By: Kayla Wyse
Few gardening frustrations are as disappointing as watching a healthy tomato develop a dark, sunken spot on the blossom end of the fruit.
Known as blossom end rot, this common disorder often appears just as gardeners anticipate their first ripe harvest.
What Is Blossom End Rot?
Despite what many gardeners believe, blossom end rot is not caused by a disease or insect. Instead, it results from a calcium deficiency within the developing fruit. Blossom end rot is a physiological disorder of the tomato.
In many cases, the soil contains adequate calcium, but environmental conditions prevent the plant from absorbing or transporting enough of this nutrient to the fruit.
The first signs typically appear on green tomatoes as a light brown, water-soaked area on the blossom end opposite the stem. As the fruit develops, the affected area darkens, enlarges and becomes leathery or sunken.
What Causes the Problem?
Inconsistent soil moisture is the leading cause of blossom end rot. Tomato plants require a steady water supply during fruit development.
Drought stress, alternating soil moisture extremes and damage to a plant’s roots all can inhibit calcium uptake, as can waterlogged or cold soils, and concentrations of ammonium, potassium and magnesium in the soil.
Movement of calcium within plants depends on active transpiration. Because leaves transpire more than fruits, calcium moves more easily into leaves where it remains. Calcium is not later redistributed from leaves to fruits.
This preferential distribution of calcium to leaves can be made worse by over-fertilizing with nitrogen, which promotes excessive production of leaves.
In addition, high relative humidity or low relative humidity in combination with hot, windy weather can limit transpiration, thus preventing calcium from reaching fruits.
Prevention Is the Best Strategy
Consistent watering is the most effective way to prevent blossom end rot. Tomatoes generally need about one inch of water per week, although more may be necessary during hot, dry weather.
Water evenly and mulch the soil to retain moisture during dry periods. Avoid practices that would damage roots. Use nitrate (NO3-) rather than ammonium (NH4+) forms of nitrogen fertilizer. DO NOT over-fertilize.
Have your soil tested periodically to determine if there is sufficient calcium in the soil. Many gardeners add eggshells or calcium sprays when blossom end rot appears. However, these remedies often provide little benefit if inconsistent watering is the underlying cause.
If blossom end rot develops, remove affected fruits and focus on maintaining even soil moisture. Once growing conditions improve, plants often produce healthy fruit for the remainder of the season.
Final Thoughts
Blossom end rot can be frustrating, but it is usually manageable with proper cultural practices. Paying close attention to moisture levels and reducing plant stress can help ensure a steady supply of healthy tomatoes throughout the harvest season.
The Scarlet Sprout will feature a new gardening topic. If you have specific questions that you would like answered in an upcoming column, please contact faes-williamsextension@osu.edu.
Until next time, stay curious about what’s growing in our corner of Ohio.
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Kayla Wyse serves as the Agriculture and Natural Resources Extension Educator for Williams County, where she has been supporting residents since 2023. She holds a Master’s in Plant Health Management from The Ohio State University. Kayla is passionate about entomology, soil science, and helping people feel confident in their gardening and growing skills.