Yellowing first and then turning black and falling off could be several different things, so without seeing a photo of the leaves or having a better descripton of how the problem advances, we're just guessing. When the leaves turn yellow and then turn black, do they develop black spots that sort of spread until they all come together or does an entire life just turn black? Do the leaves become malformed while turning black? If so, it might be fire blight, which is a bacterial infection that often infects pears and apples. If there's fruit on the tree, is it developing scabby spots? That would indicate scab. If the leaves, once they have turned black, have growths like fungal spores, you're dealing with one of the fungal diseases that affect pears and their relatives. If the foliage looks somewhat rusty before going black, then it likely is a rust disease. There's other guesses that come to mind, but the ones I mentioned are the most commonly pear issues here.