August 6, 2011

1 - Blossom end rot in tomatoes and other garden vegetables is due to a failure of water to transport calcium to the tomatoes and other fruit as they form.

*Yard and Garden - August 6, 2011


My tomatoes have finally started turning red. Every tomato I have picked thus far has a sunken, black area at the bottom of the fruit (away from the stem). I determined that this is called "blossom end rot" and that it is caused by either a calcium deficiency or by lack of water. Which one is the cause? What can I do to prevent this problem?

- Alvin S.


You have indeed described blossom end rot in tomatoes. The fruit of other plants grown in the garden can also display this disorder. Cucumbers, melons, and squash are examples of some other garden plants that may develop blossom end rot.

The ultimate cause of this disorder is a calcium deficiency in the fruit of the plant. Calcium is needed by plants as they form their fruits to "glue" the cells together. Calcium (and all other minerals) must be dissolved in water to be extracted from the soil and transported throughout the plant. On hot, sunny, windy New Mexico summer days, water may not reach the ends of the newly forming fruits of susceptible plants. If the water does not reach these forming fruits, blossom end rot begins. The fruit may continue to develop, but the cells cannot form properly due to the absence of the calcium. At first this problem is not obvious, but as the fruits enlarge the area of improperly formed cells collapses and forms the blackened area you described. We often do not see the problem until we start harvesting the fruit. Melons and squash may just stop developing and shrivel up indicating that their cells did not form properly.

Some tomato varieties are more resistant to blossom end rot, so try several varieties each year to identify those that do best in your soils and under your irrigation regimes.

Garden books for Eastern gardeners advise application of lime to the soil - Do Not Add Calcium to New Mexico Gardens! Most New Mexico gardens do not have a deficiency of calcium in the soil, the problem is the water needed to transport the calcium throughout the plant. Soil tests will determine if you have the rare calcium deficient New Mexico garden soil, but without the test, do not add lime!

Some gardeners have constructed shade structures (lath houses) over their gardens to provide shade and wind protection. This reduces blossom end rot, sun scald, and provides other benefits (water conservation, more comfortable gardening environment, frost protection, and protection from hail). Such a structure must be designed to allow adequate room for the plants and the gardener under the structure. Other gardeners choose to plant plants susceptible to blossom end rot in areas that get afternoon shade. Best adapted varieties combined with adequate irrigation combined with wind protection and shade can greatly reduce the problem.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email:, office: 505-865-7340, ext. 113.


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