Blossom End Rot

A common foe of tomato, pepper, melon, and eggplant growers alike. BER is an unsightly rotting patch at the bottom of the fruit.

This condition is often associated with a deficiency of calcium, however, it can occur even if there's adequate calcium in the soil. Some reasons for it could be:

-Inconsistent watering
-Extreme temperatures
-Stress

Simply said, it happens when a plant is unable to access calcium to create fruit. If there is a calcium deficiency in the soil, it would make itself apparent in deformities of new leaves.

It is likely your plant is stressed in some way, or, some determinate tomatoes will show BER in just the first flush of new fruit. Simply pick those off so the plant can focus on new fruit - it is usually a shock to these plants to set so much fruit at once, so the demand of calcium is too high for the plant to manage until it matures more.

If you observe and test your soil to be deficient of calcium, the best additive to fix this is gypsum (calcium sulfate).