Answer: The problem is most likely blossom end rot (BER). It is caused by a lack of calcium to the tip of the fruit. When the cells in tip of the tomato lack calcium during growth, they die and the black decay you see follows. You may have adequate calcium in the soil but still get BER due to soil moisture fluctuations (from wet to dry). This is quite common here in the low desert, where it can be difficult to maintain consistent soil moisture, which tomatoes really like. BER is best prevented by keeping soil evenly moist. Before each planting season, incorporate 4 to 6 inches of compost or other organic matter, which will help with this. Mulching will also help keep soil moisture even. You can cut off the brown portion and eat the rest of the fruit.
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