My tomatoes have dry, black patches on the botton end.  What is this and how can I stop it?
Image of blossom end rot of tomato

Blossom end rot is a common problem of tomatoes, but is also found on peppers, eggplant, squash and watermelon. It occurs as a flat, dry, sunken, brown rot that is seen on the blossom end of the fruit, opposite the stem end. The rot is first seen as a small, water-soaked spot on the base of half-developed fruits and continues to enlarge as the fruit matures. The size of the rotted area varies, but can cover up to 50% of the fruit.

On peppers, the affected area is tan and is often confused with sunscald, which causes a white lesion. Affected areas are often colonized by secondary fungi, which affect the remaining fruit making it useable. This problem is not an insect or disease problem, but is a physiological disorder associated with a calcium deficiency in the developing fruit.

Calcium is an important component for normal cell wall development; when inadequate levels of calcium are available to the rapidly developing distal tissues of the tomato, the result is cell breakdown. This condition is rarely the result of a lack of calcium in the soil, but rather occurs when plants cannot pull up calcium quickly enough for the developing tissues. Calcium is a nutrient with limited mobility in plants; it must be dissolved in water to move within a plant, so water deficits are a frequent contributing factor. The distal, or blossom end, tissues of the tomato fruit also contain fewer vascular bundles, which move water and nutrients from one part of the plant to another, so are most susceptible to a lack of calcium.

Drought stress, low daytime humidity, high temperatures and rapid, vegetative plant growth caused by excess nitrogen applications favor blossom end rot development. Drought stress and low daytime humidity cause plants to lose large amounts of water through transpiration, resulting in more water being sent to the leaves and less to the developing fruits. High temperatures contribute to this condition by causing faster fruit enlargement, and a greater need for calcium.

Cultural techniques that can be used to reduce the incidence of blossom end rot include the following:

  • Prevent drought stress on plants by providing at least 1 inch of water per week, and greater amounts for plants in sandy soils or during very hot, dry conditions
  • Use an organic mulch like wood chips, clean straw, pine straw, peat moss, compost, herbicide-free grass clippings to preserve soil moisture
  • Avoid excess fertilization, especially with ammoniacal nitrogen sources

Foliar applications of calcium have little effect on this condition, due to the poor absorption and movement of calcium from the leaves to the fruits.

More information:
Blossom End Rot of Tomato, Pepper & Eggplant , The Ohio State University Extension