Blossom end rot in tomatoes first appears while the fruit is green. First symptoms look like a water-soaked area on the blossom end of the tomato, but as the tomato ripens, the spot grows larger and darkens to a sunken leathery patch that may cover more than half of the tomato. Typically, by the time you realize your tomatoes are afflicted with blossom end rot, it's too late to correct the problem.
Causes of blossom end rot: Blossom end rot is caused by a lack of calcium supply to the tomato. Sometimes this is due to an actual calcium deficiency in the soil, but often irregular watering and an interruption in the uptake of calcium cause it. If weather is unusually dry and you notice symptoms of blossom end rot, chances are good that the real culprit is uneven watering.
Amending the soil: Perform a soil test on your soil before planting if blossom end rot is a common problem on your tomatoes. Your local extension office should offer soil testing services for a nominal fee and can advise you of the calcium level in your soil. Follow their instructions for amending the soil. Recommendations may include adding lime to adjust the pH to between 6.5 and 6.8 or adding gypsum. Amend soil in the fall to allow time for the lime to gypsum to make the proper changes.
Calcium chloride: According to Charles W. Averre and P. B. Shoemaker, Extension Plant Pathologists from North Carolina State University Extension, spray tomato foliage with calcium chloride once a week for four weeks when the first symptoms are noticed. Apply in the morning when temperatures are cool. Do not exceed the recommended strength.
Watering: Even watering is vital to the health of tomatoes and the prevention of blossom end rot, as both excessive moisture and lack of moisture may cause blossom end rot. Water tomatoes deeply to saturate the soil to the root level once or twice a week, depending on the weather and soil conditions. Those grown in containers or raised beds may require more frequent watering.
Check the soil one inch below the surface. If the soil feels dry, your tomatoes require watering. Mulching with black plastic keeps soil moist and is thought to produce a higher yield of tomatoes.
Care: Remove any effected fruit when you notice them to allow the plant to channel its energy into producing healthy fruit. Once the lesions from the blossom end rot appear on the tomatoes, it cannot be reversed. You can, however, cut away the affected areas and safely eat the tomato.
Monitor your tomato plants closely and watch for watery spots on green tomatoes, as these are your cue that blossom end rot is forming. Take corrective measures quickly to prevent your entire crop of tomatoes from developing the ugly black mark of blossom end rot.