The black spots appearing on the bottom of your tomatoes are the signature symptom of Blossom End Rot (BER). This condition is a physiological disorder caused by internal plant stress and environmental factors, not by a disease-causing pathogen. While frustrating, BER is both treatable in the short term and preventable in future seasons.
Diagnosing Blossom End Rot
Blossom End Rot is easily identified by its distinctive appearance on the fruit. The initial symptom is a small, water-soaked spot that develops exclusively on the blossom end, the part opposite the stem. This spot quickly enlarges, turning dark brown or black, and becomes sunken and leathery, often covering up to half of the tomato’s surface.
BER is not a disease, which helps differentiate it from issues like fungal infections. Fungal diseases such as Anthracnose may also cause sunken black lesions, but they often appear elsewhere on the fruit and may show signs of fungal sporulation. The firm, dry, and leathery nature of the BER lesion, localized to the fruit’s bottom, confirms it as a non-pathogenic disorder.
Understanding the Calcium Connection
The root cause of Blossom End Rot is a localized calcium deficiency within the developing fruit. Calcium is a structural mineral necessary for building strong cell walls. When a tomato fruit is rapidly growing, a lack of calcium causes the cell structure at the furthest point from the stem to collapse, resulting in the characteristic black lesion.
This deficiency is most often a transport problem, not a lack of calcium in the soil itself. Calcium moves through the plant solely in the xylem, relying on the flow of water driven by transpiration. Inconsistent soil moisture, such as a dry spell followed by heavy watering, severely disrupts this water flow and prevents calcium from reaching the fruit.
Another major factor impairing calcium uptake is soil pH and the presence of other nutrients. Tomatoes thrive when the soil pH is between 6.0 and 6.8, and values outside this range can “lock up” existing calcium, making it unavailable to the plant’s roots. Additionally, over-fertilizing with nitrogen, particularly in the ammonium form, can antagonize calcium uptake and increase the risk of BER.
Immediate Actions to Save the Harvest
Since the damaged cells cannot be repaired, any tomatoes showing the characteristic black lesion should be removed immediately. Removing affected fruit allows the plant to redirect resources toward developing healthy new fruit. The most effective immediate action is stabilizing your watering routine to ensure deep, consistent soil moisture.
Aim to provide about one to two inches of water per week, applying it deeply in one or two sessions rather than with light, frequent sprinkles. For a temporary measure, a foliar spray containing calcium chloride or calcium nitrate can be applied directly to the foliage and young fruit. Although calcium is poorly translocated from leaves, direct absorption by small fruit may help reduce the incidence of BER on the next set of developing fruit. This temporary measure addresses the symptom but does not fix the underlying soil moisture or pH imbalance.
Preventing Recurrence Next Season
Preventing Blossom End Rot relies on long-term soil and water management. Before the next planting season, performing a professional soil test is the best way to determine soil pH and actual calcium levels. If the soil pH is too low (acidic), incorporating an amendment like agricultural lime or dolomitic lime will raise the pH to the optimal range and provide calcium.
To maintain consistent soil moisture, apply a thick layer of organic mulch around the base of the plants. Mulch helps moderate soil temperature and significantly reduces water evaporation, preventing the moisture fluctuations that trigger BER. Finally, avoid high-nitrogen fertilizers that promote rapid, leafy growth, which can outpace the plant’s ability to transport calcium to the fruit.