Tending a tomato plant for weeks only to discover a large, dark spot marring the fruit is frustrating. This discoloration at the bottom of the tomato is a common issue for home gardeners. Fortunately, the problem is not caused by insect pests or contagious diseases that spread throughout the garden. This brown spot is a physiological disorder, meaning it results from an environmental or nutritional imbalance within the plant.
What is Blossom End Rot
The characteristic brown blemish on the bottom of the fruit is known as Blossom End Rot (BER). It begins as a small, water-soaked area on the blossom end, opposite the stem. As the fruit grows, this spot rapidly enlarges, sinks inward, and turns dark brown or black with a leathery texture.
BER is strictly a problem of the fruit and does not spread to the plant’s leaves, stems, or roots. It often appears on green fruit about half its full size, though it can affect ripening tomatoes as well. While the damaged part is not suitable for consumption, the rest of the fruit is technically edible, provided secondary fungi or bacteria have not invaded the tissue.
The Role of Calcium and Water Stress
Blossom End Rot is directly caused by a localized deficiency of calcium in the developing fruit tissue. Calcium is an immobile nutrient, meaning it cannot be re-mobilized from older parts of the plant to new growth. A steady supply to the fruit is necessary because calcium acts like “concrete” for cell walls; without it, cell membranes collapse in the rapidly expanding fruit.
The disorder usually results from the plant’s inability to transport calcium, rather than a lack of the mineral in the soil. Calcium is absorbed by the roots and moves through the plant’s vascular system with the flow of water via transpiration. When soil moisture fluctuates erratically between dry and overly wet conditions, this steady flow of water and calcium is interrupted, starving the fruit.
Periods of sudden drought stress or overly saturated soil impede the roots’ ability to take up water and nutrients. High levels of certain salts, such as sodium or excessive ammonium-based nitrogen fertilizer, can also interfere with calcium uptake. The fastest-growing fruits, often the first ones of the season, are the most susceptible to this transient calcium shortage.
Prevention and Treatment Strategies
The most immediate step to manage Blossom End Rot is to remove affected fruit, as it will not recover and continues to draw resources. The primary focus for prevention is maintaining consistent soil moisture. Tomatoes typically require 1 to 1.5 inches of water per week, and deep, regular watering is more effective than shallow, frequent sprinkles.
Applying a layer of organic mulch, such as straw or wood chips, around the base of the plant conserves soil moisture and moderates soil temperature fluctuations. Using a soaker hose or drip irrigation system is an excellent way to consistently deliver water directly to the root zone. Some gardeners use a foliar calcium spray for a temporary fix, but its effectiveness is limited because calcium is not easily absorbed through the leaves and transported to the fruit.
For long-term prevention, start with a soil test to determine calcium content and pH level. Calcium uptake is most efficient when the soil pH is between 6.5 and 7.0. If the pH is too low, incorporating a calcium source like garden lime or gypsum before planting the following season can correct the balance. Also, avoid using excessive amounts of high-nitrogen fertilizer, especially those containing ammonium. Rapid vegetative growth can outpace the plant’s ability to supply calcium to the developing fruit.