Blossom End Rot
Do any of your tomatoes look like this? If so, you have blossom end rot. It is an environmental disorder (not caused by a pathogen) that causes fruits of tomatoes to turn brown-black on the bottom (blossom end). Blossom end rot can also affect peppers, eggplant, cucumbers, melons, and squash.
Blossom end rot is not caused by a disease, and there are no pesticide solutions. Here are a few things you can do to alleviate or at least lessen the problem:
Avoid moisture stress. To reduce rot, monitor soil moisture to make sure that the root zone neither dries out nor remains saturated. If possible, water deeply but infrequently. Rather than giving the plants a light irrigation every day, it is better to give them a good, long soak every few days, more or less, depending on growing conditions and the type of soil in your garden. Proper irrigation is most critical during fruit set and development. Mulching with 2 to 3 inches of materials such as grass clippings, straw, and leaves prevents rapid soil drying and allows roots to take up available calcium efficiently.
Don’t over-fertilize. Follow recommended rates for fertilizers (as stated on the package directions). Too much nitrogen during early fruiting, especially with nitrogen made from ammonia, ties up calcium in the soil chemistry.
For more information on growing tomatoes, or other problems associated with tomatoes, click here.