This is a nutritional disorder called “blossom end rot”. This may result from soils being deficient in available calcium. The soil pH should be 6.0-6.8. A soil pH lower than 6.0 may not supply enough available calcium. In this case, lime must be applied to raise the pH level according to soil pH test recommendations. When the pH is in the proper range, but the level of calcium in the soil is low, applying a non-liming source of calcium (gypsum) may be helpful.

Infrequent watering will also disrupt the flow of soluble calcium to the developing fruit. The soil must be kept consistently moist to keep calcium in solution and available to plants. The other extreme, overwatering, and also severe pruning can also encourage blossom end rot.

Spray applications of soluble calcium to the foliage can help to control blossom end rot. Spray the plant with a solution of 4 tablespoons of calcium chloride mixed in 3 gallons of water. Spray twice weekly, 1 quart per plant, according to product labels.

Additional Resource

http://edis.ifas.ufl.edu/VH028