Blossom End Rot (BER) is a common physiological disorder affecting fruiting vegetables, causing frustration for home gardeners. It frequently strikes tomatoes, peppers, eggplant, and squash just as the harvest approaches. BER is not caused by pests or pathogens, meaning fungicides or pesticides are ineffective. The disorder results from a breakdown in the plant’s internal mechanism for delivering a specific nutrient, leading to localized cell death in the developing fruit.
Identifying Blossom End Rot
The visual manifestation of Blossom End Rot is distinct, always appearing at the distal end of the fruit, opposite the stem. Symptoms begin as a small, water-soaked spot that rapidly enlarges and darkens. This lesion quickly develops into a sunken, dry, leathery patch that is dark brown or black.
The damage occurs while the fruit is developing, often becoming most noticeable when the fruit is about half its full size. Although secondary fungi may colonize the affected area, the initial cause is physiological and the disorder does not spread between plants. Tomatoes are the most commonly affected, but peppers, eggplant, and squash varieties like zucchini are also highly susceptible.
The Physiological Cause
Blossom End Rot results from a localized calcium deficiency within the developing fruit tissue. Calcium is a structural nutrient used to build strong cell walls; a shortage causes the cell walls in the rapidly growing fruit to collapse. The problem is rarely a lack of calcium in the soil itself, but rather the plant’s inability to transport the nutrient efficiently to the fruit’s furthest point.
Calcium is considered an immobile nutrient, meaning it moves through the plant almost exclusively via the xylem, traveling with the water that is pulled up from the roots. This movement is driven by transpiration, the release of water through the leaves. Since fruit surfaces transpire less water than the leaves, the calcium supply is preferentially directed toward the actively transpirating foliage. Inconsistent water availability—periods of drought followed by heavy watering—disrupts this steady flow, causing a temporary calcium shortage in the fruit’s blossom end. Other factors, such as high heat, root damage from cultivation, or cold soil temperatures, can also impair the root’s ability to take up water and nutrients, exacerbating the problem.
Long-Term Prevention Strategies
The most effective way to prevent Blossom End Rot is by establishing cultural practices that ensure a consistent and uninterrupted supply of water and calcium. Consistent soil moisture is primary, making deep, regular watering a foundational strategy. Using a soaker hose or drip irrigation system delivers water slowly and consistently to the root zone, stabilizing the moisture level the plant experiences. Plants typically require about one to one-and-a-half inches of water per week, adjusted for local climate and weather conditions.
Soil health and pH also play a significant role in calcium availability. Soil testing can determine if the pH is in the optimal range of 6.5 to 7.0 for calcium uptake, since acidic soil limits absorption. If testing indicates a need, amendments like dolomitic lime or gypsum can be worked into the soil before planting, with lime being preferred if the pH also needs to be raised.
Avoiding an overload of nitrogen fertilizer is another important preventative measure. High nitrogen levels promote rapid, lush foliage growth, increasing the plant’s overall demand for water and calcium. This rapid vegetative growth can divert the limited calcium supply to the leaves, starving the developing fruit. Using balanced or low-nitrogen fertilizers once the fruit has set supports the reproductive phase without encouraging excessive leaf production.
Applying a layer of organic mulch, such as straw or shredded bark, helps regulate soil temperature and reduces water evaporation from the soil surface. This maintains the uniform soil moisture necessary for steady calcium transport to the fruit.
Immediate Management of Active Symptoms
Once a fruit exhibits the characteristic sunken, dark spot of Blossom End Rot, the damage is permanent and the affected fruit will not recover. The first step in managing active symptoms is to remove all affected fruit immediately. Removing damaged fruit redirects the plant’s energy and nutrient resources toward the production of new, healthy flowers and fruit.
The immediate focus must then shift to correcting the underlying issue of inconsistent water delivery. Establishing a strict, consistent watering schedule is the most effective acute correction, aiming to keep the soil evenly moist without becoming waterlogged. This immediate correction ensures that subsequent fruit development is not affected by the lack of steady calcium flow.
For a temporary calcium boost, a gardener may consider using a rapid-absorption solution, such as a foliar calcium spray, directly on the plant. However, these sprays are only a temporary measure that treats the symptom, not the root cause, as calcium is poorly absorbed through the leaves. The long-term solution remains the establishment of a consistent watering regimen and structural changes to soil health. New fruits that develop after water consistency is corrected should be free of the disorder.