Blossom End Rot
Blossom end rot is a common tomato problem. According to the UC Pest Note for blossom end rot, affected tomatoes will "show small, light brown spots at the blossom end of immature fruit". Typically, the damage occurs when the plants are water stressed or when the plants have had wide fluctuations in soil moisture, and when the fruits are about 1/3" to 1/2" of their full size. It's not caused by a pathogen, so there are no pesticide solutions.
The UC IPM website has cultural care information as well as a list of the most common pests and diseases for tomatoes. The most important cultural care recommendations include:
- Make sure the soil is not too dry (watering tomatoes).
- Follow recommended rates for fertilizers.
Additional information:
- UC Publication Managing Blossom-End Rot in tomatoes and Peppers.
A final note: the tomatoes are usually still edible. Just cut off the discolored part of ripe fruits.