Cedar Apple Rust:
This is a disease that requires two hosts, apple/crabapple and eastern red cedar/junipers. It cannot spread from crabapple to crabapple, or from cedar to cedar, it alternates between the two. In spring spores from orange galls on cedars or junipers are blown to apple trees, as far as 2-3 miles away. These spores germinate during warm, wet weather, and they infect the leaves and fruit. Yellow spots appear on the leaves, gradually enlarging and turning orange, and causing premature leaf and fruit drop. In the summer small cups with fringed edges develop on the undersides of the leaves. These spores are blown back to cedars and junipers, causing new infections.
Where possible, avoid planting these two hosts within several hundred yards of each other. Remove any galls and destroy them. If cedar-apple rust is diagnosed on the leaves, it is too late to treat them. Early season fungicide applications, repeated at 7-10 day intervals may be effective.