Blossom end rot in tomatoes occurs when the bottom of the tomato become sunken, with a tan or black color. Once it starts occuring, there is really nothing to be done but to remove the affected tomatoes. The key is prevention.

To keep your tomatoes from rotting on the vine, you need to make sure that your soil has plenty of calcium prior and the correct pH level (around 6.5) prior to planting your tomatoes. To improve the calcium in the soil, consider adding crushed Tums, crushed egg shells, powdered milk, or bone meal tea.