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Blossom end rot appears as a light tan water-soaked spot on the blossom end of partially grown fruits. It is most prevalent on tomatoes but also affects peppers, squash and melons. The water-soaked area becomes sunken, leathery, and turns brown to black in color. Blossom end rot is caused by calcium deficiency brought on by fluctuating soil moisture, especially moisture stress. Contributing factors are the use of excessive amounts of nitrogen fertilizer (especially ammonium) and root pruning.
Prevent blossom end rot by protecting the roots from injury; don't cultivate too close to the plants.. Keep plants uniformly moist -- avoid wide cycles of wet and dry. Use plastic or organic mulch, as this helps provide a more uniform moisture supply. Plastic mulch works well. The edges of the plastic should be weighted down with soil to prevent the wind from getting under the plastic. This can be done by digging a shallow trench around the edge of the plastic, placing the edge in the trench, then covering the edge with soil. Cut small holes in the middle of the plastic to plant the tomato transplants.
It is best to use clear plastic which warms the soil faster in spring, producing a better root system and giving an earlier crop of tomatoes. When transplanting the tomatoes, be careful that the plastic does not touch the plants as this can injure the stem. A space of at least 1/2 inch should separate the plastic from the plant stem.
Blossom end rot usually affects the first clusters of tomato fruits and then is not as bad on subsequent production. The problem is also less serious during cooler weather.