Blossom end rot (BER) is a common issue for home gardeners, appearing as a dark, sunken lesion on the bottom of the tomato fruit. This blemish is not caused by a pest or a fungal pathogen, but is a physiological disorder within the plant itself. The rot signals the plant’s inability to efficiently deliver calcium to its rapidly developing fruit cells. Preventing this disorder relies on managing the environmental and nutritional factors that interfere with this mineral’s movement.
Understanding Calcium Uptake Failure
Blossom end rot occurs because calcium is a non-mobile nutrient within the plant once incorporated into tissues. This means the plant cannot relocate calcium from older leaves to supply a new, fast-growing tomato fruit. Calcium travels almost exclusively through the xylem, the water-conducting tissue, driven by transpiration, or water evaporation from the leaves.
The developing fruit has a much lower transpiration rate compared to the leaves, making it a weaker destination for the calcium-carrying water flow. If the plant experiences a sudden change in its environment, such as drought followed by heavy watering, the flow of calcium is disrupted. This inconsistent delivery prevents the mineral from reaching the cells at the furthest point of the fruit, leading to the collapse and death of the cells at the blossom end. The disorder is most common during the initial period of rapid fruit expansion, when the demand for calcium briefly outpaces the supply.
Ensuring Consistent Water Delivery
Maintaining an even level of soil moisture is the most effective practice for preventing blossom end rot. Inconsistent water availability directly impacts the flow of calcium-rich water from the roots to the fruit. Gardeners should aim for deep, infrequent watering rather than shallow, daily sprinkles, which only moisten the topsoil and encourage shallow root growth.
A deep watering session should wet the soil profile to a depth of six to twelve inches, encouraging a robust root system. To check the moisture level, use a finger to probe the soil several inches down (the knuckle test). If the soil feels dry at that depth, apply water slowly and thoroughly.
Drip irrigation systems or soaker hoses are recommended as they deliver water directly to the root zone with high consistency, avoiding wetting the foliage. Applying a thick layer of organic mulch (straw or shredded leaves) is beneficial. Mulch reduces water evaporation from the soil surface and helps stabilize soil temperature and moisture.
Managing Soil Chemistry and pH
While water inconsistency is the primary trigger, soil chemistry plays a foundational role in making calcium available to the plant roots. The optimal soil pH for tomato plants to efficiently absorb calcium and other nutrients is a slightly acidic to neutral range, typically between 6.0 and 6.8. A soil test should be performed prior to planting to confirm existing calcium levels and pH, allowing for necessary adjustments.
If the soil test indicates a low pH (too acidic), amendments like garden lime can be incorporated to raise the pH into the desirable range while simultaneously supplying calcium. For soils with sufficient pH but lacking calcium, gypsum (calcium sulfate) is a suitable amendment because it adds calcium without significantly altering the pH. These amendments should be mixed into the soil before planting.
It is important to manage other nutrients, particularly nitrogen and potassium, which can compete with calcium for uptake. Over-fertilizing with high-nitrogen fertilizers, especially those with a high percentage of ammonium-based nitrogen, can promote excessive vegetative growth that outpaces calcium delivery to the fruit. Maintaining a balanced fertilization program prevents this nutrient competition and supports steady, healthy growth.
What to Do When Blossom End Rot Appears
Once the dark, sunken lesion of blossom end rot has formed, the damage is irreversible, and the affected fruit will not recover. The first step upon noticing symptoms is to immediately remove and discard the affected fruit so the plant can divert resources toward new, healthy development. New fruit that sets after the issue is corrected will typically develop without the disorder.
The appearance of BER signals that the plant’s management strategy needs immediate correction, usually centering on inconsistent moisture. The gardener must immediately implement preventive practices, focusing on establishing a regular, deep watering schedule. This prompt action can halt the disorder on subsequently developing fruit.
In the short term, some gardeners employ a foliar spray of calcium chloride as an emergency measure to supply calcium directly to the plant tissues. While foliar sprays can provide a quick, temporary boost to the plant’s calcium status, they are not a substitute for proper soil and water management. The long-term solution is rooted in correcting the underlying issues of water delivery and nutrient availability in the soil.