Do the dead branches turn black and curl over like a sheperd's hook?
If so, you probably have fireblight, which usually isnt a serious
problems on the Callery pears, but can look ugly. Keep in mind it is a
bacteria and not a fungus, so you are very limited in preventative
measures. Fungicides containing copper are mildly effective at
preventing it, but your best bet is agricultural streptomycin, sold as
agri-mycin. It may be hard to find, but I have seen it at retail
nurseries. You might have a different problem, but if you see black
stems as described then you can pretty much bet fireblight, especially
if the variety of pear you have is "aristocrat" or "redspire", which
are extremely succeptible to the bacteria. Hope this helps!