Thank you for the question! Rotten pear fruit on the tree is most likely due to bitter rot. Late summer is when this disease normally strikes pear and apple trees. High temperatures, significant rainfall and humidity are optimal conditions. This fungus infects only the fruit. It starts as a small, circular, light brown area. Spots enlarge rapidly and become darkened. A distinctive characteristic is a saucer-shaped depression with fruiting structures in the center and concentric rings to the periphery. The best way to combat bitter rot is to first remove any affected fruit and dispose of any fruit that have fallen to the ground. Applying a fruit tree fungicide containing captan is the next step. A copper fungicide may be helpful too. Application rates may differ from chemical composition and the manufacturer label, but usually 10 to 14 day intervals will suffice until desired result. Remember, maintaining good cultural practices such as fertilizing, watering & pruning will encourage vigorous health, and in turn help fortify trees from diseases. If you will send me some pictures, I can confirm. <personal data hidden>