Plant Doctor Archive

Brown spots on my broad beans

The leaves of my broad beans have brown spots on them. What is this and how can I treat it?

This is likely to be the fungal disease Botrytis fabae. The fungus causes brown spotting on leaves, pods and stems, and looks similar in appearance to a dusting of chocolate powder, giving rise to its common name, 'chocolate spot'.

Chocolate spot favours acidic soils and is prevalent in damp or warm, humid spring weather. Broad beans sown in winter are more susceptible.

The infection is likely to reduce your crop; if severe, it will kill the plant. Apply a fungal spray to prevent further spreading.

Burn infected plants or place in the rubbish bin at the end of cropping. Clear away any fallen leaves. You can still eat the beans, but the pods themselves may be discoloured.

Broad beans should be planted in well-draining soils — neutral to alkaline soils are ideal. If your soil is acidic, apply some lime. Space seeds or plants well apart.

Chocolate spot spores can survive in the soil for several years, so crop rotation is a must. There should be a rotation of at least two years before broad beans are grown in the same ground.

Broad beans do not need large amounts of nitrogenous fertislisers — being legumes, they take in nitrogen from the atmosphere.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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