Cedar apple rust is caused by the fungus Gymnosporangium juniperi-virginianae, and requires two hosts to complete its life cycle. The fungus must infect apple or crabapple in the spring, then an alternate host, Eastern red cedar (Juniperus virginiana) or other junipers in the late summer. The best remedy is to get rid of one of the plant hosts.

Cedar apple rust can also appear on crabapple trees and creeping juniper as in the case pictured below.