When you have plenty of tomatoes on your plants, but some of them look like they are rotting on the vine, your tomatoes are suffering from blossom end rot. The first sign is a brown discoloration near the bottom end of the fruit. These spots grow and darken until they cover up to half of the tomato, and the rotting fruit becomes vulnerable to secondary bacteria and fungi. The affected tomatoes can't be saved, but the plant can.
Causes of Blossom End Rot on Tomatoes
Blossom end rot is the result of calcium deficiency. However, that doesn't necessarily mean that you need to add calcium to your soil. Blossom end rot is most often caused by watering practices—your own or Mother Nature's. In a typical situation, the soil is allowed to dry out completely, and then the gardener over-compensates by watering heavily when the plant starts to wilt. The plant bounces back, but the damage has been done to fruit in an early state of development. During the dry conditions, the plant was unable to absorb sufficient calcium from the soil through its roots. When fruit develops, it shows the telltale rot at the blossom end.
Another less common situation occurs when plants are given too much water. If you have a cool, wet spring and summer in your parts of the country, the plants take up so much moisture that the amount of available calcium is diluted, and the result is blossom end rot.
Other factors that can affect a plant's ability to absorb calcium from the soil include:
- A lack of aeration caused by compacted soil
- Planting tomato plants early in cold soil
- Parasitic fungi and nematodes
- Overapplying nitrogenous fertilizer
If the problem is caused by too much moisture, side-dress the plants with bone meal to replenish the soil's supply of calcium. A few brands of organic blossom end rot spray are on the market. These are calcium sprays that you spray on the foliage every week or so. This doesn't save any fruit that already shows rot, but it may prevent rot on future fruit.
Prevention of Blossom End Rot
Test your soil in the fall or early spring for calcium. If it is calcium deficient, as determined by a soil test kit purchased at your local garden center, add ground limestone before setting out the tomato plants. A half-pound of ground limestone per 10 square feet of soil raises the soil pH by about one point.
If you can't locate a soil test kit that measures calcium content, contact your local cooperative extension service to learn how to collect a soil sample and send it to the lab for analysis. The report you receive alerts you to any mineral deficiencies in your soil—including calcium—as well as steps to correct the problems.
If you have tomatoes with blossom end rot on your plants, remove the damaged tomatoes—they'll keep growing and use the plant's energy, which is better put to use producing new rot-free fruit.
Blossom-End Rot of Tomato Tip Sheet. Michigan State University Extension