The appearance of a dark, sunken spot on the bottom of a developing tomato is a common and frustrating experience for home gardeners. This damage is often mistaken for a fungal disease or pest infestation, but it is actually a physiological disorder known as Blossom End Rot (BER). This issue signals a temporary breakdown in the plant’s nutrient delivery system, specifically impacting the fruit tissue during its rapid growth phase. Understanding the nature of this disorder is the first step toward preventing it and ensuring a successful harvest.
Confirming Blossom End Rot
The visible symptom of Blossom End Rot (BER) helps distinguish it from other types of fruit decay. BER begins as a small, water-soaked spot on the distal end of the fruit, which is the part opposite the stem where the blossom was originally attached.
As the fruit matures, this spot rapidly enlarges and darkens, turning a characteristic dark brown or black color. The affected area becomes flattened or sunken and takes on a dry, leathery texture. This damage appears on green or partially ripened fruit. While the lesion itself is not caused by a pathogen, the dead tissue can later be invaded by secondary fungi or bacteria, leading to a slimy, wet rot.
The symptoms are confined to the blossom end of the fruit, differentiating BER from sunscald or other types of rot that start near the stem or on the sides. Once the lesion appears, the fruit will not recover. This is not a contagious condition, and the plant itself remains healthy, often producing normal, unaffected fruit later in the season.
The Root Cause: Calcium Transport Failure
Blossom End Rot is caused by a localized deficiency of calcium in the fruit tissue. Calcium is an immobile element within the plant, meaning it cannot be moved from older leaves to newer, growing parts like developing fruit. The fruit requires a continuous supply of calcium to build strong cell walls and maintain tissue integrity.
This calcium supply is dependent on the flow of water through the plant, a process known as transpiration. Calcium is passively carried with water from the soil, through the roots, and up into the plant structure. If the plant’s access to water is interrupted, the delivery of calcium to the fruit ceases, causing the cells at the tip of the fruit, the furthest point from the water supply, to collapse.
Inconsistent soil moisture is the primary trigger for this transport failure. Periods of drought stress followed by heavy watering create a cycle that starves the fruit of calcium. Excessively wet or cold soil can also inhibit root function, making it harder for the plant to absorb the mineral, regardless of the amount present in the soil.
The disorder rarely indicates that the soil is completely lacking in calcium, but rather that the plant cannot utilize the available supply. High rates of growth, often encouraged by over-application of nitrogen fertilizer, can exacerbate the issue. This rapid vegetative growth increases the demand for calcium in the leaves, diverting the limited supply away from the slower-transpiring fruit.
An imbalance in soil nutrients can actively interfere with calcium uptake. High concentrations of other positively charged ions, particularly potassium and magnesium, can compete with calcium for absorption sites on the plant roots. Using fertilizers with an ammonium form of nitrogen can also reduce calcium uptake, as the ammonium ion competes for the same pathways into the root system.
Strategies for Prevention and Recovery
The most effective method for preventing Blossom End Rot is to maintain a consistent watering regimen throughout the growing season. Tomatoes require approximately 1 to 1.5 inches of water per week, and the goal is to keep the soil evenly moist, not saturated or completely dry. Watering deeply and less frequently encourages the development of strong, deep root systems that handle minor fluctuations in moisture better.
Applying a 3- to 4-inch layer of organic mulch, such as straw or shredded leaves, around the base of the plant helps significantly. Mulch acts as a barrier, reducing water evaporation from the soil surface and preventing the rapid moisture swings that disrupt calcium transport. This also keeps the soil temperature more stable, which benefits root health and function.
Proper nutrient management is another preventative measure that begins with a soil test to confirm existing calcium levels and pH. Tomatoes thrive when the soil pH is in the range of 6.5 to 7.0, which optimizes calcium availability. If fertilization is necessary, choose a balanced product that provides moderate nitrogen, ideally in the nitrate form, to avoid promoting excessive leafy growth and nutrient competition.
Once a fruit shows the characteristic sunken lesion, the damage cannot be reversed, and it will not recover. It is beneficial to remove and discard these affected tomatoes immediately so the plant can redirect resources toward developing new, healthy fruit. While foliar calcium sprays are available, they offer only a temporary measure and are not a substitute for correcting inconsistent water and nutrient management.