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 economically 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. The canker phase of the disease is rare. For more information, see this article on Bitter Rot of Apple.
Answer provided by Alan R. Biggs, West Virginia University.