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, calcium deficiency in the soil is rarely what causes blossom end rot.

Tomato with blossom end rot (Kate Russell)

Most soils contain plenty of calcium. There are exceptions and the only way to know for sure is with an affordable, lab-based soil test. 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 very difficult (i.e., uses a lot of water) to move around inside the plant after it has been absorbed. Regular, frequent irrigation during the growing season provides plants with the water they need to get calcium where they need it.


Blossom end rot conditions are made worse when salt levels are too high due to over-fertilizing. Lime can be added to provide calcium. When watering, be sure that the roots are neither 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, the rotten part can be cut out and the rest of the fruit is fine for eating.


​Now you know.