Disease
Chocolate spot
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Watch for | Watch for in February | Watch for in March | Watch for in April | Watch for in May | Watch for in June | Watch for in November |
What it is
Chocolate spot is a fungal disease of broad beans, closely related to the grey mould that attacks many other plants. Almost every broad bean crop shows a few spots in a wet spring, and most still give a decent picking. It becomes serious when plants are soft, crowded or growing in cold, wet soil.
Symptoms
Small, round, chocolate-brown spots appear on the leaves, often with a reddish edge, and later on stems, flowers and pods. In the mild form they stay separate and the plant carries on. In the aggressive form, usually after a spell of warm, humid weather, the spots merge into large dark blotches, flowers drop, and whole stems blacken and collapse. Rust on broad beans gives raised orange-brown pustules that rub off, rather than flat spots.
Cause
The fungus survives on old bean debris and is spread by airborne spores and rain splash. Infection needs wet leaves and high humidity, so it is commonest from late winter to early summer. Plants under stress suffer most: waterlogged or acidic soil, too much nitrogen, low potassium, sheltered spots with still air and close spacing all raise the risk. Overwintered autumn sowings are particularly exposed.
Control
Pick off and bin the worst-affected leaves, and remove collapsed plants. Mild attacks need no treatment; a dry spell often lets plants grow away cleanly. After harvest, clear all haulm and fallen leaves rather than leaving them on the bed. Garden fungicides are not a sensible option.
Prevention
Sow broad beans in well-drained soil, ideally limed if acidic, and avoid heavy nitrogen feeding. Give plants generous spacing, around 20 to 25 cm, and weed well so air moves between rows. On heavy, wet soil, spring sowing is often more reliable than autumn sowing. Rotate beans around the plot and avoid sowing next to last year's crop.
Plants often affected
Could it be something else?
These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.
Months to keep an eye out: February, March, April, May, June and November.
Sources
Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.