Blossom-End Rot Strikes Again

Although blossom-end rot is often listed as a disease, it is not caused by disease organisms such as fungi, bacteria, and viruses. It is a problem related more closely to moisture stress and fertility. The first sign of blossom-end rot is a small water soaked area at the center of the tomato where the blossom developed. Gradually, as the rest of the tomato grows and the tissue continues to die, the dead area becomes blackened and depressed.

When conditions are right for its development, blossom end rot tends to become most evident at this time of year as weather conditions shift from ideal growing temperatures and adequate rainfall to less rainfall and higher temperatures. This condition, in combination with excess fertilization, low soil calcium, and insect or disease damage, often leads to blossom-end rot. Varieties also may vary in their tendency to develop this problem.

One of the most important parts of combating the problem is to maintain uniform soil moisture. In a garden where water is easily available, the problem is fairly easy to control. Regular thorough watering when there is not adequate rainfall help considerably. In early summer when temperatures are still no high for long periods, tomatoes need only about an inch of water applied per week.

As temperatures rise and are maintained for longer periods, they may need as much as two inches week, either as natural rainfall or irrigation, or most likely a combination of both. When irrigation is not possible, a heavy mulch can help keep soil moisture more uniform as well as help the soil retain moisture longer. Mulches also discourage the practice of deep cultivation which may damage roots and add to the problem. Where mulches are not used, scrape the surface only enough to remove weeds, but not deeply enough to cut roots.

Blossom-end rot may sometimes develop even though there is apparently adequate moisture. This may be a result of fertility levels, or the type of fertilizer used. Excess nitrogen and potassium fertilization as well as excesses of soluble ammonium, magnesium, or sodium seem to add to the problem. When soluble calcium levels are low, the disease becomes more severe.

In dry weather, excesses of any of these nutrients may result in their uptake by the plant in preference to the movement of calcium even though calcium may be adequate in the soil. Low calcium in the tissue leads to collapse of cells at the blossom end of the fruit. Water stress increases the damage. Damaged tomatoes should be promptly picked and discarded so that they do not compete with normal fruit.

Low calcium in the soil can be eased for future years by the addition of lime. However, since ground limestone is slow, acting, there is little benefit for the present situation. A soil test can determine whether lime is needed, and how much to apply. It should be plowed in fall or early spring. If available, calcium nitrate may be used for side dressing plants as a nitrogen and calcium fertilizer while blossom-end rot is a problem. Also, avoid ammonium forms of nitrogen fertilizer such as ammonium nitrate or ammonium sulfate during the fruiting season.

http://en.wikipedia.org/wiki/Blossom_end_rot