Blossom-end rot is a fruit disorder that affects members of the nightshade family, especially tomatoes, eggplants and peppers. Although this nasty ailment looks like an infection or disease, it’s actually caused by a nutrient deficiency.
Blossom-end rot begins as a small, watery lesion on the blossom end of tomatoes and eggplants, and on the side of the fruit near the blossom end of peppers. As it progresses, the lesion grows and turns dark and leathery. In some cases it takes over the whole bottom half of the fruit.
Calcium deficiency is the main cause of blossom-end rot. A relatively large concentration of calcium is required to support normal fruit cell growth. When rapidly growing fruit do not receive enough calcium, the cell structure weakens and breaks down causing rot on the blossom end of the fruit. Once fruit has blossom-end rot, it is no good, but you can take measures to prevent it from occurring in the first place.
You can prevent blossom-end rot by ensuring that the soil contains enough calcium. Add lime to the soil to increase calcium content, and choose nitrate nitrogen fertilizers over ammonium nitrogen fertilizers—the ammonium reduces the plants ability to take up calcium from the soil. Maintain the soil pH around 6.5. Moisture fluctuations also affect the plant’s ability to uptake nitrogen. Provide about one inch of water weekly on a consistent schedule.