This could be European Pear Rust, a fungal infection. Its best if you can remove the affected leaves and destroy them to prevent the spores spreading. If this isnt possible then ensure you rake all the leaves up in the autumn and burn them (dont compost them). Its also worth noting that the fungus over-winters on Junipers so if you have any in the vicinity, look for rust infection on the stems and prune out any that are infected (or if necessary and remove the Juniper entirely). There are a few fungicides available in garden centres that you can spray on the tree to control the rust look for Difenoconazole or Myclobutanil as the main ingredients such, as Westlands Plant Rescue Fungus Control. Use a liquid concentrate and make sure that you read the manufacturers instructions regarding the time period required between applications and harvesting the fruit.