Tomatoes That Are Blackening on the Bottom

Damage

• Blossom-end rot causes more than blackening on the bottom of the tomato. Affected fruits may become malformed or fail to reach their full mature size. Not all affected tomatoes develop a black spot. The condition may cause rotting internally on the blossom end but the tomato looks healthy from outside. Pressing on the end reveals the fruit is soft and mushy inside. Mold and mildew may also grow around the black, rotted spot on the tomato.

Nutrient Deficiency

• Nutrient deficiencies lead to blossom-end rot. Tomatoes cannot absorb the necessary nutrients from soil unless the pH is between 6.5 and 6.7. Performing a soil test and adding lime to the soil as recommended by the test prior to planting prevents some rot problems. Calcium-deficient plants also exhibit blossom-end rot. Applying a calcium spray once the second flush of flowering occurs helps provide the necessary calcium to prevent the tomatoes from blackening. Also avoid over-fertilization, as too much nitrogen fertilizer in a single application can cause issues on the fruit.

Drought Stress

• Periods of drought and dry soils weaken the tomato plant. Blossom-end rot occurs when a dry period is followed by heavy irrigation. Repeated drought followed by heavy soaking makes the tomatoes even more susceptible to the condition. Keep plants evenly watered by providing 1 to 2 inches of water weekly via irrigation or rain. Water the plants more frequently and deeply during drought periods so the soil retains an even moisture level. Drought isn't the only cause of blackened tomato bottoms, so even properly watered plants can be afflicted.

Prevention

• Calcium sprays, irrigation and proper fertilization aren't the only methods for preventing or controlling blossom-end rot. Mulching the garden bed prevents water loss and further alleviates drought stress. Plant the tomatoes outside only after the soil has warmed and after frost danger is past. Plants grown in soil that is too cold put on new growth slowly and become more susceptible to end rot. Perform yearly soil tests to determine the lime and fertilizer needs so the tomatoes do not suffer from over- or under-fertilizing.