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.