Experts and BHG readers answer.
How do I stop blossom-end rot on tomatoes?
This condition is called Blossom End Rot. It's caused by inconsistent moisture. To keep the plants from going dry and then getting wet from too much irrigation or rainfall, it's best to mulch around the plants. The mulch will help keep the soil moisture constant and will minimize the Blossom End Rot, but if you are having a lot of rain, you still may see this happen to some extent. Mulch will help."