Blossom End Rot
This is mainly a problem on the fruit of tomatoes and peppers and manifests itself as sunken brown and leathery patches on the blossom end of the first few fruit of the season. These patches may eventually turn black and rot, as the damaged skin facilitates the entrance of rot-causing organisms. Blossom End Rot is a physiological problem rather than a disease and is caused by a lack of calcium in the plant. Calcium is rarely deficient in the soil because it is commonly added to adjust pH, so there is usually some other factor at work. Most often it is due to irregular watering inhibiting calcium absorption, but it can also be caused by too much nitrogen or magnesium.
To reduce the incidence of Blossom End Rot, keep the soil evenly moist and well mulched. Don’t give the plants too much nitrogen. If your soil is acidic then lime it, as this will add calcium too. Foliar feeding with seaweed or compost tea is a quick way to supply extra calcium. Indeterminate varieties are less commonly affected by Blossom End Rot than determinate varieties. You should remove affected fruit immediately, so the plant doesn’t waste energy on them (they won’t be of any use anyway).
Image: David B. Langston, University of Georgia, Bugwood.org