Blossom End Rot

Blossom end rot is the bane of tomato growers. It starts as small brown spots on the bottom of the fruit and expands to a large, sunken brown or black leathery area.


Contrary to popular belief, soil calcium deficiencies rarely cause blossom end rot.

Blossom end rot of tomato (Mark) CC BY 2.0

Most soils contain plenty of calcium, but there are exceptions. An affordable, lab-based soil test is the only way to know what is in your soil. I urge everyone to get their soil tested every few years. The information is invaluable. But back to blossom end rot.


Blossom end rot occurs when calcium and irrigation are out of balance. Calcium is an “immobile” nutrient, which means it is difficult (i.e., uses a lot of water) to move around inside the plant after it is absorbed. Regular, frequent irrigation during the growing season provides plants with the water they need to get calcium where they need it.


High salt levels due to over-fertilizing increase the chance of blossom end rot. Lime amendments provide calcium when needed. And when watering, be sure that the roots are not dried out or saturated.


Blossom end rot can also affect summer squashes, such as zucchini. A similar problem, called bitter pit, affects apples. In either case, rotten bits can be removed and the rest of the fruit is fine for eating.


Now you know.​