A tomato rotting exclusively at its base is a frequent issue for many home gardeners. This localized decay is not caused by insects or disease, but rather a physiological disorder known as Blossom End Rot (BER). BER affects tomatoes and other fruiting vegetables like peppers, squash, and eggplant. Understanding the mechanism behind BER is the first step toward preventing it and ensuring a bountiful harvest.
Identifying Blossom End Rot
Blossom End Rot is easily recognizable by its location and appearance. The condition exclusively affects the blossom end of the tomato, the bottom portion farthest from the stem. Symptoms typically begin on green or half-ripe fruit as a small, water-soaked spot.
This spot rapidly enlarges and darkens, transforming into a sunken, leathery patch of tissue. The color changes to dark brown or black, eventually covering up to half of the fruit’s surface. The dry, necrotic tissue is often invaded by secondary fungi or bacteria, leading to further decay and a mushy texture.
BER is a non-infectious, abiotic problem. Affected fruits will not heal, but their presence does not mean the entire plant is lost or that later-forming fruits will be damaged. The disorder is particularly common on the first fruits that set, as the plant is under the greatest stress during this initial period of rapid growth.
The Root Cause: Calcium Imbalance
The fundamental cause of Blossom End Rot is a localized deficiency of calcium within the developing fruit tissue. Calcium plays a structural role in plant cell walls, acting like cellular cement. Without sufficient amounts, the newly forming cells at the fruit’s end collapse. The issue is rarely a lack of calcium in the soil, but rather the plant’s impaired ability to transport the nutrient to the fruit.
Calcium is an immobile nutrient, moving passively through the plant’s xylem along with water via transpiration. Leaves are the primary site of transpiration, meaning they receive a preferential distribution of calcium over the fruit. Any factor that disrupts the steady flow of water to the fruit will restrict calcium delivery, even if the soil contains ample calcium.
The greatest trigger for BER is inconsistent soil moisture, particularly cycles of drought followed by overwatering. When the soil dries out, roots cannot take up water or dissolved calcium, instantly cutting off the nutrient supply.
Other factors exacerbate this delivery failure. Excessive use of high-nitrogen fertilizers promotes rapid vegetative growth, diverting calcium away from the fruit and toward the leaves. High concentrations of other positively charged ions, such as potassium, magnesium, or ammonium nitrogen, can also compete with calcium for root uptake.
Prevention and Correction Strategies
Managing Blossom End Rot focuses on stabilizing the plant’s environment to ensure a continuous calcium supply. The most effective strategy involves maintaining consistent soil moisture throughout the growing season. Deep, regular watering that moistens the soil to a depth of at least six inches is better than shallow, frequent watering. Applying a thick layer of organic mulch, such as straw, helps regulate soil temperature and reduces moisture evaporation, preventing damaging wet-dry fluctuations.
Correcting soil chemistry is a preventative measure best done before planting. A soil test confirms if the soil pH is within the optimal range of 6.0 to 7.0, as highly acidic soil limits calcium availability. If calcium levels are low, amendments like agricultural lime or gypsum can be incorporated into the soil, preferably in the fall, to slowly raise the calcium content.
Gardeners should also pay attention to fertilization practices by avoiding products high in nitrogen. Over-fertilizing, particularly with ammonium-based nitrogen sources, encourages vigorous leaf growth and inhibits calcium uptake. If BER symptoms appear on existing fruit, remove the affected tomatoes immediately, as they will not recover, allowing the plant to focus resources on newer, unaffected blossoms.