Halo blight

What you see

Small water-soaked spots on the leaf, each surrounded by a broad, pale yellow-green halo far larger than the spot itself. Pods develop greasy, water-soaked patches that later ooze a cream-coloured bacterial slime.

Conditions

Cool, wet weather, 16-20C, and any handling of wet plants. Splashing rain moves it along a row fast.

Treatment

Remove affected plants where practical and stop working in the bed while foliage is wet. Copper limits spread but does not cure infected tissue.

Prevention

The control is clean seed - buy it, or save only from a crop that showed no halo blight at all. Rotate three years and clear debris.

Months: May, June, July, August

Plants

Green bean, Runner bean, Broad bean

Remedies

Bordeaux mixture (copper)

The thing people get wrong

The oversized yellow halo is the whole diagnosis and it separates this from every fungal leaf spot - which matters because sulphur, bicarbonate and milk are useless against bacteria. It is also seed-borne, so the single most valuable thing you can do is refuse to save seed from an affected row, however good the beans looked.