Blossom-end rot is a tomato disorder that I've been asked about a couple of times on Twitter and via Email. It isn't a disease caused by garden pests or something transmitted through infected plants. Blossom-end rot generally occurs when a developing tomato fruit doesn't get the necessary amount of calcium.
Fertilizing your tomatoes may not be of much help because even if your tomatoes are growing in nutrient rich soil, the problem is aided by uneven watering that don't allow the tomato plants to make good use of the nutrients in the soil. Prolonged droughts or heavy watering followed by long dry periods can also lead to blossom-end rot.
The symptoms of blossom-end rot can start off as a small, tan-colored spot on the blossom-end of the fruit. It can first appear when the fruit is green or while it is ripening. The problem worsens and develops into a dry, leathery lesion.
If your fruit shows signs of blossom-end rot simply remove the affected fruits and make sure your plant is properly watered from here on out to prevent the rest of your fruits being affected. There is no need to discard the whole plant, just the affected fruits.
When you water, provide a slow and steady drink at the soil level avoiding water getting on the stems,leaves and fruit-- which will lead to other problems. This is especially important if you're growing your tomato plants in containers. If your schedule makes it hard to keep a constant eye on the moisture
levels of your soil try making a slow drip irrigation system from
plastic soda bottles.
Blossom-end rot isn't just a tomato problem, it can also affect peppers and eggplants.