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 on unripe tomatoes (Scot Nelson) Public Domain

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. If a soil test indicates low calcium levels, lime amendments can temporarily correct the problem.

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. Lime amendments provide calcium when needed. Over-fertilizing can also lead to blossom end rot by increasing soil salt levels.

Blossom end rot can also affect summer squashes, such as zucchini. A similar problem, called bitter pit, affects apples. In either case, you can remove the rotten bits and enjoy the rest of the fruit.

Now you know.​