A common disorder in tomatoes is blossom-end rot in which the fruit becomes sunken and blackened. This condition is caused by lack of calcium in the developing fruit.
Over-watering should be avoided, however, because root absorption may be less efficient.
Black and sunken rotted sections at base of fruit.
How to Protect your plants
Blossom-end rot is aggravated by moisture stress in very hot weather, so regular watering and mulching of the surface will help control the problem. An application of lime or gypsum (calcium sulfate) to the bed before planting will lessen the incidence of this disorder.