Definition of apple rust
1 : any of several diseases caused by fungi of the genus Gymnosporangium; especially : a destructive fungal 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 Juneperus, especially a common red cedar (J. virginiana)
2 : the fungus causing apple rust —called also cedar apple rust, cedar rust — compare cedar apple
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