Common bacterial blight (French bean)
Water-soaked leaf spots turning brown with a narrow yellow halo, and greasy, sunken pod spots — carried into the field inside the seed itself, so certified seed matters more than any spray.
- Pathogen
- Bacterium (Xanthomonas axonopodis pv. phaseoli)
- Spreads by
- Infected seed; rain splash; wet-foliage handling
- Favoured by
- Warm, humid weather
- Main risk
- Leaf spotting and defoliation; unsellable spotted pods
Overview
Common bacterial blight of French bean is caused by the bacterium Xanthomonas axonopodis pv. phaseoli — a distinct pathovar specific to Phaseolus vulgaris, different from the Xanthomonas pathovars that cause bacterial blight in cowpea or guar. It begins as small, water-soaked spots on the leaves that enlarge and turn brown, typically ringed by a narrow yellow halo, and merge into large dead patches as the disease spreads.
On pods, infection shows as water-soaked, greasy-looking spots that turn brown and sunken — the symptom that does the most direct damage to a crop picked and sold fresh every 2–3 days. The bacterium is genuinely seed-borne, carried both inside and on the seed coat, so a field can start the season already infected even with no other visible source nearby, and further spreads by rain splash and by handling wet, infected foliage. There is no cure once a plant is infected.
How to identify it
- Small, water-soaked spots on leaves that enlarge and turn brown, usually with a narrow yellow halo around the edge.
- Spots merging into large, irregular dead patches as the disease progresses, sometimes defoliating the plant.
- Water-soaked, greasy-looking spots on pods that turn brown and sunken — unsellable for the fresh market.
- Symptoms often appear first on the crop's edges or low points, wherever wet foliage lingers longest.
What causes it and when
- The bacterium Xanthomonas axonopodis pv. phaseoli, carried inside and on the seed coat of infected seed.
- Spread further within a field by rain splash between plants, and by people or tools moving through wet, infected foliage.
- Favoured by warm, humid weather with frequent rain or heavy dew — conditions common in a rainy-season crop or a humid hill planting.
- Crop debris left in the field after harvest can carry the bacterium over to infect the next bean crop.
Cultural & organic control
- Sow only certified, disease-free seed of a known source every season — the single most effective step against a seed-borne disease, and far more effective than any spray.
- Avoid working in or irrigating the field when the foliage is wet, since handling wet leaves spreads bacteria on hands, tools and clothing.
- Rotate away from French bean and other beans for at least two seasons on a field with a blight history.
- Clear and destroy infected crop debris after harvest rather than leaving it to carry the bacterium into the next planting.
Chemical control
- There is no chemical cure for an already-infected plant — bacterial diseases do not respond to fungicides.
- A copper-based bactericide applied preventively from the first spots can slow further spread in a high-pressure season.
- Confirm the current recommended product and dose for your area with your local KVK before spraying.
Prevent it coming back
- Start every season with fresh, certified, disease-free seed rather than seed saved from an unknown-health field.
- Rotate beans out of a blighted field for at least two seasons and destroy infected residue.
- Scout early and rogue out badly affected plants before spots spread widely across the field.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
I bought seed from a trusted shop — how could my French bean already have bacterial blight?
The bacterium that causes common bacterial blight lives inside the seed coat itself, not just on its surface, so seed can carry it without any visible sign and without any other infected plant nearby. This is exactly why buying certified, tested seed each season — rather than farm-saved seed of unknown health — is the single biggest lever against this disease.
Can I spray something to cure my French bean plants that already show blight spots?
No — like all bacterial diseases, there is no chemical cure once a plant is infected. A preventive copper-based bactericide applied at the very first sign can slow further spread in a bad season, but real protection comes from certified seed and field sanitation before symptoms ever appear.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.