Blossom-End Rot: How to Identify, Control, and Prevent Diseases

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Blossom-End Rot: How to Identify, Control, and Prevent Diseases

Blossom-end rot on tomato plant

Catherine Boeckmann

Here are tips on how to identify, control, and prevent blossom-end rot on your tomatoes and garden plants.

What is Blossom-End Rot?

Why are your tomatoes rotting on the bottom? Unfortunately, they probably have blossom-end rot. It is a common problem on tomatoes, eggplants, and squash caused by a low concentration of calcium in the fruit. (Calcium is needed for normal cell growth.) Blossom-end rot usually occurs when there are wide fluctuations of moisture, which reduces uptake and movement of calcium into the plant. When the demand for calcium exceeds the supply, the tissues break down.

Calcium deficiency during fruit formation can also be caused by too much nitrogen fertilizer, high salt levels in the soil, or damage to plant roots during cultivation. Be aware of these causes when caring for tomatoes in order to prevent blossom-end rot.

Some blossom-end rot is more or less normal in the first tomatoes of the season, since the plants are usually under stress during the initial fruit set. If the damaged portion is small, you can trim it off and enjoy the rest of the tomato. Hopefully the problem will ease, because after all, no one wants rotten tomatoes!

Identification

How to Identify Blossom-End Rot Damage

Usually blossom-end rot occurs when the fruit is green or ripening. It starts with a small, depressed, water-soaked area on the blossom end of the fruit. As the spot enlarges, it becomes sunken and turns black or dark leathery brown in color.

Control and Prevention

How to Control Blossom-End Rot

Remove the affected fruit. There is not much that can be done. If the affected fruit is pinched off, the plant might blossom again and set normal fruit.
Apply a liquid calcium fertilizer after removing the affected fruit.

Prevent Blossom-End Rot

The key is soil preparation. Maintain a soil pH around 6.5. Lime the soil with calcium and increase the ratio of calcium ions to other competitive ions in the soil. Add crushed eggshells, gypsum, or bone meal to the transplant hole to fortify calcium intake.
Maintain a more uniform moisture supply. Use mulches and/or irrigation to avoid drought stress. If it’s rainy, ensure plants have good drainage and soil dries out (but do not cease to water). Overall, plants need about once inch of moisture per week.
Avoid cultivating, or hoeing, near the roots of tomato plants.
For fertilizer, use nitrate nitrogen instead of ammoniacal nitrogen (as the latter increases blossom-end rot).
Avoid over-fertilizing during early fruiting when blossom-end rot is more likely to occur.
Staking the plants when they’re young can also be helpful. See our video on properly staking tomatoes.

Share your advice, questions, and comments on blossom-end rot below.