Why Are My Field Tomatoes Going Black?

Blossom End Rot

• Blossom end rot is a disease characterized by tomatoes turning black and rotting on the blossom end of the fruit. The earliest symptom of this disease is the appearance of tan colored lesions on tomato fruit that look glossy or water-soaked. The lesions eventually grow in size and turn black. The cause of blossom end rot is not exactly known, but scientists suspect the cause is typically a combination of improper watering schedules and a serious calcium deficiency in the soil in which the tomatoes are being grown.

Preventive Measures

• Since the primary cause of this condition is improper watering, the first and most important step in preventing it is to use an ideal watering schedule for your tomatoes. When you purchase your tomato plants, inquire as to the most ideal watering schedule for that particular variety of tomato or consult a university extension service. If you know your soil is seriously deficient in calcium, it may be advisable to add a calcium-rich soil amendment prior to planting. It is important to recognize that the twin causes of blossom end rot -- improper watering and calcium deficiency -- often go hand in hand. Tomatoes grown in calcium-rich soil, if improperly watered, will not be able to transport the calcium they absorb from the soil as they should.

Natural Control Methods

• For tomatoes that are already infected, adjusting your watering and fertilization schedules in response to blossom end rot is the first step to managing the disease. Again, watering and fertilization requirements are highly dependent both on your individual geographic conditions and on the specific variety of tomato you are growing. University extension services are often the most reliable sources of information for regionally focused watering and fertilization recommendations.

Other Considerations

• Blossom end rot is the most likely cause of rotting tomato fruit, but there are other possible diagnoses, such as insect feeding damage or other tomato diseases. If adjusting the watering and fertilization schedule does not result in healthy harvests, look for other symptoms on the tomato plants or for signs of insect feeding damage. If the fruit does not return to normal health, replant tomatoes with new seeds rather than saving and planting the previous season's seeds, since diseases can persist in seeds and be carried over from one growing season to another.