Why Are My Runner Bean Leaves Curling?

Black bean aphid

Aphis fabae forms very dense, shiny-black, often waxy-powdery colonies particularly at the growing shoot tips and on the undersides of the developing young leaves at the top of the plant. Aphid feeding at the growing tip causes the developing young leaves to curl very tightly around the dense colony. Colonies arrive from spindle tree in late May to early July and can be so massive they visibly weigh down and distort the entire growing point.

What to do

• Pinch out the growing shoot tip along with the enclosed aphid colony (the most immediate effective action, with the added benefit of encouraging the plant to throw out productive side laterals); squash colonies lower down the plant by hand or knock off with a forceful jet of water from a hosepipe; plant open-structured flowers (marigolds, phacelia, poached egg plant) near the beans to attract hoverflies and lacewings; avoid broad-spectrum insecticides that kill natural predators; apply insecticidal soap spray directly to the colonies if needed, checking the edible crop withholding period.

Bean mosaic virus

Bean common mosaic virus (BCMV) and bean yellow mosaic virus (BYMV) are transmitted by aphids feeding on infected plants. The characteristic symptom is a mosaic pattern of alternating dark green and pale yellow-green or yellow areas on the leaf blade with moderate leaf blistering, puckering, and curling. Plants are often noticeably stunted and produce reduced pod yields. No cure; remove and destroy infected plants.

What to do

• Remove and dispose of infected plants in general waste (do not compost); do not save seed from infected plants; control aphids rigorously from the earliest stages of plant growth to reduce virus transmission risk; choose BCMV-resistant French bean varieties where available (many modern French bean varieties are bred with BCMV resistance); cover young plants with fine insect-proof mesh to exclude aphids during the most vulnerable early growth period.

Halo blight

Pseudomonas savastanoi pv. phaseolicola produces small angular water-soaked spots on leaves, each surrounded by the very characteristic wide, bright yellow halo that gives the disease its name. Spots turn brown; leaves may yellow, wilt, and curl in severe infections. Seed-borne; spreads in cool, wet UK spring and early summer conditions via rain splash.

What to do

• Remove infected plants and dispose of in general waste (do not compost); do not save seed from infected plants; buy seed from a reputable supplier (certified seed should be free of the halo blight bacterium); rotate beans to a different bed each year, not returning to the same ground for three to four years; avoid touching or working among bean plants when the foliage is wet to reduce spreading the bacterium between plants via handling.

Bean rust

Uromyces appendiculatus is very common on runner beans and French beans in warm, humid UK late summer conditions from August to October. Small, bright orange-brown pustules appear on the undersides of the bean leaves; corresponding pale yellow spots appear on the upper leaf surface directly above each pustule. Leaves may curl and die in a severe infestation.

What to do

• Remove and dispose of the most severely rusted leaves in general waste (not compost); ensure adequate spacing for good air circulation between plants; avoid overhead watering; clear away all bean plant debris at the end of the season and dig or compost it (away from the bean growing area) rather than leaving infected debris over winter; sulphur-based and other registered fungicide sprays may reduce rust spread when applied at the first signs of infection; check the edible crop withholding period before harvesting beans after treatment.

Frequently asked questions

Why are my runner bean leaves curling?

Runner bean and French bean leaves curl most commonly because of black bean aphid (Aphis fabae; very dense shiny-black often waxy-powdery colonies at growing shoot tips and on undersides of developing young leaves; heavy feeding at growing tips causes developing young leaves to curl very tightly around the dense colony; pinch out growing shoot tip along with enclosed aphid colony; squash colonies lower down by hand or knock off with forceful jet of water from hosepipe; encourage natural predators by planting open-structured flowers nearby; apply insecticidal soap spray directly to colonies if needed; check edible crop withholding period), bean mosaic virus (BCMV and BYMV transmitted by aphids; mosaic pattern of alternating dark green and pale yellow-green or yellow areas on leaf blade with moderate leaf blistering puckering and curling; plants noticeably stunted and produce reduced pod yields; no cure; remove and dispose of infected plants in general waste; do not compost; control aphids rigorously; choose BCMV-resistant French bean varieties), halo blight (Pseudomonas savastanoi pv. phaseolicola; very distinctive small angular water-soaked dark spots each surrounded by a very characteristic wide bright yellow halo; seed-borne; spreads in cool wet UK spring and early summer conditions via rain splash; remove infected plants in general waste; do not save seed from infected plants; buy certified seed; rotate beans to different bed each year), or bean rust (Uromyces appendiculatus; small bright orange-brown pustules on undersides of bean leaves with corresponding pale yellow spots on upper surface; leaves curl and die in severe infestation; remove most severely rusted leaves in general waste; ensure adequate spacing; clear all bean plant debris at end of season).

