Blossom End Rot
Blossom end rot, a common malady on tomatoes, is showing up now that large-fruited tomatoes have set fruit and sizing up. Both green and red fruit develop a black, dry rot area on the bottom or base of the fruit.
This is not a disease but a physiological problem. The following is taken from the Texas A&M Plant Disease Handbook:
Blossom end rot is a physiological disorder of tomatoes that can appear on fruits at any time in their development, but most commonly appears when fruits are one-third to one-half grown. The initial symptoms are water-soaked spots on the blossom end of the fruit. These spots later enlarge and become black. Secondary infection by other decay causing organisms usually follows. The cause of this disorder is considered to be calcium deficiency in the developing fruit. Extreme fluctuations in moisture, root pruning and excessive nitrogen fertilization can also result in blossom end rot. Best means of control is to maintain a uniform supply of moisture through irrigation and soil mulches. Calcium chloride can be used as a spray for control under some conditions.
For pictures and other descriptions, click here for more blossom end rot photos, etc.