Tomato bottom rot, also called blossom end rot, appears as a black sunken spot on the tomato's end opposite of the stem. This disorder may become disastrous to your tomato crop, with a loss of 50 percent or more of your crop, estimates the Ohio State University Extension. It begins as a small area that looks water-soaked, but gets larger and turns black and leathery. It's not a parasitic organism, but instead a physiologic disorder caused by a low calcium concentration. Stopping the condition involves applying a quick dose of calcium and following with measures you should take each season to prevent bottom rot.
Spray the tomato plant foliage with a calcium chloride solution product in the morning or late afternoon about once a week. Follow the specific application instructions provided with the product, especially since calcium chloride is toxic to plants when applied too frequently or excessively.
Water your tomato plants once or twice a week when they are not receiving sufficient rainfall. Water deeply to provide sufficient water. Tomato plants require about 1 inch of moisture per week for sufficient growth.
Add a layer of mulch around your tomato plants to hold in moisture.
Test your soil pH with a pH tester or have your soil analyzed. Purchase a soil test from any garden center or have your local extension office test the soil for you. Aim for a soil pH around 6.5.
Fertilize your tomato plants with a fertilizer based on the results from your soil test. Follow the specific application directions provided with the fertilizer. Do not use a high nitrogen fertilizer; instead use one that has more potassium, such as a 5-5-15. Avoid over-fertilizing, as that can cause additional problems.