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Blossom-end rot can occur when sunny days follow a cloudy, wet period. It is a calcium-deficiency disorder in the plant and not a pathenogenic disease. Some cultivars are more prone to this condition.
Calcium moves slowly in plants and even slower in the fruit, so deficiencies can occur even when soil tests indicate that calcium is adequate. Blossom-end rot often occurs on the tomato first. It may also be a problem on peppers, squash and watermelons. It is more common on fruit that is one-third to one-half grown, and it occurs on the blossom end of the fruit.
It begins as a small, water-soaked spot and develops into a dark brown, leathery spot that may involve half the fruit. The surface of the spot shrinks and becomes flat or sunken. Blossom-end rot is caused by a lack of calcium in the developing fruit. The uptake of calcium from the soil by the tomato plant can be reduced by fluctuations in soil moisture – either excessively wet soil or excessively dry soil.
The disease commonly occurs when plants are growing rapidly and luxuriantly early in the season and are then subjected to prolonged dry weather. The disease may be more serious on the windward side of a garden and on staked tomatoes rather than on unstaked or bushy plants.
Prevent blossom-end rot by maintaining a soil pH around 6.5 and uniform soil moisture by irrigating and mulching, and avoid heavy applications of nitrogen.
Control blossom-end rot by spraying foliage with 2 level tablespoons of 96 percent calcium chloride in 1 gallon of water at seven- to 10-day intervals. Do this for three to four applications. Begin spraying with first appearance of symptoms. Overdosing plants with calcium chloride may result in leaf burn. Spray on cloudy days or wait until the sun is low.
Calcium nitrate also may be used. Use 1 rounded teaspoon per plant. Apply calcium nitrate into the soil about 8 inches from tomato plant stems. A second application may be needed several weeks later as well. If foliar applied, use 2 level tablespoons per gallon of spray applied late in the day.
