Q: What causes young tomatoes to get a black bottom that turns to rot?

A: Blossom end rot is caused by lack of calcium in the sap when the small fruit is growing. Calcium is distributed to the fruit in tomatoes via the sap. But if the roots get dry calcium does not get out to the fruit.

Applying calcium (lime) to the soil doesn’t help much. Calcium sprayed on the leaves doesn’t work either because calcium can’t move into the sap that way.

The real cause of blossom end rot is inconsistent watering, and perhaps over-fertilizing. Mulch your plants and keep the soil moist but not soggy to keep blossom end rot at bay.

Blossom End Rot Myths