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 and can typically affect tomato, peppers, eggplant, watermelon, pumpkin, and squash. Drought, salinity and high temperatures have been found to promote blossom-end rot. 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 the younger, growing tissues, which need calcium for their normal development.
Blossom-end rot is more likely to develop when fruiting vegetables are grown in raised beds, especially when planted in the southern- or southwestern-facing corners of raised beds where the soil moistures levels can be more quickly depleted. Potted tomato, pepper, eggplant and squash plants are generally more susceptible to this nutritional problem than when grown in the ground. Blossom-end rot more often occurs when soil moisture levels greatly fluctuate.
Foliar applications of calcium have been reported to decrease the incidence of blossom-end rot up to a certain point; excessive levels of calcium can decrease fruit yields and high soil calcium levels can increase incidence of blossom-end rot. In one study, CaCl2 solutions (0.0%, 1.0%, 1.5%, and 2.0%, w/v) were applied to four tomato varieties starting seven days after fruit initiation on weekly intervals until the first harvest. Plants receiving 2.0% CaCl2 solutions had about 10% lower fruit yields overall but little to no blossom-end rot. The non-treated tomato plants and the 1%.5% CaCl2 treated plants had slightly more than 1% fruit with blossom-end rot. There have been potted plant studies on tomato showing that applications of nickel can reduce the levels of blossom-end rot even under conditions of limited calcium supply to the plant; but other studies using nickel at higher rates have reported lower nutrient levels inside tomato plants evaluated. Applications of nickel at 0.5 and 1.0 mg dm-3 (milligram/cubic decimeter) increased tomato fruit yields under both sufficient and deficient calcium conditions.
Symptoms A water-soaked, light tan spot develops at the blossom end or bottom side of a fruit. The spot enlarges, becoming dark brown or black and can take on a leathery look. Typically, spots are dry but these areas may become soft if secondary bacteria and fungi invade the fruit. Affected fruit ripen faster than normal.
Cultural control
- 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 to help maintain more even soil moisture levels.
- 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 that water penetrated that far. Home gardeners should provide even moisture levels throughout the fruiting period if possible by controlled watering and mulching.
- Ensure good air movement by plant spacing and pruning practices. By 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. An over-developed shoot that is out of balance with root volume will increase the issue with calcium deficiency if calcium or water levels are insufficient in the soil.
- 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 to developing fruitwhen needed. Too high of calcium application can suppress fruit yields. Phytotoxicity (injury to foliage) may result but making applications in early morning can help avoid spray injury.
References Brubaker, V. 2016. Calcium: It does a plant good. Greenhouse Grower, January.
Macedo, F.G., José de Melo, W., et al. 202) Nickel reduces blossom-end rot even under calcium deficiency conditions; evidence from physiological responses of the NI - CA interaction. J. Plant Nutrition 46(12):893-2904. DOI: 10.1080/01904167.2022.2160759.
Mazumder, M.N.N., Misran, A., Ding, P., Wahab, P.E.M., and Mohamad, A. 2021. Preharvest foliar spray of calcium chloride on growth, yield, quality, and shelf life extension of different lowland tomato varieties in Malaysia. Horticulturae:7(11):466. https://doi.org/10.3390/horticulturae7110466.
Reitz, N.F., Shackel, K.A., and Mitcham, E.J. 2021. Differential effects of excess calcium applied to whole plants vs. excised fruit tissue on blossom-end rot in tomato. Scientia Horticulturae 290:https://doi.org/10.1016/j.scienta.2021.110514.