Answer: What you describe is blossom end rot. It is not a disease, but a physiological disorder of tomato 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 regular irrigation and with soil mulches. Some gardeners report success with calcium chloride, a preventative spray you can purchase at garden centers. You can't save the affected tomatoes but you can try to stop the disorder from affecting other fruit by maintaining constant soil moisture. Good luck with your plants!
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