Answer: It sounds as if you have blossom end rot. It looks like a small, water-soaked lesion that appears on the blossom end of the fruit. It grows larger and becomes dark. Blossom end rot is caused by a lack of calcium at the growing tip of the fruit. While your soil may have adequate calcium, fluctuations in soil moisture from dry to wet really increase the problem. It is especially bad on the early fruit each summer and in sandy soils.
To control the problem, water regularly and deeply, maintaining even soil moisture. Apply 2-3 inches of mulch to reduce evaporation from the soil. Incorporating 2-4 inches of compost into the tomato bed before planting next season can help maintain even moisture. You may want to have a soil test done to ensure that calcium is present. If your soil is deficient, there are a number of amendments that can be added, such as gypsum. Check with the soil tester to see what they recommend.
Q&A Library Searching Tips