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Plant Pathology Infobytes

May 31, 1996

What's Wrong With My Tomato Fruit?

Dark brown or black blotches on the bottom of tomato fruit is an indication of blossom-end rot, a problem which is currently showing up in many vegetable gardens across Mississippi.

The first symptom of blossom-end rot is a slight discoloration, water-soaked in appearance, occurring at the blossom-end (bottom) of the fruit. This area enlarges rapidly, producing a dark brown or black sunken area. The skin over the affected area becomes dry and leathery.

Blossom-end rot is caused by a shortage of calcium in developing fruit. This may be due to a lack of calcium uptake from the soil or to extreme fluctuations in water supply. Tomato plants growing in soils low in calcium and soils which are alternately wet and dry during fruit development are more likely to show blossom-end rot. This problem also tends to be more severe when plants are over- fertilized with nitrogen fertilizer.

The following control recommendations will reduce the amount of tomato fruit lost to blossom-end rot:

  • Since this problem is closely related to water supply, it's important to monitor soil moisture and irrigate before signs of moisture stress are apparent. In general, tomato plants need at least one inch of water per week in the form of rain or supplement- al irrigation.
  • If your plants aren't mulched, consider doing so, since this practice will help maintain uniform soil moisture conditions.
  • Apply several sprays of calcium chloride (available at garden supply stores under a variety of trade names) if the disease begins to develop. Follow label directions. Sprays containing calcium chloride will help to prevent further development of the problem but will not cure fruits already affected.
  • Follow a recommended program of fertility and avoid excessive application of nitrogen fertilizer.
  • Collect a soil sample to determine levels of calcium and correct any deficiency problem through application of lime this fall.
  • Remove fruit showing symptoms of blossom-end rot when the problem is first observed. This practice will reduce the drain of food and nutrient materials which otherwise would be available for develop- ment of other fruit not affected by blossom-end rot.

Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.