Answer: It sounds like you are describing blossom end rot which occurs at the end of the tomatoes (and peppers) opposite the stem. While it is attributed to calcium deficiency in the soil, it often appears in cases of fluctuating soil moisture levels and often occurs during periods of drought. 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 plants with a blossom end rot spray (contains calcium) which can usually be purchased from your local garden center. Usually, more attention to keeping the soil evenly moist will solve the problem. Next summer, you might want to dig down into the soil a bit and see how effective your watering actually is.
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