Cause A localized calcium deficiency due to any soil or growing condition that affects calcium uptake. This physiological problem is common, especially in home gardens. Blossom-end rot often occurs when soil moisture fluctuates. If too little calcium is in the soil or if the soil is high in salts, calcium uptake will be impeded, especially under periods of sudden drought stress. Excessive nitrogen applications, especially in the ammonium form, can increase a plant's demand for calcium. Once calcium is used in the plant, it becomes immobilized and cannot be translocated from older tissues to younger, growing tissues, which need calcium.
Symptoms A water-soaked, light tan spot at the blossom end or side of a fruit. The spot enlarges, becoming dark brown or black and leathery. Normally, spots are dry but may become soft if secondary bacteria and fungi invade the fruit. Affected fruit ripen faster than normal.
- Do not plant tomatoes where drainage is poor, surface water accumulates, or soil is droughty.
- Use a soil test to determine if there is a calcium shortage. If so, add lime, preferably in fall, to adjust soil pH to 6.8 to 7. Mix lime thoroughly in top 8 to 12 inches of soil.
- Mulch plants with black plastic or loose organic material.
- Ensure uniform soil moisture. Wet all soil in the root zone at least every 7 to 10 days. About 24 hours after watering, dig a small hole one foot deep to be certain water penetrated that far. Home gardeners should provide even moisture throughout fruiting by controlled watering and mulching.
- Ensure good air movement by plant spacing and pruning practices. Bu reducing relative humidity, plant transpiration increases, aiding in the uptake of calcium when sufficient amounts are present in soil.
- Fertilize only moderately to keep plants normally green and vigorous but not luxuriant. Side-dress with nitrogen fertilizer only if required to maintain green color and moderate growth.
- Foliar applications of calcium nitrate fertilizers or calcium chloride dihydrate (1.5 teaspoons per gal water plus 0.5 teaspoon of a surfactant such as CapSil) on a weekly basis during fruiting can provide necessary calcium when needed. Phytotoxicity may result but making applications in early morning can help avoid spray injury.
Reference Brubaker, V. 2016. Calcium: It does a plant good. Greenhouse Grower, January.