Cedar Apple Rust disease on crab apple leaf

By Littleinfo (Own work) [Public domain], via Wikimedia Commons

This is a fungus caused disease which affects cedar or juniper as well as apples and hawthorns.  It is interesting because it is an alternate host disease.  The fungus needs two completely different host plants to complete its life cycle.  The initial infection is on cedar trees or juniper.  It generally is not a serious problem there, but it produces an interesting red/orange glob with protruding horns on twigs.  The fungus spores then arrive at a susceptible host such as a crab apple and develop red/orange circular spots on leaves.  These can get large and extensive, resulting in partial or full defoliation.  This can be managed with spring and early summer fungicide sprays.  More effective management would be to remove one of the needed host plants from close proximity with each other.  Look for rust resistant trees when purchasing new plants.