The fungus Guignardia aesculi was introduced accidentally into the UK from North America in the last century.
The life cycle has not been studied in detail, but the fungus is known to pass the winter in fallen leaves, releasing airborne spores in the spring. These infect the leaves and produce a second type of spore in the infected tissues. These spores are responsible for most of the infections, which are worse in wet conditions.
Although it can be unsightly, the disease is not seriously damaging. Severe attacks tend to build up in late summer and may be mistaken for early development of autumn colour. The disease is more prevalent in the south and west of the UK.