Blossom-end Rot in Tomatoes: Causes and Prevention

Tomato growers must take care to avoid blossom-end rot (BER), a disorder that can dramatically reduce both quality and quantity of tomato fruit.

By using a few simple irrigation and fertilization practices, damage due to blossom-end rot can be reduced.

Blossom-end rot is a physiological disorder, not a disease. It is easily identified as a brown, leathery rot developing on or near the blossom-end of the fruit. It starts with a dry brown lesion the size of a dime, generally increasing in diameter as the condition worsens. In time, lesions often become covered with a black mold.

Blossom-end rot is caused by calcium deficiency, usually due to fluctuations in water supply. Because calcium is not a highly "mobile" element in the plant, even brief changes in the water supply can cause blossom-end rot. Droughty soil or damage to the roots from excessive or improper cultivation, for example root pruning, restricts water intake and can prevent plants from getting the calcium they need from the soil. Also, if plants are growing in highly acidic soil or are getting too much water from heavy rain, over-irrigation or high relative humidity, they can develop calcium deficiency and blossom-end rot.

To control blossom-end rot, take the following steps:

  • Keep the soil's pH at 6.0 to 6.5. Perform a soil test and apply the recommended rate of lime, using dolomitic or high-calcium limestone. Be sure to apply lime two to four months before planting tomatoes.
  • Apply the required amount of fertilizer based on soil test results for tomato. Applying too much fertilizer at one time can induce BER. Following soil test recommendations is the best way to fertilize properly.
  • Use mulches to conserve moisture. Use pinestraw, straw, decomposed sawdust, plastic or newspapers. Mulches conserve soil moisture and reduce incidence of blossom-end rot.
  • Give your plants adequate water. Tomato plants need about 1.5 inches of water a week during fruiting. Extreme fluctuations in soil moisture can result in a greater incidence of blossom-end rot.
  • If your plants develop calcium deficiencies, spray them with a calcium solution at the rate of 4 pounds of calcium nitrate or calcium chloride per 100 gallons of water (or four level tablespoons per gallon of water). Be careful with calcium chloride. If day temperatures are greater than 85-90 F , calcium chloride can burn plants. Under high temperatures, use calcium nitrate. You should spray two or three times each week, beginning when the second fruit clusters are blooming. Spraying calcium is not a substitute for proper irrigation and fertility management.
  • Some varieties of tomatoes tend to be more sensitive to conditions that cause blossom-end rot. Try growing several varieties and keep notes as to their performance under your growing conditions.
  • If you experience severe problems with blossom-end rot, remove the infected fruits. Once a fruit develops blossom-end rot it will not re-grow or repair the infected area. Leaving the damaged fruit could serve as an entry point for disease-causing bacteria, fungi and insects.
    SOURCE: Dr. Joe Kemble, Extension horticulturist, Alabama Cooperative Extension System (334) 844-3050.