The sight of perfectly ripening tomatoes suddenly developing a dark, sunken spot on the bottom is a common frustration for home gardeners, especially those growing elongated varieties. This issue is known as Blossom End Rot (BER), a physiological disorder, not a disease caused by a pathogen. Roma tomatoes, along with other paste-type and plum varieties, are particularly susceptible to this condition, which impacts the fruit’s ability to develop properly. Understanding the underlying mechanisms of BER is the first step toward prevention.
Identifying Blossom End Rot Symptoms
The symptoms of Blossom End Rot are distinctive, helping to differentiate it from fungal or bacterial diseases. The disorder begins with a small, water-soaked spot at the blossom end of the fruit, opposite the stem. This lesion rapidly enlarges and darkens, changing from light tan to deep brown or black. The affected tissue becomes dry, shrunken, and characteristically leathery in appearance.
The sunken area can eventually cover up to half of the fruit’s surface. Although BER is not caused by infection, the damaged tissue can become a secondary entry point for fungi or bacteria. This secondary infection often causes the fruit to rot completely. The defining feature is that the problem always starts exclusively at the bottom of the tomato.
Why Calcium Uptake Fails
Blossom End Rot is fundamentally caused by a localized lack of calcium within the developing fruit tissue. Calcium provides structural integrity, helping to hold plant cell walls together. When a fruit is rapidly expanding, a constant supply of calcium is required for new cell formation; without it, the cell walls collapse, leading to the sunken spot.
The problem is rarely a simple deficiency of calcium in the soil, but rather an issue with the plant’s ability to transport it. Calcium moves almost exclusively through the xylem, the plant’s water transport system, driven by transpiration. Leaves, which transpire heavily, receive calcium preferentially, while the fruit, which transpires very little, is often shortchanged.
The most significant factor disrupting this transport is inconsistent soil moisture. Periods of drought stress followed by heavy watering cause fluctuations that interfere with the steady uptake of calcium from the roots. Other conditions that can block uptake include overly acidic or alkaline soil pH, excessive levels of nutrients like potassium or magnesium, or high nitrogen fertilization that promotes rapid leaf growth.
Practical Steps for Management
Addressing Blossom End Rot requires focusing on immediate relief and long-term soil health.
Consistent Watering
The most immediate action is ensuring consistent water delivery to the plant, which enables steady calcium movement. Deep, uniform watering keeps the soil evenly moist without becoming waterlogged, stabilizing the transpiration stream. Mulching around the base of the plants further aids this effort by regulating soil temperature and reducing moisture loss.
Temporary Measures
A calcium chloride spray can be applied directly to the foliage and developing fruits for a temporary, localized measure. While this is not a permanent solution, it provides a quick boost to newly forming tomatoes. Any fruit already showing the dark, sunken spot will not recover and should be removed to redirect the plant’s energy.
Long-Term Soil Health
Long-term prevention is achieved by optimizing soil chemistry before planting. Soil testing is recommended to confirm calcium levels and pH, as calcium is most available in the slightly acidic to neutral range. If supplementation is needed, applying gypsum (calcium sulfate) is beneficial as it adds calcium without significantly raising the soil pH. Alternatively, dolomitic lime can be incorporated if the soil is too acidic. Avoiding excessive nitrogen fertilizer is also helpful, as high levels promote rapid vegetative growth.