Quince rust is a fungus disease caused by Gymnosporangium clavipes. The fungus infects fruit, but not leaves, of most apple cultivars. It infects both leaves and fruit of hawthorn. Typically, an infection on apple fruit shows up first as a slightly raised purplish area on the calyx end of the fruit. On mature fruit, the lesion is sunken and dark green or purple. Usually, the entire calyx end of the fruit is involved, and the flesh is deformed to the core. The fungus may produce powdery, fluorescent orange spores in tube-like structures (aecia) on the calyx end of apple fruit. Similar structures are produced on hawthorn leaves and fruit. These spores infect young stems of the red cedar, resulting in cankers in branches 2 to 5 years old. Spores produced on these cankers serve as inoculum for apple and hawthorn. For more information and images, read this article on Quince Rust on Apple.
Answer provided by Alan R. Biggs, West Virginia University.