How do I control black bean aphid on runner beans?

Managing expectations: a well-established runner bean plant will usually survive a severe black bean aphid infestation and continue to produce pods particularly if the colony is controlled before it becomes massive; the aphid colonies tend to decline naturally in late summer when natural predator populations build up. Pinching out shoot tips: the most immediate and most effective control action is to pinch out the growing shoot tip along with all the enclosed aphid colony; this removes the main cluster of aphids in a single action and has the additional horticultural benefit of encouraging the plant to throw out side laterals (increasing the number of flowering and pod-producing branches); pinch out the tip of the main shoot when the plant reaches the top of its support anyway (as is standard UK runner bean practice). Squashing and water: for colonies on the undersides of leaves lower down the plant (rather than at the growing tip) squash the colonies by hand or knock them off with a forceful jet of water from a garden hosepipe on a jet setting directed at the undersides of the infested leaves; check lower leaves regularly for re-infestation. Encouraging natural predators: ladybirds hoverfly larvae lacewing larvae and parasitic wasps provide very effective biological control; avoid broad-spectrum insecticides that kill these natural enemies; plant marigolds phacelia and open-structured flowers near the runner beans to attract hoverflies and lacewings.

What causes halo blight on runner beans?

Halo blight is caused by the bacterium Pseudomonas savastanoi pv. phaseolicola; it is the most distinctive and most easily identified bacterial disease of runner beans and French beans in UK kitchen gardens because of the very characteristic bright yellow halo surrounding each water-soaked dark spot on the leaf. Symptoms: first symptoms are small water-soaked greasy-looking dark spots on the leaves; spots expand slightly and become more angular and brown; the most distinctive and diagnostic symptom is the very wide bright yellow halo of affected leaf tissue surrounding each spot; in a severe infection many leaves are affected with multiple spots and halos; leaves may eventually yellow wilt and curl; pods can also be infected; the seeds inside infected pods can be infected. Conditions: spreads most readily in cool wet UK spring and early summer conditions; spread between plants in the garden via rain splash; infection is also seed-borne. Management: no chemical cure; remove infected plants and dispose of in general waste (do not compost infected material); do not save seed from infected plants; buy seed from a reputable supplier (certified seed tested and should be free of the halo blight bacterium); do not grow beans in the same location more often than once every three to four years; avoid touching or working among bean plants when the foliage is wet.

Why do my runner bean flowers drop before setting pods?

Hot dry conditions at flowering: runner bean pollen is very sensitive to high temperatures and very low humidity; when temperature exceeds approximately 27 to 30°C combined with very low humidity the pollen becomes non-viable and flowers drop without setting pods; this is the most common cause of UK runner bean flower drop in hot UK summer spells. Inadequate pollination: if bee activity is very low in a very exposed windy or very shaded position the flowers may not be pollinated and will drop; this is unusual in most UK gardens where bees are present. Too-dry soil: soil that is very dry at the root zone during the critical flowering period can cause flower drop even if the temperature is not excessively high; water regularly and deeply at the base of the plants throughout the flowering and pod-setting period. Solutions: water deeply and regularly at the base of the plants (not overhead) throughout the flowering period; mulch the root zone with a thick layer (7 to 10 cm) of well-rotted garden compost in June before the soil dries out; mist the flowers with water in early morning and evening during very hot UK summer spells to raise local humidity around the flowers; choose a sheltered but not overly enclosed growing position that receives adequate bee visits.