What is causing black leaves on my pear tree?
Thank you for the question. Your pear tree may be suffering from fire blight which is a bacterial infection that that is very common in pears. It is difficult to be certain from the photograph you submitted.
For pears infected with fire blight, prune away and discard twigs and branches at with visible symptoms. You'll want to prune about 12 inches below the decay. Be sure to disinfect your equipment frequently to avoid spreading the disease.
The bactericide streptomycin has demonstrated limited success but will probably not cure the infection. It may perform better as a preventative measure for trees that have not been infected.
Avoid using fertilizers during periods of infection. High nitrogen fertilizers may worsen the infection, and cause non infected trees to become more easily infected.