Blossom End Rot

Why are tomatoes black on the bottom?


By Sandy Feather
Penn State Extension

Q. Last summer, the bottom of many of my tomatoes had black, rotten spots. Although the rest of the fruit was usable, these spots were not very appetizing. What can I do to avoid the problem this year?

A. What you are describing is blossom end rot.  It appears as dark brown or black leathery areas on the blossom end of maturing tomatoes. Despite the name, it is a nutrient deficiency rather than a disease. Blossom end rot develops due to one or more of the following factors: calcium deficiency, drastic fluctuations in soil moisture, overfertilization, or root damage caused by cultivation.

Calcium may be present in your soil mix in sufficient quantity, but is unavailable to the plant due to lack of soil moisture, or because the fine feeder roots have been severed by hoeing near tomato plants.


To avoid a recurrence of the problem, have your soil tested to make sure that calcium levels are adequate. Soil test kits are available from your local Penn State Extension office for a nominal fee. In Allegheny County, consumer soil test kits cost $12 each, and come with detailed instructions for taking a good soil sample and information to help you understand your soil test results. Customers ordering multiple kits at one time pay $12 each (check with them for their current price) for the additional kits. Send a check made payable to Penn State Extension and mail it to: 400 North Lexington Street, Pittsburgh, PA 15208. Write Attn. Soil Test Kit in the lower left corner of the envelope. You may also pick them up in person, Monday through Friday from 8:30 a.m. to 4:30 p.m. Take the sample according to the directions in the kit, and send it to the soil analysis laboratory in University Park, PA.  Follow their recommendations for limestone and fertilizer applications.


Above: Soil test results


Maintain even soil moisture by watering regularly and mulching your tomato plants. You may use an organic mulch such as straw, or an inorganic mulch such as black or red plastic. Mulching helps maintain soil moisture, moderate soil temperature, and it also keeps weeds under control. Be very careful about cultivating near tomato plants as you can sever fine feeder roots and make the plant think that it is under moisture stress - which will cause blossom end rot.

Although tomatoes are classified as heavy feeders - which means they require additional fertilizer applications through the growing season - do not apply more fertilizer than your soil test results suggest.  Too much nitrogen results in a lot of vegetative growth and little or no fruit, and it also can cause blossom end rot because excessive nitrogen blocks the plant from taking up calcium.


Blossom end rot can be a big problem for gardeners who grow tomato plants in pots. If you ever allow container-grown plants to dry out to the point of wilting after they have set fruit - even one time - all of the fruit present when wilting occurred will have blossom end rot. (As long as it does not dry out again, fruit that forms after will be undamaged). To avoid this, be sure to plant tomato plants in large containers - at least equivalent to a five-gallon bucket. Remember to water tomato plants thoroughly every day. The leaves of larger plants will shed rainwater outside the pots, so you have to water even when it rains. You can use a fine-textured mulch over the soil surface to help conserve water. Some gardeners use sphagnum moss for decoration as well as conserving moisture.


Vegetable garden topics

How to take a soil sample (video)

Interpreting soil test results (video)