This afternoon in Churchland Wood, Sedlescombe, I found a fallen sweet chestnut (Castanea sativa) leaf spotted all over with pale patches bearing the fruiting bodies of a microfungus, easily identifiable as Mycosphaerella punctiformis.
This is what is known as a 'plurivorous' species that is also found on leaves of oaks, beeches and other trees. It has been recorded from several places in West Sussex, but this may be the first formal record from East Sussex (which only goes to show there is much recording work to be done on the lower plants).
Like the ash dieback fungus it is described by Wikipedia as a a pathogen. It is probably found wherever suitable leaves occur in our part of the world and, with many other microfungi, is important in breaking down and returning fallen leaves to the soil, so of great benefit rather than harm.
The number of spores dispersed by this and the myriad other micro and macro fungi is, of course, astronomical and the proportion of ash dieback spores in this ever present cloud must be vanishingly small.