Blossom End Rot is a common tomato problem in the Carolinas. As the fruit starts to mature, the bottom of the tomato turns brown or black, ruining the tomato. This can happen to multiple tomatoes on the same plant.
Blossom End Rot is not caused by insects or a soil disease… it is a simple calcium deficiency in the plant. While it is easy to diagnose Blossom End Rot, the fix is not as straight forward. The problem is usually not a calcium deficiency in the soil, but an issue with the calcium getting into the plant… and this all due to uneven watering and the soil drying out between waterings.
Calcium is taken up into the plant with water through the roots. If the water supply is interrupted, so is the source of calcium. The easiest way to treat Blossom End Rot is to maintain a steady soil moisture. Drip irrigation on a timer can help provide even watering, and mulch around the plant can slow evaporation from the soil.
An easy way to avoid Blossom End Rot is to choose tomato varieties that are resistant. If you grow a variety and have an issue, avoid that variety in the future. I have noticed that plum or Roma style tomatoes are more susceptible to Blossom End Rot.
Adding calcium to the soil is another method to fight Blossom End Rot. A soil test will let you know if your soil needs extra calcium. Some people add eggshells to their soil where tomatoes are grown. Garden lime is also a great way to add calcium to your soil. It is water soluble and will increase the calcium in the soil rapidly. Calcium sprays will not work in preventing Blossom End Rot. The skin of a tomato will not absorb calcium, and calcium absorbed by the leaves will not make its way to the fruit.
While Blossom End Rot is common in tomatoes, it can also occur with peppers, watermelon, eggplant, squash, and cucumbers.
Remember that while Blossom End Rot is a calcium deficiency in the plant, it does not always mean that there is calcium deficiency in the soil. The first step in treating should be making sure the soil does not dry out. Most times even watering, and stable soil moisture will cure Blossom End Rot.