Stop Tomato Blossom - End Rot Before it Starts

Although the tomato blossom-end is called a disease, it is not caused by any organism such as a bacteria, virus or fungus. The time to keep it from developing is now. Blossom-end rot is often most severe on plants that have grown rapidly and luxuriantly during the early part of the season. As the weather becomes hotter and drier, plants are suddenly under more stress, and if the dry period is prolonged, young fruit develops the problem. First sign is a small water-soaked spot at the blossom-end.

In addition to abrupt changes in moisture, other conditions can cause or add to the severity of the problem. These include excess fertilization, type of fertilizer, or extreme fluctuation in soil moisture. Most important in combating the problem is to maintain uniform soil moisture. In a garden where water is easily available, the problem is fairly easy to control. Occasional thorough watering help considerably. However, avoid frequent light waterings. Thoroughly soak the soil. Then, after several days when the surface has dried, but there is still moisture evident an inch beneath the surface, another watering should be given if no rainfall has occurred.

Where irrigation is not possible, mulches can help keep soil moisture more uniform as well as help the soil retain moisture for longer periods. Also, avoid any practices that might injure roots. Avoid deep cultivation. Scrape the surface of the soil only enough to remove weeds. Even when moisture conditions are adequate, blossom-end rot sometimes develops. This may be due to fertility levels, or perhaps to the type of fertilizer used.

Excess nitrogen and potassium fertilization, as well as excesses of soluble ammonium, magnesium or sodium, add to the problem. When soluble calcium levels are low, the severity of the disease increases. In dry weather these nutrients may be taken up in excess and thereby exclude the movement of calcium, even when adequate amounts are available in the soil. A calcium deficiency in the tissue leads to collapse of cells at the blossom-end of the fruit. Water stress in the tissue may increase the amount of damage.

Depending on conditions, the injury may be a small spot or engulf a large portion of the tomato. The damaged spot finally becomes blackened and leathery. At that state the tomato is useless and should be picked so that it does not compete with normal fruit. Low calcium in the soil can be eased by the addition of lime. Ground limestone is relatively slow reacting; most benefit occurs later. A soil test helps determine proper amounts to apply. If needed, application should be made when the garden is plowed in the fall or early spring.

Avoid ammonium forms of nitrogen fertilizer such as ammonium nitrate or ammonium sulfate. If available, calcium nitrate is a beneficial fertilizer where blossom-end rot has been a problem. In the small garden, it is also helpful to plant tomatoes where they get shade during the hottest part of the day. Drying and water stress will then be reduced somewhat.

http://pubs.caes.uga.edu/caespubs/horticulture/blossom-rot.html