Blossom end rot (BER) is a common physiological disorder affecting many garden fruits, particularly tomatoes, peppers, eggplant, and squash. It is caused by the plant’s inability to properly distribute nutrients to the developing fruit, not by a pest or disease. The classic symptom is a dark, water-soaked spot that appears at the blossom end—the bottom—of the fruit when it is about half its full size. This spot enlarges, eventually becoming a sunken, brown-to-black, leathery patch.
Identifying the Root Cause
Blossom end rot is caused by an insufficient supply of calcium within the developing fruit tissue. Calcium is a structural component of cell walls; without it, the cells at the fruit’s farthest point collapse, creating the characteristic lesion. The disorder is rarely due to a true lack of calcium in the soil, which is often present in adequate amounts. The problem lies in the plant’s ability to take up and transport calcium to the rapidly expanding fruit.
Calcium is moved through the plant exclusively with water via the xylem, a process driven by transpiration. This system makes calcium transport highly vulnerable to inconsistent soil moisture; both drought and overwatering inhibit root function and calcium uptake. Root damage from cultivation or high concentrations of competing nutrients like ammonium, potassium, or magnesium can also impair uptake, creating a calcium delivery failure.
Immediate Treatment Options
When blossom end rot appears, the immediate focus shifts to ensuring the next set of fruit develops without the disorder. One quick, reactive measure is to apply a foliar calcium spray, such as a calcium chloride solution, directly to the foliage and young fruit. While calcium is not easily redistributed from leaves, this application provides a short-term, localized boost to the developing fruit. The recommended application is two to three sprays at one-week intervals, starting when the first blooms appear.
A fast-acting soil drench can also provide available calcium to the root zone immediately. Garden lime or gypsum, mixed with water to create a slurry, can be poured around the base of the plant to make calcium more readily available for root uptake. This approach works best when combined with an immediate deep watering if the soil is dry, which helps activate the calcium and encourages normal nutrient transport.
Long-Term Soil and Watering Management
The most effective long-term solution for preventing blossom end rot is maintaining consistent soil moisture. Plants require about one to one-and-a-half inches of water per week; deep, less frequent watering is better than shallow, daily sprinkles. Applying a layer of organic mulch, such as straw or wood chips, helps stabilize soil temperature and reduces moisture evaporation, preventing the sharp wet-to-dry fluctuations that trigger the disorder.
Proper soil chemistry is also important, as calcium uptake is optimized at a slightly acidic soil pH between 6.5 and 7.0. A soil test can confirm existing calcium levels and guide the use of amendments like pulverized limestone or gypsum to adjust the pH and ensure calcium availability. Fertilization must be managed carefully: high-nitrogen fertilizers, particularly those containing ammonium, can promote excessive leaf growth and interfere with calcium uptake. Using balanced fertilizers low in nitrogen and avoiding over-fertilization helps maintain a healthy nutrient balance.
Salvaging the Harvest
Once a fruit develops the characteristic dark, sunken spot, the cellular damage is permanent. It is important to remove affected fruits immediately from the plant, as they will continue to draw water and nutrients that could otherwise be directed to healthy, developing fruit. Removing the damaged fruit redirects the plant’s resources to newer blooms and fruit set, which have a better chance of developing without issue once corrective measures are implemented.
If the rot spot is minor and localized to the blossom end, the unaffected portion of the fruit is safe to eat after the damaged area has been trimmed away. Severely affected fruit should be discarded or composted. Promptly removing spoiled fruit and correcting the underlying moisture and nutrient issues ensures a successful harvest from the subsequent fruit produced by the plant.