Blossom-end rot of tomatoes

Lately, I have received a lot of phone calls about this particular disorder, so I thought I would share some information and management strategies for this particular disorder.

Blossom-end rot is not caused by living causal agent, like a pathogen, but rather, a deficiency in calcium. This disorder is common in tomatoes, peppers, squash, and watermelons. If you had your soil tested, and the results suggested you have an adequate level of calcium in your soil, how did your tomatoes get a deficiency in calcium?

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Calcium moves quite slowly in plants and even slower in fruit, so adequate levels of calcium could still result in this deficiency. Indirectly, this disorder is caused by excessive levels of moisture or not enough moisture. Considering the excessive rainfall New Orleans received last month (about the time your tomatoes were in flower), it is not surprising to see a lot of blossom end rot. This disorder can affect all ages of plants, but is more common in 1/3-1/2 grown tomatoes. 

It begins as a small, water-soaked spot and develops into a dark brown, leathery spot that may involve half the fruit. The surface of the spot shrinks and becomes flat or sunken. Blossom-end rot is caused by a lack of calcium in the developing fruit. The uptake of calcium from the soil by the tomato plant can be reduced by fluctuations in soil moisture – either excessively wet soil or excessively dry soil.

The disorder commonly occurs when plants are growing rapidly and luxuriantly early in the season and are then subjected to prolonged dry weather. The disorder may be more serious on the windward side of a garden and on staked tomatoes rather than on unstaked or bushy plants.

Prevent blossom-end rot by maintaining a soil pH around 6.5 and uniform soil moisture by irrigating and mulching, and avoid heavy applications of nitrogen.

Control blossom-end rot by spraying foliage with 2 level tablespoons of 96 percent calcium chloride in 1 gallon of water at seven- to 10-day intervals. Do this for three to four applications. Begin spraying with first appearance of symptoms. Overdosing plants with calcium chloride may result in leaf burn. Spray on cloudy days or wait until the sun is low.

Calcium nitrate also may be used. Use 1 rounded teaspoon per plant. Apply calcium nitrate into the soil about 8 inches from tomato plant stems. A second application may be needed several weeks later as well. If foliar applied, use 2 level tablespoons per gallon of spray applied late in the day.