Answer: Blossom end rot is basically caused by irregular watering and/or a calcium deficiency. It tends to occur when a long dry spell hits after a period when the plants have grown very fast or after a very wet rainy stretch. In other words, wide fluctuations in water availability can cause it. Try to ensure your plants receive a steady supply of moisture (if in doubt you can check the soil with your finger to see if it is moist or dry), mulch them well, and the problem should subside.
In addition, you might want to check your soil. Often, the best way to solve the deficiency problem is simply to keep the soil pH between about 6 and 6.8 as measured by a soil test. Soil test kits are commonly avalable at garden centers and through the County Extension. Your County Extension staff can help you with the test and interpreting the results and determining how to adjust the pH or add calcium if it is necessary. Your local County Extension can be reached at 863-0110.
The spot usually occurs when the tomatoes are about a third to half way ripe, so if you check them early you can remove the marred ones to preserve the plant's energy. Good luck with your tomatoes!
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