Chocolate spot is one of the most common fungal diseases of broad beans. The fungus causes dark, chocolate-coloured spots on all parts of the plant.
What is chocolate spot?
Chocolate spot is caused by two species of the fungus Botrytis. Botrytis fabae is the most common cause and only affects broad beans. Botrytis cinerea can cause very similar symptoms, and this fungus also causes grey mould on a very wide range of plants. Chocolate spot can be very damaging, in some cases causing flower loss, with severe attacks resulting in whole plant collapse. It is worse in cool, damp, overcrowded conditions. This disease appears from late winter on autumn sown crops, but is seen from mid-spring on spring-sown broad beans.
No other beans including runner and French beans are affected, although common vetch and agricultural field beans sometimes used as green manures can be infected.
You may see the following symptoms:
- Small round, red-brown spots on all parts of the plant
- Under favourable conditions these spots expand aggressively, turn a darker, chocolate-brown colour, and leaves shrivel
- Stem infections may cause the plants to collapse
- Maximise air flow around the plants by wider spacing and avoid damp, humid sites
- Destroy infected plant material at the end of the season
- Eliminate common vetch from the vicinity
- Avoid using seed from infected plants
There are no fungicides available to amateur gardeners for the control of broad bean chocolate spot.
The fungus produces masses of air-borne dispersal spores, which spread the disease during wet conditions. It may also produce overwintering seed-like resting structures (sclerotia) in dead tissues, but their role in the disease cycle is unclear. The disease can be transmitted on infected seed. It also infects common vetch and these infections may also carry the fungus over the winter months, as can lesions on autumn-sown broad beans.
Under dry conditions the pathogen remains contained within the small round spots, but under wet conditions it spreads outwards and the lesion expands rapidly. Alternating wet and dry weather may lead to a spot with a series of concentric growth rings, but in longer periods of wet weather the leaf is rapidly killed.
Another fungal pathogen, Didymella fabae (syn. Ascochyta fabae), causes spots on leaves and pods, but is usually less damaging than chocolate spot.
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