Why Are There Water-Soaked Rings on My Bean Leaves?

Small, water-soaked, greasy or translucent-looking spots on bean leaves — each surrounded by a pale yellow or cream ring (the "halo") — are the diagnostic symptom of halo blight, caused by the bacterium Pseudomonas syringae pv. phaseolicola. This is one of the most recognisable bean diseases because the halo is distinctive and unlike any fungal disease. The yellow ring is caused by a bacterial toxin that inhibits chlorophyll production in surrounding cells, creating a clearly defined pale zone around each lesion.

How to confirm the diagnosis

The combination of water-soaked lesions (appearing greasy or translucent, especially when held up to light) and the surrounding pale halo is diagnostic — no common fungal disease produces this pattern. In cool, wet conditions the spots spread rapidly and coalesce. Pod symptoms are equally distinctive: pods develop water-soaked, greasy patches that turn reddish-brown, and seeds inside may be infected (showing a shrivelled, discoloured appearance). Infected seeds can pass the disease to the next generation of plants.

How it spreads in the garden

Halo blight spreads primarily by rain splash from infected leaves to healthy ones, and by water contaminated with bacterial cells. It is most active in cool, wet weather (10–20°C with high humidity). Overhead watering in the evening, which leaves foliage wet overnight, is a significant risk factor. The bacteria can persist on crop debris in the soil over winter, though they do not form the long-lived spores that some fungi produce. Once established in a planting, removing affected plants reduces spread but does not eliminate it.

Management and prevention

There is no curative treatment for halo blight — affected plants cannot be cured. Remove and bin (do not compost) severely affected plants to reduce bacterial load. Avoid overhead watering; water at the base. Practice a three-year rotation — do not grow beans in the same bed in successive years. Most importantly, source seed from reputable suppliers selling certified disease-free stock. Do not save seed from plants that showed any blight symptoms. In gardens with a history of halo blight, look for resistant varieties.