French Beans: Pests & Diseases
French beans thrive in a warm, sheltered British summer and provide crisp pods over several weeks when picked regularly. Dwarf types suit beds and containers, while climbing beans need substantial support. Wait for reliably warm soil before sowing, as seedlings dislike cold, wet conditions and may fail before growth begins.
Common pests
Blackfly can gather on tender shoots, flower buds and pod stems, causing curling and sticky honeydew. Slugs are a serious threat to newly emerged seedlings, particularly after wet weather. Under glass, red spider mite may cause pale speckling, bronzing and fine webbing beneath leaves. Mice may take seed before germination, while pigeons and other birds can disturb or peck young plants. Occasional caterpillars may chew leaves and pods.
Common diseases
Halo blight causes water-soaked leaf spots surrounded by pale yellow halos and can spread rapidly when foliage is wet or handled. Anthracnose makes dark, sunken lesions on stems, pods and sometimes seed. Grey mould affects flowers, damaged pods and crowded damp growth, producing fuzzy grey growth. Bean rust appears as orange-brown pustules on leaves. Root rots can cause weak, yellow seedlings in cold, saturated soil.
Prevention
Sow only when soil has warmed and drain poorly drained sites before planting. Use healthy seed, rotate beans away from previous legumes, and give plants generous spacing and strong supports for climbing types. Water at soil level rather than over foliage. Avoid handling, picking or tying plants when wet, especially where halo blight is suspected. Remove fallen leaves and old crop material, and ventilate greenhouse crops well to limit grey mould and mites.
Non-chemical control
Pick off small blackfly colonies or remove badly infested shoot tips, while encouraging ladybirds, hoverflies and lacewings nearby. Protect seedlings with slug barriers and regular hand collection after dusk. In a greenhouse, raise humidity around plants briefly with dampened paths, improve ventilation, and remove leaves badly affected by red spider mite. Use fine mesh against birds. Remove and dispose of plants with clear halo blight symptoms rather than working through them when wet.
What to look for
Inspect emerging seedlings daily for slug damage and missing plants. From flowering onwards, check shoot tips and leaf undersides for aphids, mite speckling and webbing. Examine pods and leaves after wet weather for haloed spots, sunken lesions, rust pustules or grey mould. Check greenhouse plants more often during hot, dry spells.
Tips
Harvest slender pods frequently before the seeds swell; this encourages more flowers. Water consistently during flowering and pod formation, but never leave roots sitting in water. If nights are cool, use temporary fleece until plants establish and temperatures improve.
French Beans pests & diseases: frequently asked questions
What are the common pests of french beans?
Blackfly can gather on tender shoots, flower buds and pod stems, causing curling and sticky honeydew.
What diseases affect french beans?
Halo blight causes water-soaked leaf spots surrounded by pale yellow halos and can spread rapidly when foliage is wet or handled.
How do I prevent pests and diseases on french beans?
Sow only when soil has warmed and drain poorly drained sites before planting.
General guidance for UK home gardens, focused on prevention and non-chemical control. The RHS or a local garden centre can help with tricky cases. Page updated 18 Sep 2026.