This is cedar rust.
It is the effect of the fungus gymnosporangium juniperi-virginianae on an apple tree leaf in my orchard. This fungus has infected a particularly lovely Yarlington Mill tree that would otherwise make a rich English-style single-varietal cider.
I can slow cedar rust down.
I can spray the tree with copper or sulfur, and it’ll kill some spores. I can spray the tree with something ‘organic’, and it’ll make the spores smell like whatever ‘organic’ goop I sprayed them with. Neither strategy will stop them. They’re in the air, on the bark and on the ground. Any leaf on this tree that has been infected with cedar rust this season will eventually curl, yellow and die. Any new leaf on the same branch will still almost certainly become infected. Even on new growth on a different branch, the prognosis isn’t very good. I’ll lose every leaf on this tree this season before its time.