Quick answer: Blossom-end rot is a dark, sunken, leathery patch on the bottom of tomatoes, peppers, eggplants, and squash, caused by a lack of calcium in the fruit — usually from uneven watering rather than poor soil13. Fix it by watering consistently and mulching, not by adding more calcium2.
What blossom-end rot looks like
The first sign is a small, water-soaked spot that looks like a bruise on the bottom of the fruit, the end farthest from the stem3. On tomatoes, this spot enlarges and turns dark brown to black, becoming sunken and leathery3.
On peppers, the spots can resemble sunscald and sometimes appear on the sides of the fruit near the blossom end, not just the bottom3. When cut open, an affected fruit's exterior may look fine while the interior is discolored and shrunken3.
As the damaged tissue sits exposed, bacteria and fungi often move in and cause further decay3. It tends to show up on the very first fruits a plant produces in a season3.
What causes it
Blossom-end rot is not caused by a disease or a pest1. It is a physiological disorder driven by adverse growing conditions, and it is caused by a lack of calcium reaching the fruit123.
Sometimes that shortfall really is low calcium in the soil13. Far more often, the soil has plenty of calcium, but soil moisture swings — too wet, then too dry — stop the plant from moving that calcium into the fruit13.
Which plants get it
Tomatoes are affected most often, along with related crops such as eggplants and peppers2. Vine crops including cucumber, pumpkin, squash, and watermelon can also develop blossom-end rot3.
How to prevent it
Because the root cause is usually watering, not soil chemistry, steady moisture is the main defense12. Making minor adjustments to watering habits will usually protect the fruit that forms afterward, even if earlier fruit was already affected2.
Steady moisture from a sub-irrigation reservoir — the design GardenStack planters use — helps prevent the wet-dry cycling that drives calcium delivery problems in fruiting vegetables. Checking soil moisture regularly through the growing season, rather than waiting until fruit already shows spots, helps catch watering swings before they affect the fruit.
Mulching around plants helps even out soil moisture between waterings, which supports steadier calcium uptake through the season1. Match your watering plan to your local growing season and climate, since heat and rainfall patterns both affect how fast containers and beds dry out.
What to do once you see it
Blossom-end rot is a physiological problem tied to growing conditions rather than a spreading disease, so it will not move from an affected fruit to healthy ones on the same plant23. Correcting watering going forward is what protects the next round of fruit2.
Frequently asked questions
Can I still eat tomatoes with blossom-end rot?
The rot is a physiological problem, not a pathogen taking over the whole fruit, so many gardeners cut away the affected end and use the rest.
Will blossom-end rot spread to other plants?
No. It isn't a disease or pest, so it doesn't spread the way a fungal or bacterial infection would123.
Does adding calcium spray fix blossom-end rot fast?
Consistent watering addresses the underlying cause, since most gardens already have plenty of calcium in the soil and the real problem is calcium not reaching the fruit123. Focus on evening out soil moisture first — that's what makes the biggest difference12.
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