Answer: It sounds like your tomatoes are suffering from blossom end rot which often happens in dry summers. It occurs at the end of the tomato opposite the stem as you described. While it is attributed to calcium deficiency in the soil, it often appears in cases of fluctuating soil moisture levels. Remedies include adding organic matter to the soil to increase its moisture holding capacity, keeping the plants evenly moist, using a mulch, and in some cases spraying the plants with a blossom end rot spray (contains calcium) which can usually be purchased from your local garden center. More care with the watering is usually the most effective immediate measure, and in the meantime you can cut away the ugly part and still use the tomatoes.
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