At the back of our property is a 'heritage hedge' of approx 60 m in length, consisting of elder, hawthorn, blackthorn, ash (healthy, thank goodness), rowan and field maple. The latter is my concern today. The leaves appear to have grown a 'rash' - definitely not bugs - that looks like bright pimples all over, turning to red and then just leaving an 'embossing' as the red fades. The trees are healthy, as are all of those in the 'hedge'. Is this a fungus of some sort or a disease? Is it going to affect the field maples and, possibly, the other trees (although no sign in them) or is it merely a cosmetic problem for this year? Would it have been caused by the excessive rains of last year (which, I might add, have had an enormous, largely beneficial, effect on the trees, lush good growth, masses of huge blossoms on the elders and huge racemes of seed heads on the ash). Any help, as usual, will be most gratefully received.