The appearance of a dark, sunken spot on the bottom of a tomato fruit is a common problem. This symptom typically develops on the first tomatoes of the season. The discoloration starts small, opposite the stem end where the blossom once was, and quickly becomes a leathery, brown or black patch. This issue can dramatically reduce the quality and quantity of a tomato harvest.
Identifying Blossom End Rot
The disorder is known as Blossom End Rot (BER), a physiological problem rather than a disease caused by a pathogen like a fungus or bacteria. It is characterized by the spot always appearing on the blossom end of the fruit. The affected tissue initially looks water-soaked before drying out, becoming flattened, dark, and distinctly leathery.
This presentation distinguishes BER from other tomato troubles, such as sunscald (white or yellow patches on the fruit shoulder) or fungal spots (scattered across the sides). The fruit is not diseased and the damage does not spread to other tomatoes. The damage is irreversible on the affected fruit, but the rest of the plant remains healthy.
Understanding the Root Cause: Calcium Mobility
Blossom End Rot occurs due to a calcium deficiency within the developing fruit tissue. Calcium is an immobile nutrient in the plant, meaning it cannot be easily moved from older leaves to rapidly growing parts like the fruit. It serves as a structural component for cell walls. When its supply is interrupted during fruit expansion, the cells at the blossom end collapse, causing the characteristic decay.
The root cause is the plant’s inability to transport the nutrient efficiently, not usually a lack of calcium in the soil itself. Calcium moves through the plant’s vascular system, called the xylem, with the flow of water via transpiration. When the water flow is inconsistent or reduced, calcium delivery to the fruit is compromised, even if the soil has ample reserves.
Inconsistent Watering
Rapid fluctuations in soil moisture are the primary trigger for BER. If the soil dries out and is then heavily watered, the disrupted flow of water through the plant limits calcium uptake by the roots. During periods of high heat, the plant’s leaves transpire rapidly, but the fruit’s transpiration rate is lower. This means calcium is preferentially delivered to the leaves instead of the fruit. This imbalance is made worse by cycles of drought followed by saturation.
Soil pH and Nutrient Imbalance
Soil acidity affects calcium availability. Calcium is most easily taken up by tomato roots when the soil pH is in the range of 6.0 to 6.5. If the soil is too acidic (below pH 5.5), calcium becomes unavailable for root uptake.
Over-fertilizing with nitrogen, particularly in the ammonium form, promotes excessive leafy growth. This growth acts as a stronger sink for calcium, diverting it away from the developing fruit. High concentrations of other nutrients like potassium or magnesium can also interfere with the plant’s ability to absorb calcium.
Prevention and Mid-Season Management
The most effective strategy for managing Blossom End Rot is maintaining a steady, consistent water supply to support continuous calcium transport. Tomato plants require about one to one-and-a-half inches of water per week, delivered evenly. Using a soaker hose or drip irrigation system is recommended to keep soil moisture levels stable without saturating the roots.
Applying a two to three-inch layer of organic mulch, such as straw or shredded leaves, around the base of the plant helps significantly. Mulch reduces the rate of water evaporation from the soil, preventing the extreme moisture swings that disrupt calcium uptake. Once a fruit shows the sunken spot, remove it from the plant to prevent it from drawing resources away from developing tomatoes.
A foliar calcium spray directly onto the fruit is sometimes used as a temporary fix. However, this is often ineffective because calcium is poorly absorbed through the fruit’s skin and does not address the underlying issue of inconsistent water movement. For a long-term solution, soil testing is the best first step to confirm both the actual calcium level and the soil pH. If the pH is too low, incorporating a slow-release amendment like dolomitic or calcitic lime before the next planting season can adjust the pH to the optimal range of 6.0 to 6.5, ensuring calcium is readily available.