What causes blossom end rot in tomatoes?
Written by
Benjamin Miller
Reviewed by
Prof. Martin Thorne, Ph.D.Tomato blossom end rot begins with watery blemishes on the bottom end of the fruit (the blossom end), which eventually becomes leathery and black. The problem, as always, is not a fungus or an insect, but a calcium deficiency inside the developing tomatoes. Your plants are experiencing difficulty transporting calcium to the fruit during critical developmental stages.
Tomatoes develop strong cell walls due to the presence of calcium. Insufficient calcium can reach the fruit quickly, leading to the collapse of the cell structure. Tomato plants need a consistent flow of calcium during the 2-week period when the tomatoes are growing and can double in size. Interruptions can lead to those annoying dark patches.
Water Stress
- Drought conditions make calcium insoluble and unavailable for root absorption
- Overwatering drowns roots reducing oxygen needed for nutrient uptake
- Inconsistent moisture levels disrupt steady calcium transport to fruits
Soil Chemistry
- Low pH acidic soils below 6.0 lock calcium in insoluble forms
- High phosphorus or potassium levels compete with calcium absorption
- Excess magnesium creates chemical barriers preventing calcium uptake
Root Damage
- Tilling near plants severs fine feeder roots critical for absorption
- Nematode infestations reduce root surface area for nutrient intake
- Compacted soil restricts root growth and oxygen circulation
Environmental conditions worsen calcium transport. When air temperatures exceed 90°F (32°C), plants allocate cellular energy storage to the survival of leaves versus fruit pathology. High humidity can further accelerate transpiration, pulling more water away from fruits. Cold soil, with a temperature below 55°F (13°C), significantly reduces root activity.
Healthy roots optimally absorb calcium. Protect them by providing adequate spacing and using no-till methods. Your mulch helps preserve the soil's temperature and moisture. I typically use straw mulch 3 inches thick around my plants. Mulch helps protect the roots against temperature fluctuations caused by the weather.
A proper soil pH between 6.5 and 7.0 allows access to calcium. Test your soil each spring with inexpensive kits. If acidic, add lime; if alkaline, add sulfur. I do this early in the spring before planting each season. This is a basic step to make other treatments effective.
Select resistant varieties, such as Roma tomatoes, which have thicker cell walls. Utilize drip irrigation to establish uniform moisture management. Inspect new fruits daily for early symptoms. You can manage blossom end rot by practicing diligent gardening practices.
Read the full article: Tomato Blossom End Rot Explained Simply
