There are a couple of leaf spot diseases that affect pear trees. To be sure which one you have, you need to send a sample to Penn State Extension plant clinic. They will diagnose the problem and give you a management plan. You need to know if it's a fungal infection or a bacteria before you can treat it. You can take a sample of infected leaves to your local Penn State Extension or you can go online to the plant clinic website and follow directions for submitting a sample. Here is the link. http://plantpath.psu.edu/facilities/plant-disease-clinic/instructions