What Is Causing the Bottoms of My Tomatoes to Turn Black?

Blossom End Rot

• If the bottoms of the tomatoes are turning black, the most likely problem is blossom end rot. This is caused by plants not getting enough calcium --- usually from the soil being too dry, wet or cold. The fruit will develop a stained look near the spot where the blossom was sealed off from the fruit. As the fruit continues to develop, the stain will turn black and curve inward. Blossom end rot will not infect other fruit.

Remedy and Prevention

• To remedy blossom end rot, make sure the plant is well-irrigated. This means not too dry or too wet. Try to maintain a moderate level of moisture that will nurture the growth of the plant. The fruits that are already infected will not get better, but irrigating properly will help the plant get the calcium it needs to develop new fruits. Adding too much nitrogen to the soil can inhibit the calcium, so a low-nitrogen fertilizer will help fruit development. To prevent blossom end rot, plant tomatoes in a well-drained soil, and make sure they are watered regularly. The soil can be prepared with lime, bone or some other material that contains calcium. Eggshells pulverized to a fine powder and added to the soil during planting provide a convenient source of calcium.

Fruits Touching the Ground

• A few other likely possibilities for black spots on the bottom of tomatoes are buckeye rot and Rhizoctonia soil rot. Both of these problems come from fruits that touch the soil. Buckeye rot can infect both green and red tomatoes, and will form a dark circle that is slightly hardened. Rhizoctonia soil rot looks like small, black, round lesions and usually attacks ripened fruit. As the infection breaks down cell walls, the fruit will turn dark colors and sometimes grow a visible mold.

Prevention of Soil Rot

• Both buckeye rot and Rhizoctonia soil rot can be prevented by keeping the fruits off the ground with trellises, stakes or cages. Soil rot is more likely to occur when the weather is too hot and wet. Pruning the bottom leaves may help create airflow around the plant and alleviate an environment that is too moist. Infected fruits should be picked and discarded to reduce rot.