Have you seen a tomato with a blackened bottom and wondered what it was? This unsightly condition is called blossom end rot. It affects not only tomatoes, but also peppers, eggplants, squash, cucumbers, melons and watermelons. It can occur at any stage of growth. Although less than appetizing, any normal-looking part of the fruit is still edible.
Blossom end rot is a disorder due to a shortage of calcium in young fruit. Moisture imbalances or water stress instigate the problem by interfering with calcium uptake in the plant.
The calcium deficiency is particularly likely when plants are water-stressed at night, which decreases the root pressure that would normally result in forcing an uptake of calcium and other nutrients.
With insufficient or uneven amounts of water, only part of the fruit gets what it needs. The other part does not develop end rots.
Other factors that can decrease calcium uptake include excessive salts in the soil, either naturally occurring or possibly from over-fertilization.
Damaged roots caused by improper cultivation or excess soil water may also lead to blossom-end rot.
Rarely, incomplete pollination may also result in blossom end rot.
The incidence of blossom end rot varies by variety and is greatly influenced by environmental conditions. It is more likely to occur when humidity is low, days are hot, and winds are high.
“The first symptom of the disorder is a slight, water-soaked discoloration on the blossom end (opposite of the stem) of the fruit.” This area turns leathery and dark brown or black. Disease organisms may invade the damaged area, producing a soft watery rot (https://utahpests.usu.edu/ipm/agricultural/vegetable/blossom-end-rot).
There is no “silver bullet” cure for blossom end rot. Adding calcium to the soil rarely solves the problem. There is usually adequate calcium in most soils; it’s just not getting to the fruit.
Buy varieties that are less prone to blossom end rot. Then, good cultural practices that encourage calcium uptake from initial planting through maturity will decrease its occurrence.
Keep the soil evenly moist with deep irrigation. This will reduce water stress. Do not allow plants to be water-stressed at night. Putting mulch around plants will decrease soil moisture fluctuations. However, do not overwater, especially in clay soils, because overwatering causes roots to rot, which reduces the number of roots available to absorb water. Fewer roots means more water stress for the plant.
Avoid over-fertilizing; do not use ammonium-based nitrogen fertilizers. Avoid hoeing near plants to avoid injuring roots.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.