Blossom end rot: why it happens and how to stop it
Last reviewed September 2026 · written from public-domain USDA/NOAA data and Cooperative Extension references — see how we source data
Blossom end rot is one of the most common problems on home-grown tomatoes, peppers and squash, and one of the most misunderstood — the usual advice to simply add calcium to the soil rarely fixes it.
What it looks like
A flat, dark, leathery patch develops on the bottom (blossom end) of the fruit, typically early in the fruiting season on tomatoes, peppers and summer squash.
It is not a disease
Nothing is infecting the fruit. Blossom end rot is a calcium-delivery problem — the developing fruit is not getting enough calcium to its cells as it grows, even in soil that has plenty of calcium in it.
The real cause: inconsistent water
Calcium moves through the plant dissolved in water, pulled along by transpiration. A dry week followed by a heavy watering interrupts that flow to the fruit far more often, in practice, than a genuine calcium shortage in the soil does.
Preventing it
Water consistently — mulch and drip irrigation both make that a lot easier. Avoid heavy nitrogen feeding early on, which pushes leafy growth faster than the roots that need to keep pace with it. Expect it to fade as watering settles into a rhythm through the season.