Dark brown spotting on broad beans is the very common disease known as chocolate spot. This disease can occur at any time from mid winter onwards but is a more serious problem when mild, wet weather follows late frosts in spring.
Round, dark, chocolate brown spots appear usually on the the upper leaf surface, leaf stalks, pods, seed coats and flowers. The spotting on the stem can be more elongated. The spots may merge to kill an entire leaf, flower pod or the whole plant. A light infection can result in a reduced crop.
The disease is more likely to occur in damp, shaded, over-crowded conditions. Therefore grow the crop in a free draining soil, avoiding a sheltered site which could lead to poor air movement around the plants, if necessary increase the spacing. If the soil is acid apply lime to give a pH of 7. Apply sulphate of potash and avoid high nitrogen fertilisers which encourage soft growth that is more prone to infection. Avoid autumn sowing if this disease is a regular problem as spring sown plants are more likely to recover.