Tomato dry rot, also called blossom-end rot, is the sign of a calcium imbalance, according to Gardener's Supply Company. The condition frequently occurs when the plant starts out in wet soil that later becomes too dry, interfering with calcium absorption.
Blossom-end rot causes a portion of the fruit to turn dark and dry, and the tomato eventually decays if it is left on the vine. Gardeners can address several issues to prevent rot, including making sure that the soil has low nitrogen, low salt and balanced acidity, Gardener's Supply Company notes. Deep watering of up to 6 inches twice a week during dry spells also helps tomatoes absorb calcium consistently.