If calcium is deficient in developing tomato fruits, an irreversible condition known as blossom-end rot will develop. Blossom-end rot occurs when cell wall calcium is deficient during early fruit development, and results in cell wall membrane collapse. Blossom end rot is a physiological disorder of tomatoes and not caused by a plant disease.
Symptoms include water-soaked spots on the blossom end of the fruit. These spots enlarge and become black. Often, rot symptoms will occur as far as one-third to halfway up the fruit, but will never start at the stem end. Secondary infections caused by decay-causing organisms usually follow. Most fruit diseases will occur randomly on other sides of the fruit and are not limited to the blossom end.
The cause of this disorder is a calcium deficiency in the developing fruit. Extreme fluctuations in soil moisture (too much rain or not enough), insufficient soil calcium, root pruning from nearby cultivation, and excessive ammoniacal nitrogen, potassium, or magnesium fertilization can also increase the chances of blossom end rot, especially early in the season.
To prevent blossom-end rot, the soil should be limed according to the recommendations of a soil analysis report to bring the soil pH up to 6.5, and to provide adequate calcium levels in the soil. Limestone is best applied three to six months in advance and tilled into the garden soil prior to planting. Follow the soil report for recommendations about pre-plant fertilizer applications. If calcium levels are not sufficient, but the soil pH is correct, then gypsum (calcium sulfate) should be tilled into the soil before planting at one to two pounds per 100 square feet.
Avoid excessive potassium or magnesium fertilization as these nutrients will compete with calcium for uptake by the plants. Epsom salts is an example of a magnesium source, so do not apply to soil unless a recent soil report indicates a magnesium deficiency. Avoid ammoniacal nitrogen fertilizers for side dress applications (beside or around the plants), as ammoniacal nitrogen also will compete with calcium for uptake. Examples of fertilizers with ammoniacal nitrogen are ammonium sulfate, ammonium nitrate and most complete fertilizers, such as 10-10-10. A calcium nitrate (15.5-0-0) sidedress fertilizer is usually the best choice, applied monthly at two pounds per 100 feet of row.
Maintain a uniform supply of moisture through supplemental irrigation and adding a few inches of mulch. As a general rule, most vegetables need the equivalent of one inch of rain every week. If rainfall does not provide this quantity, water plants thoroughly once or twice per week to supplement as needed. One or two deep soakings per week are better than many light water applications. Consider using drip irrigation or soaker hoses around your plants with mulch on top. Mulches will not only keep the soil cooler and more evenly moist, but will suppress weeds, thus reducing the need for nearby cultivation that may damage tomato roots.
Although some people believe foliar calcium sprays can correct a deficiency in developing fruits, research is very inconclusive on this issue. If you decide to purchase a calcium spray, realize that it will not have any effect on fruit that have already started to form. Be sure to remove fruit with blossom-end rot symptoms from the plants. This will allow the plant to redirect any calcium resources in the plant to new fruit that are starting to develop.
As the plants become more mature, they will have more roots to take up calcium in the soil later in the season. The problem may self-correct with older plants, assuming consistent soil moisture conditions and calcium is adequate in the soil. Applying gypsum at one to two pounds per 100 square feet as a side dress supplement during the growing season has also proven beneficial.
Paul Pugliese is the Extension Coordinator and Agricultural & Natural Resources Agent for Bartow County Cooperative Extension, a partnership of The University of Georgia, The U.S. Department of Agriculture, and Bartow County. For more information and free farm, lawn, or garden publications, call (770) 387-5142 or visit our local website at ugaextension.org/bartow .