Halo Blight is a common disease that affects french beans and runner beans.
What does it look like?
Small, angular, water-soaked spots appear on the underside of the leaves, surrounded by a light green to yellow halo. Pods get pinprick-sized water-soaked spots that become sunken and reddish-brown, and the toxin the bacterium produces can yellow newer leaves. The cause is a bacterium, so there is no insect or visible organism to look for.
What causes it
The disease is transmitted through the seed, and infested seed is the most important source of infection. From there water drops carry the bacteria from leaf to leaf and plant to plant, which is why overhead watering is best avoided. US advice gives humid, moist conditions at 16 to 23°C as the favourable range.
Is this disease active now?
Watch through the May-to-September stretch when beans are growing, which is also when the 16 to 23°C and humid conditions the bacterium favours occur.
🌱 Plants at risk
Symptoms
Affected foliage develops the spotted, haloed lesions. A badly infected plant is a loss, not a cosmetic blemish, and is usually removed and destroyed rather than nursed through.
Life cycle
The bacteria can overwinterKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → in infested debris or seed, and halo blight can persist in the soil, which is why a gap of at least three years before growing beans on the same ground again is advised.
Why this matters in Scotland
Humid, moist weather at 16 to 23°C favours halo blight, and Scottish summers can supply it: Edinburgh's mean daily maximum is 17.4 to 19.3°C from June to August, with 10 to 11 days of 1mm or more of rain a month (Met Office, 1991 to 2020).
The disease does not need heat, so a cool summer does not protect a bean crop from it. No Scottish incidence figures were found.
Controls: what works
Buy certified, disease-free seed and never save seed from an affected plant
The bacterium is transmitted through the seed, so clean seed stops the problem before it begins. Don't save seed from plants with similar symptoms.
Water at the base of the plant rather than over the foliage
The bacteria are spread by water drops, so keeping water off the foliage removes the main way they move between leaves and plants.
Remove and destroy badly infected plants; pick off individual affected leaves on a lightly infected one
Destroy badly affected plants. Where the whole plant isn't affected, individual leaves can be picked off.
Use a 3-year minimum rotation before growing beans in the same ground again
Halo blight can persist in the soil, so wait at least three years before planting beans in the same spot. US advice for dry beans gives two to three years with a non-legume crop.
Clean tools and boots between plants or beds
Clean tools and boots between plants or beds, and avoid working among the plants while the foliage is wet.
Myths about halo blight
“It's fine to save seed from a bean plant that had halo blight, as long as the pods themselves look clean and unspotted.”
The bacterium that causes halo blight starts inside the seed itself, not on its surface, so a pod's outward appearance doesn't confirm the seed inside is clean. Where a pod has been infected, the seed it carries can come out shrivelled, discoloured or undersized, but that's a sign of infection rather than a reliable screening test a gardener can apply by eye before those changes show. The advice is never to save seed from a plant with similar symptoms.
Our read Judging seed safety by pod appearance treats a visible symptom as a pass/fail test when the infection route is internal, which is exactly the gap the seed-borne mechanism creates.
What to do: Buy certified, disease-free seed each year and don't save seed from any plant that showed halo blight symptoms anywhere on it, pods included.
2 sources
- Garden OrganicAdvises gardeners to never save seed from a plant with symptoms similar to halo blight.
- UC IPM (University of California)States that infested seed is the most important source of infection, and that pod infections can transfer to the seeds inside, making them shrivelled, discoloured or smaller than normal.
“Pods with a few halo blight spots are still fine to eat once you cut the marked parts off.”
The sources on halo blight cover identification, spread and crop management, and none states whether infected pods are safe to eat. That is a gap rather than an implied yes: food safety for a plant pathogen isn't automatically covered by advice written for growers managing a crop disease.
Our read No agricultural extension or growing-advice source addresses human consumption of affected pods either way, so there's nothing to cite for a safe-to-eat verdict; the silence itself isn't evidence of safety.
What to do: Treat harvest decisions as a crop-management question rather than a food-safety one: the standing advice is to remove and destroy badly infected plants and pick off affected leaves, not to selectively harvest around lesions, so err toward discarding visibly affected pods.
“Growing a halo blight-resistant bean variety means you can skip buying certified clean seed and crop rotation.”
A US university extension programme lists genetic resistance as one strand of an integrated approach alongside certified seed, crop rotation and avoiding overhead irrigation, not a stand-alone fix. UK growing advice aimed at home gardeners doesn't mention resistant varieties at all, building its entire control programme around clean seed, watering practice, plant removal, rotation and tool hygiene instead.
Our read The source that treats resistance as real still keeps it inside a combined programme rather than a replacement for the other measures, and the UK-facing source skips resistant varieties entirely, which together don't support dropping clean seed or rotation.
What to do: Where a resistant variety is available, treat it as one more layer on top of certified seed and rotation, not a substitute for either.
2 sources
- University of Idaho ExtensionPresents genetic resistance as a long-term part of an integrated approach that also includes certified disease-free seed and cultural controls.
- Garden OrganicGives a full halo blight control programme, clean seed, watering practice, plant removal, rotation and hygiene, with no mention of resistant varieties.
“Brushing past wet bean plants, or working among them with wet tools and boots, can carry halo blight from plant to plant.”
A US university extension programme names mechanical contact directly alongside rain splash and irrigation as one of the ways this seed-borne bacterium moves between plants, not just water droplets falling from above. UK growing advice backs this with a concrete instruction to clean tools and boots between plants or beds, treating hygiene as its own separate control measure rather than an afterthought to watering practice.
What to do: Avoid working among bean plants while the foliage is wet, and clean tools and boots between an affected patch and the rest of the plot.
In Scotland: Foliage that stays wet for long stretches, as it can in a damp Scottish summer, widens the window in which this route stays open.
2 sources
- University of Idaho ExtensionDescribes the halo blight bacterium as spreading through infected seed, rain splash, irrigation and mechanical contact.
- Garden OrganicRecommends observing good hygiene by cleaning tools and boots as a distinct control measure.
Frequently asked questions
What should I do first if I find halo blight in my garden?
The first thing to try: Buy certified, disease-free seed and never save seed from an affected plant. Then: Water at the base of the plant rather than over the foliage.
Is there a spray that works?
No. There are no amateur chemical treatments available for halo blight. The controls that work are cultural: the measures listed above.
When should I watch for halo blight?
Watch through the May-to-September stretch when beans are growing, which is also when the 16 to 23°C and humid conditions the bacterium favours occur.
Sources
Show the 4 sources
- Garden Organic8: identification, symptoms, the seed-borne origin, the water-splash spread mechanism, life cycle and soil persistence, clean seed, watering practice, removing infected plants, crop rotation, tool hygiene, absence of a chemical control
- RHS1: halo blight named, with rust, as a bacterial disease that can develop on runner beans
- RHS1: the broad bean growing guide does not name halo blight, so broad beans are not listed
- UC IPM: water-soaked leaf and pod spots with halos, 16 to 23C and humid conditions, infested seed as the most important source, overwintering in debris or seed, two to three year rotation for dry beans; a US source