What causes the bottom end of my tomatoes to rot?

ASK THE HORT AGENT

Question What causes the bottom of my tomatoes to rot?

Answer The bottom of a tomato is the part closest to the ground. The top of a tomato has the stem attached to it. The bottom is also known as the blossom end. The condition you are describing is appropriately known as “blossom end rot” or BER. This problem begins when little tomatoes are developing. The affected area may end up being a mere spec or ¾ of the fruit.

BER is caused by a lack of calcium in the developing fruit. This may be a widespread problem like low calcium levels in the soil. It can also be a temporary calcium interruption caused by fluctuations in water supply. Incidence of BER is also increased where there is a low ratio of calcium to certain nutrients such as potassium and nitrogen.

If calcium levels are low in the soil, then the addition of lime will turn things around. Soil should be limed according to soil sample report recommendations, which try to obtain a pH between 6.5 and 6.8. Home gardens not limed in the past 2 to 3 years will need 2 cups of lime for each plant. The lime should be worked into the soil 12 inches deep.

The use of gypsum (land plaster) will also help. Gypsum adds calcium without raising the pH. A typical rate of gypsum is 1 to 2 pounds per 100 square feet. Lime and gypsum are best utilized when added 6 months prior to planting. If you add it now, you will get some benefit. Better late, than never.

Dry spells followed by wet spells cause the fruit to grow faster than the plant can take in calcium. The end result is a half rotten tomato. Clusters of tomatoes prior to or after the affected fruit may not show any symptoms of BER at all. Cultivation (with tractor implements or a hoe) can also damage roots enough to temporarily interrupt water supply and produce BER symptoms.

Veggies need 1 to 1.5 inches of water per week during fruit production. Whenever possible, use mulch around tomatoes to moderate moisture loss and prevent weeds. Mulch is most useful when added after the soil temperature has warmed up. Usually the middle or end of May is good. The addition of mulch early in the growing season will keep the ground cool and slow plant growth.

Small, quick-growing fruit like cherry tomatoes don’t have nearly as much incidence of BER as large, slow maturing fruit. For more info on BER, visit http://www.ces.ncsu.edu/depts/pp/notes/oldnotes/vg19.htm If you don’t have internet access, call me at 910-893-7533 or email me at gpierce@harnett.org

Tomatoes with BER are perfectly fine to eat. Personally, I have never acquired the taste for the rotten section.

Gary L. Pierce

Horticulture Extension Agent