There is nothing more frustrating to a vegetable gardener than to have a large, beautiful tomato ripening on the vine, only to have the base of the tomato start to rot. This is called blossom end rot. While it can strike at any stage of the development of the tomato, it most commonly strikes when the fruit is one-third to one-half size. Blossom end rot starts as a small, water soaked spot on the blossom end of the tomato. This spot becomes larger and larger as the tomato develops. It may engulf up to half the tomato. Large spots dry out and become flattened, black, and leathery in appearance and texture.
Blossom end rot is a common disease of tomatoes. Unfortunately, it is frustratingly hard to prevent. It is also impossible to treat the individual tomato once it begins showing symptoms.
Blossom end rot does not spread from tomato to tomato. It is caused by a physiological problem, so the usual control methods for fungal problems and other diseases will not help.
What does effect blossom end rot? The availability of water and calcium to the tomato. The less water and calcium that are taken up by the plant, the greater the amount of blossom end rot. The most common scenario is when tomato plants that are getting lots of water and really growing tomatoes suddenly experience a period of drought. Since water carries the calcium from the soil throughout the plant, there is suddenly a shortage of calcium available to the tomato. Blossom end rot is often the result.
Anything that effects the root system and its ability to bring water and calcium to the rest of the plant can cause blossom end rot. Cultivating too close to the plant and damaging the roots of the tomato plant can cause the problem. Too many soluble salts in the soil can cause this problem as well. The calcium may still be in the soil, but the availability of the calcium to the plant decreases as the amount of soluble salts increase.
The best way to prevent blossom end rot is to prevent it. Make sure the tomato gets a steady supply of water and calcium. When it rains for several days in a row or a large amount of rain falls quickly, supplemental calcium may be needed. This is because the rain dilutes the amount of calcium available to the tomato and can set it up for blossom end rot.
Starting tomatoes too soon, when the soil is cold, can cause blossom end rot. Waiting until the soil warms for the season can help.
Mulch around your tomato plants to help keep soil moisture high during a drought, although you will still have to water the tomato plant regularly. Making sure you do not cultivate too close to the roots of the tomato helps, too. Finally, the use of fertilizers low in nitrogen and high in phosphate, such as 4-12-4 or 5-20-5, can help prevent blossom end rot
– Stephanie Suesan Smith, PhD, Hunt County Master Gardener