sounds like "blossom end rot". You can Google that and read all about it. I save eggshells for the calcium and crush them, put a handfull in each hole before transplanting the seedling, has been working fine ::::knock on wood:::::. There is also a calcium spray.

Nicole, it happens to even the experts. About the time that I claim that I won't have it this year, two fruit on one of the Medford's showed up with it. It's the plant's inability to take up calcium and may happen even if they are growing in a limestone quarry. Everyone has a theory as to exactly why it happens but none have ever been proven 100%. It's common with some varieties, mostly the early types, and rare with others, mostly late types. It seems that the plants need more time to develop a proper system for processing calcium. No matter what variety is affected, there is one thing for certain. You can throw many dollars worth of cure at the plants and it will go away. You can do nothing and it will go away just as fast!

Martin

Thank you for the follow ups. I have a tomato turning color right now, and so far no rot. So, if I understand what you are saying, is even if some fruit on the plant has rot, I can produce more fruit off the same plant that may be fine?

Nicole, you are correct in your assumption. Unless you are growing a variety which is notorious for Blossom End Rot, your plant has probably quickly outgrown the problem. Usually it is the Roma or paste types which are affected but it supposedly should happen to almost any variety. Why it doesn't has not yet been explained.

Martin