Blossom End Rot (BER) is a common physiological disorder that frustrates many home gardeners growing tomatoes, as well as peppers, squash, and melons. This condition results from a failure of the plant to deliver sufficient nutrients to the developing fruit. It is not caused by a fungus or bacteria, but rather by environmental factors that interfere with a specific mineral’s movement within the plant. Fortunately, once the underlying cause is understood, this recurring problem is often treatable and largely preventable.
Identifying Blossom End Rot
The first sign of Blossom End Rot is a small, water-soaked spot appearing on the distal end of the fruit. This spot rapidly expands and darkens, turning into a large, sunken area that is dark brown or black with a leathery texture. The fruit often begins to ripen prematurely. While the affected area is not infectious, it provides an entry point for secondary decay organisms. This problem is typically observed on the first fruits that set, when the plant’s nutrient demands are rapidly increasing.
Calcium Transport and Cell Development
The direct cause of Blossom End Rot is a localized deficiency of calcium within the tissue of the developing fruit. Calcium plays a fundamental role in the plant by strengthening cell walls, where it acts structurally to stabilize the cell membranes. Without an adequate, continuous supply of this mineral, the cells in the fastest-growing part of the fruit—the blossom end—begin to break down and collapse. BER is not usually a sign of calcium depletion in the soil itself, but rather a problem with the plant’s ability to move the nutrient.
Calcium is considered an immobile nutrient within the plant, meaning it cannot be redistributed from older leaves to the new, growing fruit once it has settled. Its movement from the roots to the fruit relies entirely on the transpiration stream, which is the flow of water pulled upward through the xylem tissue and evaporated through the leaves. Since leaves transpire at a much higher rate than fruit, calcium is preferentially deposited in the foliage, often leaving the fruit with inadequate supplies during its rapid expansion phase.
Environmental Triggers for Calcium Failure
The plant’s inability to deliver calcium to the fruit is most often linked to inconsistent soil moisture levels. Fluctuations between dry soil and heavy watering disrupt the steady flow of the transpiration stream, momentarily halting calcium transport. Reduced water uptake during dry periods limits calcium absorption, depriving the developing fruit of the mineral. Both overly dry conditions and waterlogged soil, which causes poor root aeration, prevent the steady uptake of calcium by the roots.
Soil Chemistry and Nutrient Competition
The condition of the soil can also interfere with calcium availability, even if the mineral is present in abundance. High soil acidity or alkalinity (pH levels outside the ideal range of 6.0 to 6.8) chemically lock up calcium, making it unavailable for root absorption. Furthermore, high levels of nitrogen fertilizer encourage rapid vegetative growth. This rapid growth diverts the limited calcium supply to the leaves and stems, creating competition that starves the developing fruit.
Damage to the root system, whether from aggressive cultivation or cold soil temperatures early in the season, limits the plant’s absorption efficiency. High concentrations of elements like magnesium or potassium in the soil can also compete with calcium for uptake at the root level, reducing the overall amount transported throughout the plant.
Actionable Prevention and Correction
Once a fruit shows the symptoms of Blossom End Rot, the damage is irreversible. Affected tomatoes should be removed immediately to direct the plant’s resources toward the development of subsequent, healthy fruit. The most significant long-term remedy is establishing a consistent watering schedule to ensure a uniform supply of water and calcium to the plant.
Applying a thick layer of organic mulch around the plant base is highly effective for maintaining even soil moisture and regulating temperature, supporting steady calcium transport. For soil concerns, a soil test can determine if the pH is within the target range of 6.5.
Soil Amendments
Using a balanced fertilizer that is low in nitrogen during the fruiting phase can prevent excessive vegetative growth that diverts calcium away from the fruit. If soil amendments are needed:
- If the soil pH needs to be raised, garden lime can be worked into the soil, which adds calcium while increasing the pH.
- Gypsum can be used to add calcium without significantly altering the soil’s existing pH balance.