Identifying and Controlling Blossom End Rot

Blossom end rot on beefsteak tomatoes (Solanum lycopersicum) caused by irregular watering
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Blossom end rot (BER)  can most easily be identified by a discolored, sunken spot at the blossom end of the fruit, most commonly tomatoes, but also squash, melons, and zucchini. The spot will start out small, and grow larger and darker as the fruit continues to grow. Generally, blossom end rot causes the fruit to ripen prematurely, resulting in inedible fruit.

Damage to Plants

The plant will generally show no signs of damage to leaves or stems.

A plant can appear to be quite healthy, yet the fruit will show the tell-tale signs of blossom end rot. Blossom end rot is more common with the first few fruits of the season if you planted in cold soil. It's also common when your garden experiences extremes in soil moisture levels, either too dry or too wet.

Life Cycle

There really is no life cycle to blossom end rot. Each fruit should be examined for signs of rot. If it is an issue that occurs year after year, the soil should be tested to check the calcium level. If there is generally no problem growing tomatoes in your area, look to your watering practices as the likely cause. 

Treatment and Prevention

Blossom end rot is caused by calcium deficiency. While this may be a result of low calcium levels in the soil, more often it is the result of erratic watering. When the plant is allowed to get too dry, or given too much water over a period of time, its ability to absorb calcium from the soil is diminished.

Blossom end rot can also be caused if a gardener is too zealous in fertilizing. Too much nitrogen in the soil can cause fruit to grow so rapidly that it cannot uptake calcium fast enough, resulting in blossom end rot. 

If your soil is low in calcium (as determined by a soil test) the easiest solution is to add lime several times per year, according to the directions on your soil test results.

A common, and erroneous, treatment for blossom end rot is a spray treatment of a calcium solution applied to the leaves, stems and fruit will do little for the plant, since calcium really is not absorbed this way. If calcium deficiency truly is the problem, then soil amendment with lime or bone meal is the solution.

If the issue is erratic moisture (much more likely) here are some tips:

  • Pay closer attention to watering. If blossom end rot has been an issue, try to make sure that your soil isn't allowed to dry out. The best defense against blossom end rot is a nice, consistent soil moisture level.
  • Mulch. By adding a three-inch layer of organic mulch, you can help maintain adequate soil moisture levels, even during dry spells. It is best to add the mulch after your soil has warmed in the spring; adding it too early can result in your soil staying cold longer than it should.
  • Plant susceptible crops (such as tomatoes, melons, squash, peppers, and eggplants) in well-draining, deep soil that has been amended with compost or well-rotted manure. Soil amended with plenty of organic matter will retain moisture better and supply plenty of nutrition (including calcium) to your plants.