Answer: What you describe sounds like Blossom End Rot. Blossom-end rot is a physiological disorder of tomatoes and is caused by a calcium imbalance within the plant. Fluctuations in soil moisture, excessively wet or dry soil, excessive nitrogen fertilizer, roots damaged by cultivation, very high or low pH, or soils high in salts can all prevent roots from taking up enough calcium to satisfy the plant's rapid cell development. The result is a water-soaked spot at the blossom end of the fruit that enlarges, turning dark brown and leathery. Rot may set in at the spot. Blossom-end rot is common when plants grow rapidly in the beginning of the season, then set fruit during dry weather. As little as 30 minutes of water deficiency at any time can cause blossom-end rot.
To prevent blossom end rot, keep plants uniformly watered throughout the season. Water deeply; wet the soil at least 6 inches down. Apply mulch to maintain soil moisture. Keep soil pH around 6.5. Try spraying with a calcium based preventative such as Rot-Stop, a ready-to-use spray available in garden centers. Use a fertilizer that's formulated for tomatoes and other vegetables.
This problem does not spread to other tomaotes and not all the fruit on your plants will be affected, especially if you follow the above growing guidelines.
Best wishes with your tomatoes!
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