Blossom end rot is difficult to control

Gardeners can start growing tomatoes early. There is a select fraternity of gardeners infatuated with picking the earliest tomato of the season.

Vegetable gardeners get really depressed when they look at their tomatoes and see the fruit turning black on the bottom. Many people have experienced this common problem, called blossom end rot, this year. Though it happens to some degree almost every year, the extended drought we had earlier and hot weather make a recipe for disaster on the tomatoes.


Terry Kelly, a vegetable specialist for the University of Georgia Cooperative Extension, recently shared more information to county agents on blossom end rot. As I have discovered, there is more to the problem than I once thought.

Blossom end rot is a calcium deficiency in the tomato. There is normally plenty of calcium in the soil, and a tissue analysis would probably show that there is plenty in the leaves, too. The lack of calcium is actually in the fruit, but by the time you see it on the tomato, it's too late to do anything for it.

Worse, it's also hard to correct the problem on the developing fruit.

Tomatoes are predisposed to blossom end rot very early in their development. When they're hardly visible, they have a critical need for calcium. If the calcium does not get to the fruit at that critical phase, the stage has been set for the blossom end rot.

Calcium is an immobile element in plants. After it becomes part of the leaves or stems, it isn't going to move to newly developing parts of the plant, such as the fruit.

As the plant takes up water, it pulls calcium from the soil. Under hot, dry conditions, the plant is getting great amounts of water, which is quickly transpired through the leaves to keep the plant alive.

The plant is acting almost like a chimney as it sucks water from the soil and moves it out through the porelike structure in the leaves. Unfortunately, this doesn't allow for a lot of lateral flow of calcium to the developing fruit, so they can become deficient.

What is a gardener to do? Sometimes there isn't much that will actually help.

The overriding factor is soil moisture. It's hard to keep the soil moist when it is hot and dry. This is particularly true for those who grow tomatoes in pots - the plants dry out so quickly. Sometimes even once-a-day watering is not enough.

Too much water can be just as bad for blossom end rot as too little.

The best way to help avoid blossom end rot is a consistent supply of moisture. Try to avoid cycles of very wet followed by excessive drying. You can now see how blossom end rot can happen a lot when we go through rainy periods, because the soil will get very wet, then it may dry out more than it should when it turns dry again.

For years, I have suggested that gardeners use a foliar calcium chloride spray, sold as Stop Rot, to reduce blossom end rot, but Dr. Kelly says there is little evidence that these applications help.

Some people add gypsum to the soil at the start of the season to make sure there is enough calcium. Most gardeners know to add lime periodically to their garden because it contains calcium and magnesium and raises the soil pH to a desirable level.

How often lime is needed depends mostly on the soil structure. Usually every two to three years is adequate. The problem remains getting it to the fruit.

Blossom end rot is very hard to control and very frustrating if your tomatoes get it.

In some years, you almost just have to live with it and hope you'll get enough good fruit to make it worth it.

Sid Mullis is the director of the University of Georgia Extension Service office in Richmond County. Call (706) 821-2349 or send him an e-mail at



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