Cedar Apple rust is a fungus that is in full fruiting mode it early May. It can be found on cedar trees and junipers. It needs an alternate host ( Apple/crabapples ) to complete its life cycle. It will not kill the tree but on apples it makes spots on both the leaves and the apples themself.
The picture below shows it in it different stages. It will go from a small brown ball to an orange, slimy, gross fruit over night. So really only a problem unless your trying to grow apples and there are cedar tree within site. When your go to the apple orchard picking in the fall look around I’ll bet you don’t see any cedar tree.