Blossom end rot is a disease which strikes melons, tomatoes, peppers, and eggplants. It is characterized by the development of a sunken, pitted area at the blossom end of the fruit, with the end of the fruit ultimately turning brown or black as it rots and withers away. The rest of the fruit may also fail to mature once blossom end rot develops. This condition is caused primarily by a lack of calcium, which inhibits the growth of the fruit, and a number of steps can be taken to control and prevent it.
In addition to calcium deficiency, two other things contribute to the development of blossom end rot: watering habits and temperatures. If plants are watered unevenly, they are at increased risk of developing the disease, and likewise if temperatures fluctuate radically. Gardeners should always water plants deeply, evenly, and regularly, and plants should be covered in cold weather to prevent blossom end rot and other problems.
The best way to deal with blossom end rot is to prevent it. Before planting, the soil should be amended with compost and fertilizer to provide lots of nutrients to the growing plants. It should also be amended with materials which will promote drainage, and some gardeners like to include lime to up the calcium levels. Once plants are established, mulching to help them retain moisture is recommended, as is providing support to keep fruit, vines, and leaves off the ground.
If blossom end rot does develop, the diseased fruits should be removed. Normalizing watering habits and providing supplemental nutrition to the plant can help to arrest the spread of blossom end rot, although sometimes the situation may progress so far that it's better to rip out the plant entirely.
Blossom end rot does not spread, although plants grown together will all develop blossom end rot if the growing conditions are poor. If blossom end rot does develop in your crop, it's a sign that you need to add more fertilizer and amendments to the soil next year in order to prevent it. These crops also like to be rotated, and you should not plant tomatoes, melons, peppers, and eggplants in the same spot year after year, because they strip the soil of nutrients. Planting beans or herbs for a few years can help restore the soil, especially if these crops are mulched back in during the winter.