Greenhouse Tomatoes

Why do many fruit have a blackish area on the bottom?

Blossom-end rot (BER) is probably the most common physiological disorder in tomatoes (greenhouse, field, or home garden). It appears as a dry (not mushy) dark brown or black area, usually at or near the bottom of the fruit. It is not a disease. While BER is caused by lack of adequate calcium in the fruit, the culprit is often lack of water. Without a steady stream of water, the plants have difficulty getting enough calcium into the fruit, resulting in BER. Incidentally, BER can sometimes occur on the side of fruit, and occasionally only on the inside of fruit without even showing signs of it on the outside. So, be sure plants are getting enough calcium and don't let plants wilt.