Published on 02/14/01

Blossom-end Rot Even Uglier Than Name

Photo: Wayne McLaurin

Blossom-end rot on tomatoes isn't a pretty sight.
To a gardener in mouth-watering anticipation of the fruits of his labors, blossom-end rot looks even worse than the name sounds.

"It's a common disorder on tomatoes, peppers, eggplants and some melons," said Wayne McLaurin, an Extension Service horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.

"Blossom-end rot is a symptom of calcium deficiency in the fruit," McLaurin said. "It's usually most severe following extremes in soil moisture (either too dry or too wet)."

It shows up first as a small, darkened or water-soaked area around the blossom end of the fruit -- hence the name. As the fruits mature, the spot darkens, enlarges and sinks in. It may end up as a mere speck or involve more than half of the fruit.

Test Your Soil

A key to preventing blossom-end rot is to test your soil, McLaurin said. If it's low in calcium, applying lime several months before you plant can help.

Once blossom-end rot shows up on the fruit, there's little you can do for those fruits. Spraying calcium on the foliage, though, may help prevent it on developing fruit.

Keep the water supply uniform and regular, too, he said. Irrigate plants thoroughly and often enough to maintain a constant water supply without water-logging the plants.

"Tomato plants require 1 to 1.5 inches of water per week during growth and fruiting, depending on the soil type and weather," he said. "Extreme fluctuations in soil moisture can cause an increase of blossom-end rot."

Cut Back on Nitrogen

Fast-growing plants with lots of foliage tend to be more susceptible to blossom-end rot, McLaurin said. So cutting back on nitrogen levels can help. Use 5-10-10 fertilizer in place of 10-10-10 or 13-13-13 on tomatoes.

Tomatoes, peppers and watermelons planted unusually early, while the soil is still cold, are likely to have blossom-end rot on their first fruits. Planting a bit later helps reduce the problem.

Another thing that helps, McLaurin said, is to remove affected fruits when you first see symptoms.

"By the time a second set of fruit begins developing," he said, "the plant has an expanded root system better able to gather and deliver calcium to the fruits."

Other Prevention Steps

Other steps to control blossom-end rot:

  • Select sites with deep, well-drained soils. A large well-formed root system is better able to take up calcium and other minerals.
  • Soil test annually. Maintain the soil pH between 6.0 and 6.5 and soil calcium levels at or above 200 pounds per acre in the coastal plain and 400 pounds per acre in the rest of Georgia. If the soil calcium is low and the pH correct, incorporate gypsum (CaSO4) into the soil at 1 to 2 pounds per 100 square feet.
  • Work all fertilizer, lime and/or gypsum into the soil 8-12 inches deep. Avoid ammonium nitrate if possible. Ammonia inhibits the uptake of calcium.
  • Fertilize tomatoes with several smaller side-dressings, and then only after the tomatoes are the size of a nickel.
  • Mulch plants to conserve moisture and provide a more uniform water supply. Straw, pine straw, ground leaves or newspapers are all good mulches.
  • Avoid cultivation and hoeing. If you have to cultivate, keep it shallow to avoid root pruning.
  • Avoid severe pruning. Severely pruned tomato plants are more prone to develop blossom-end rot.
"Remember," McLaurin said, "controlling blossom-end rot is based mainly on proper calcium nutrition and proper irrigation scheduling."

For more on blossom-end rot, check McLaurin's publication on the Web at blossom-rot.html. Or contact the county Extension Service office.

Dan Rahn is a news editor with the University of Georgia College of Agricultural and Environmental Sciences.