Experts and BHG readers answer.
rotting bottoms of tomatoes
This is a condition called blossom end rot. Roma tomatoes are rather susceptible to it; other varieties less so. It is caused by lack of calcium in the fruit, and is usually precipitated by fluctuating soil moisture conditions. Mulching the soil to maintain more even moisture can help. You may also like to try other varieties of tomato. A paste type that I like a lot is called Pompeii.