Quince Rust (Gymnosporagium clavipes) on English Hawthorn fruit (Crataegus laevigata).

Quince rust alternates between Rosaceous hosts and Juniper (Juniperus/Sabina). The Rosaceous hosts of hawthorn rust include Serviceberry [aka Juneberry, Saskatoon & Amelanchier (genus Amelanchier)], apple (Malus), Pear (Pyrus) and Mountain Ash ( Sorbus).

The Juniper host is the upright form (genus Sabina according to some taxonomists).

More photos and information on the Quince Rust can be found at this site.