noun
1. : any of several diseases caused by fungi of the genus Gymnosporangium ; especially : a destructive fungous disease especially of the apple but also attacking pears and quinces that is caused by a rust ( G. juniperi-virginianae ) having its pycnial and aecial stages on the leaves and fruit of the apple and the telial stage on species of Juniperus, especially a common red cedar ( J. virginiana )
2. : the fungus causing apple rust — called also cedar apple rust, cedar rust ; compare cedar apple