Bitter rot is a fungus disease that causes fruit rot during the summer and preharvest period. The bitter rot fungi are almost worldwide in distribution and cause an especially important disease in the southern areas of the United States. Outbreaks can occur rapidly, and losses can be severe, especially under prolonged warm, wet weather. Several host species can be affected. On peach and nectarine, the same fungus causes a disease known as anthracnose, and on grape it causes ripe rot. The primary hosts, however, are apple and pear. More information and a table of resistant and susceptible cultivars can be found in this article on Bitter Rot.
Answer provided by Alan R. Biggs, West Virginia University.