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.