Tiny fruiting bodies develop during winter on disease-affected fallen leaves at the base of the tree, and produce microscopic spores that are released in April and May. The spores are carried upwards by air currents to infect the newly-emerging leaves.
No long-term damage is done to affected trees. Even if the leaves are shed slightly prematurely, by the time that they fall they have already made the bulk of their contribution to the growth of the tree for that year.
There are a number of genetically-distinct ‘races’ of the fungus, which vary in the species of Acer that they affect.