Why Are My Broad Bean Leaves Curling?

Black bean aphid

Black bean aphid (Aphis fabae) is the most damaging and most common pest of UK-grown broad beans. The shiny, jet-black aphids colonise the growing tips of broad bean plants from late April or May onward, forming dense clusters that cause the young leaves at the shoot tip to curl tightly around the colony. The colony grows rapidly in warm spring weather, and a single tip can host thousands of aphids within a week of the first colonisation. Heavy infestations stunt plant growth, reduce pod set, and produce sticky honeydew that attracts sooty mould.

What to do

• Pinch out the growing tips of broad bean plants once 4 to 6 flower clusters have formed and the first pods are beginning to set. This removes the primary feeding site and the aphid colony concentrated there in one action, and the removed tips can be cooked and eaten.
• Squash small aphid colonies by hand as soon as they appear, before they have the opportunity to reproduce and spread. Regular inspection of the growing tips from late April is the key to catching colonies when they are still manageable.
• Spray established colonies with insecticidal soap or a dilute washing-up liquid solution, covering all surfaces of the growing tip thoroughly. Repeat every 4 to 5 days until the colony is eliminated.
• Encourage natural predators: ladybirds, lacewing larvae, and parasitic wasps all attack black bean aphid and can reduce populations significantly in gardens with a good diversity of flowering plants nearby.

Pea leaf weevil

Pea leaf weevil (Sitona lineatus) attacks both peas and broad beans, eating characteristic scalloped notches from the margins of the leaflets. The notching is most obvious in early spring when young plants are most vulnerable, and the leaflets curl slightly around the damaged edges. The above-ground leaf damage is cosmetic and rarely causes significant crop loss, but the larvae of the same weevil feed on the nitrogen-fixing nodules on the roots underground, reducing the plant's ability to utilise atmospheric nitrogen.

What to do

• Cover young plants with fine insect mesh to exclude the adult weevils during the most vulnerable early weeks after planting or germination.
• In most UK gardens the leaf damage is tolerable and the plants grow through it without intervention. Focus protective measures on the critical early weeks rather than managing the pest throughout the season.

Chocolate spot

Chocolate spot (Botrytis fabae) causes reddish-brown spots on the leaves, stems, and pods of broad beans. In its non-aggressive form it produces isolated spots scattered across the leaf surface, which are unsightly but do not spread significantly. In its aggressive form, which develops in wet, humid conditions, the spots merge and the entire plant collapses rapidly. The aggressive form is most common in autumn-sown plants that have been through a wet winter, in dense plantings with poor airflow, and in wet spring conditions.

What to do

• Space plants adequately: at least 20 to 25 centimetres between plants in the row and 45 centimetres between rows. Good airflow between plants significantly reduces the humidity within the crop and limits the spread of chocolate spot.
• Remove and destroy infected plant parts promptly to reduce the spore load available to infect neighbouring plants. Do not compost infected material.
• For autumn-sown plants, a potassium-rich fertiliser applied in late winter can improve the plants' resistance to chocolate spot.

Bean rust

Bean rust (Uromyces viciae-fabae) causes orange-brown powdery pustules on the underside of broad bean leaves, with corresponding pale spots on the upper surface. In heavy infections the leaves may curl and yellow. Rust is most common in late summer on plants that have been in the ground since autumn. It rarely causes serious crop loss but can reduce the vigour of plants and contribute to premature foliage decline.

What to do

• Remove and destroy severely affected leaves to reduce the spread of rust spores. The remaining pods typically continue to develop normally even if the leaves are partially affected.
• Crop rotation and removal of all plant debris at the end of the season reduce rust inoculum in the garden from one year to the next.

Drought

Drought causes broad bean leaves to droop and the leaflets to curl inward along their length during the hottest part of the day. Pod set is particularly sensitive to drought: irregular watering during flowering and pod filling causes pods to set incompletely or abort entirely, dramatically reducing yields. In the UK, the critical period for broad bean watering is typically from late May through June when the plant is in full flower and beginning to set its pods.

What to do

• Water thoroughly and consistently from when the first flowers appear until the pods are fully formed. A deep watering every 10 to 14 days in dry weather is more beneficial than frequent shallow watering.
• Apply a mulch of compost or straw between the plants to retain soil moisture through the critical pod-setting period. Broad beans have deep root systems that can access soil moisture at depth, but surface mulching significantly extends the time between waterings.

Frequently asked questions

Why are my broad bean leaves curling?

Broad bean leaves curl most often from black bean aphid infestation or pea leaf weevil damage. Black bean aphid (Aphis fabae) colonises the growing tips and young leaves of broad beans from late April, causing the leaves at the shoot tip to curl tightly around the feeding colonies. This is the single most common broad bean pest in UK gardens and in a warm spring can cause the entire growing tip to become encased in a dense mass of black aphids within days. Pea leaf weevil (Sitona lineatus) eats distinctive scalloped notches from the leaf margins, causing the affected leaflets to curl slightly around the damaged edges.

What is chocolate spot on broad beans?

Chocolate spot is a fungal disease of broad beans caused by Botrytis fabae, producing small, reddish-brown spots on the leaves, stems, and pods. In severe cases, the spots merge and the leaves and stems turn brown and collapse. There are two forms: a non-aggressive form that produces isolated spots and does not spread significantly, and an aggressive form that causes rapid collapse of affected plants in wet conditions. Chocolate spot is most common in wet springs and on plants growing in dense, poorly ventilated conditions. It is more severe on autumn-sown plants that have been growing through a wet winter.

Should I pinch out broad bean tips?

Pinching out the growing tips of broad beans once the first pods have set (typically when there are 4 to 6 flower clusters on the stem) is one of the most effective ways to reduce black bean aphid infestations. The soft, tender growing tip is the preferred feeding site for black bean aphid: removing it eliminates the primary target and the colony of aphids concentrated there. The pinched-out tips can be cooked and eaten as a vegetable in their own right. Pinching out also directs the plant's energy toward pod filling rather than continued stem extension, potentially improving yield.

When should I sow broad beans in the UK?

Broad beans can be sown in the UK in two main seasons: autumn sowing from October to November for overwintering plants that crop from late May to June, or spring sowing from February to April for plants that crop from July to August. Autumn-sown broad beans are generally more productive as they establish a larger root system before winter, but are more susceptible to chocolate spot in wet winters. Spring-sown plants are less susceptible to chocolate spot but may encounter black bean aphid pressure earlier in the growing season. Hardy varieties such as 'Aquadulce Claudia' are specifically bred for autumn sowing.