Pear Rust (Gymnosporangium sabinae)
This is Gymnosporangium sabinae, a rust fungus, found on a Pear tree in Christchurch Park (TM16464519) on 24 October 2015. ID confirmed by Keith Palmer of the British Plant Gall Society. It has a remarkable life-cycle. The fungus causes red-orange spots on the upper surface of infected leaves of Pear trees (top picture). Swellings grow on the other sides of the leaves (bottom picture) which release another type of spore. These cannot re-infect Pear trees, but can cause a perennial fungal infection in Juniper bushes. An infected Juniper produces spores each spring. These spores infect Pear trees and complete the cycle.