When you see a dark, sunken spot on the bottom of your ripening tomatoes, the plant is suffering from Blossom End Rot (BER). This is not a disease caused by a fungus, bacteria, or insect pest, but a physiological disorder. It occurs frequently in home gardens and is the plant’s reaction to environmental stress. Its appearance signals that the plant is struggling to move a particular nutrient.
Identifying Blossom End Rot
The first sign is a small, water-soaked spot that appears on the distal end of the fruit, opposite the stem where the blossom was attached. This lesion quickly enlarges, changing color from light tan to dark brown or black. The affected area becomes dry and leathery, sometimes covering up to half of the fruit’s surface.
This tissue breakdown is caused by a localized deficiency of calcium within the developing fruit. Calcium is required by the rapidly dividing cells at the fruit’s blossom end, acting like a cellular “glue” to maintain structural integrity. When these cells are deprived of calcium, their membranes break down, leading to the characteristic rot. The deficiency is specific to the fruit tissue; the rest of the plant, including the leaves, may appear perfectly healthy.
Cultural Practices That Cause the Problem
The underlying issue is rarely a lack of calcium in the soil; instead, the plant cannot properly transport existing calcium into the fruit. Calcium is an immobile nutrient, moved through the plant primarily by water uptake and transpiration. Any factor that disrupts the flow of water through the plant’s xylem can prevent calcium from reaching the fruit, where transpiration is naturally low.
Inconsistent watering is the most frequent trigger, as periods of drying out followed by heavy watering cause extreme fluctuations in moisture and nutrient uptake. Water stress lasting as little as 30 minutes can initiate the disorder. Other factors that compete with calcium absorption include high concentrations of salts from excessive fertilization, especially high-nitrogen or ammonium-based fertilizers.
Soil pH plays a role in nutrient availability; soil that is too acidic (below 5.5) or too alkaline limits the roots’ ability to absorb calcium. Damage to the plant’s feeder roots from aggressive cultivation can also inhibit water and nutrient intake. The combination of these cultural practices with rapid fruit growth often accelerates the disorder’s appearance.
Immediate and Future Management
Once a tomato fruit shows the dark, sunken lesion, the damage is irreversible because the affected cells cannot be repaired. Immediately remove and dispose of any fruit showing signs of Blossom End Rot so the plant can divert energy toward producing healthy new fruits. For a temporary fix, some gardeners apply a calcium chloride foliar spray, which provides a quick boost to developing fruit, though its efficacy is limited since calcium is poorly absorbed through leaves.
The most effective long-term management focuses on stabilizing the plant’s environment to ensure consistent calcium transport. Establish a regular, deep watering schedule to maintain even soil moisture, aiming for about 1.5 inches of water per week. Applying a two to three-inch layer of organic mulch (like straw or shredded leaves) helps regulate soil temperature and significantly reduces moisture evaporation.
For future prevention, avoid excessive nitrogen application, especially during the fruiting stage. Utilize a soil test to confirm your soil’s pH is in the optimal range of 6.0 to 6.8. If the test indicates a true deficiency, incorporating a calcium source like dolomitic lime or gypsum can correct the issue over time. Focusing on consistent moisture and balanced nutrition creates the conditions for the plant to properly move calcium into every developing tomato.