Pear rust is a fungal disease of pears, causing bright orange spots on the leaves during summertime. If severe it can cause cankering of the stems. The only 2 methods, which are to remove and destroy all of the affected leaves, or give them a spray with difenoconazole (Westland Plant Rescue Fungus Control concentrate).
Like all rusts they are biotroph, which means that they cannot live on dead material. So they must find a living host to over winter on, which in Pear Rusts case is Juniper.
On pears, the brown fungal growths produced on the underside of infected leaves release spores which cannot reinfect pear, but instead are wind-dispersed and infect several juniper species, causing perennial stem infections. In spring these produce orange, horn-like outgrowths, which in turn produce wind-blown spores that reinfect pears.