.