This common affliction of tomatoes is the result of a physiological disorder rather than a disease. First described over a century ago, blossom end rot is linked to low levels of calcium in the young fruits as well as to environmental stress. Blossom end rot also occurs in watermelons, peppers, and squash. Other calcium-related disorders include blackheart of celery, internal browning of brussels sprouts, cavity spot of carrots, and cork spot of apples and pears.
Calcium is essential in the formation of new cells. Among other functions, it maintains the structural integrity of plant cells as well as the permeability of cell membranes. Calcium is absorbed from the soil by young root tips and is transported to the growing tips and fruits, but once deposited, it is immobile. It cannot be translocated to growing leaves or developing fruits.
A combination of low calcium levels at the blossom end of the fruit, coupled with stress, causes cell membranes to break down and release their contents, creating what is termed blossom end rot.
SYMPTOMS: Blossom end rot is first visible on young, rapidly expanding fruits within two weeks of the blossom's fertilization. Initially, it is a tiny, slightly sunken, dark green and watery lesion. Eventually this turns tan, shrinks, and collapses to produce an enlarged dark brown leathery region. Often the lesion turns black after being colonized by a saprophytic black mold (Alternaria spp.). Although the external damage is most noticeable, internal blossom end rot also exists, in which the damage occurs inside the fruit.
CONTROL: Ensuring an adequate supply of calcium is the first step to reducing the incidence of this disorder. Where soils are acid, the application of lime to achieve a pH of 6.5 to 7.0 will provide sufficient calcium. Elsewhere, gypsum should be used to provide calcium without changing the pH.
Equally important, however, is reducing the stress on the developing plant. Keep moisture levels uniform by watering regularly and by maintaining a mulch layer around the base of the plants. Any cultural practice or environmental condition that affects the growth of new roots can be expected to impair the uptake of calcium. Avoid damaging the delicate feeder roots during transplanting or when cultivating near the plants.
Finally, avoid overfertilizing, because an excessive accumulation of salts in the soil will inhibit the uptake of calcium and will also encourage rapid, excessive vegetative growth that will compete with the fruits for calcium.
Tomato varieties differ in their susceptibility to blossom end rot. Large-fruited varieties tend to be more prone to the disorder than smaller-fruited ones. ‘Daybreak’–an early, determinate, medium-size, red-fruited variety—is touted as especially resistant to blossom end rot.