Blossom end rot on fruits such as tomatoes occurs because the plant is deficient in calcium. It also attacks peppers, watermelon, squash, eggplant and sometimes cucumbers.
Blossom End Rot (left)-University of Kentucky, Blossom End Rot on watermelon-Tom Isakeit
It starts out as a small watery looking spot that turns brown and begins widening as the fruit ripens. The tissues then look dry and leathery. Calcium deficiency can be lessened with the application of lime or gypsum.