Blossom end rot
A dark, sunken, leathery patch at the flower end of a fruit. It is not a disease and nothing is attacking the plant: the fruit could not move enough calcium into its fastest-growing tissue while it was forming, almost always because water supply was irregular.
Described view
Two green tomatoes side by side at the same scale: one with the flat, dry, leathery, sharply-bounded sunken patch of blossom end rot at the flower end, and one with the greasy, spreading, brown-marbled lesion of late blight that has no clear edge and is not confined to the flower end
What brings it on
Fluctuating water availability during fruit development. Calcium moves with the transpiration stream, so a plant that dries out and is then flooded cannot deliver it evenly to the fruit tip — even when the soil holds plenty of calcium. Containers and grow bags are the commonest setting.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Blossom end rot
- Home and Garden Information Center — Clemson Cooperative Extension: Tomato diseases and disorders — blossom end rot
Checked 2026-08-13
Preventing it
Ordered by how much difference each measure actually makes, not by how often it is repeated.
- Water consistently rather than heavily. A smaller volume more often beats a soaking after a dry spell, and mulching evens out what the roots experience between waterings.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Blossom end rot — prevention
Checked 2026-08-13
- Do not let a container crop wilt. In hot weather a fruiting tomato in a pot may need water more than once a day.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Growing tomatoes in containers
Checked 2026-08-13
- Avoid heavy nitrogen feeding during fruit set, which pushes leafy growth that competes with the fruit for water.
source
Horticultural consensusmoderate confidence
- University of Minnesota Extension — University of Minnesota: Growing tomatoes — nutrition
Checked 2026-08-13
Dealing with it once it is there
- Remove affected fruit so the plant puts its resources into sound ones. Later trusses on the same plant are usually unaffected once watering is steady.
source
Horticultural consensusmoderate confidence
- Royal Horticultural Society: Blossom end rot
Checked 2026-08-13
Widely repeated, and not supported
Recorded so GrowingHQ can address them. These are not alternative approaches — they are things the evidence does not back.
“Blossom end rot is fixed by adding calcium — eggshells, lime, or a calcium spray.”
Most soils and composts already contain ample calcium, and the problem is transport rather than supply. Adding more calcium to soil that has enough changes nothing; correcting the watering does. Lime also raises pH, which can cause a separate problem on ground where it was not needed.
Horticultural consensusCommonly said, not supportedhigh confidence
- Royal Horticultural Society: Blossom end rot
- Home and Garden Information Center — Clemson Cooperative Extension: Tomato disorders — blossom end rot
Checked 2026-08-13
What it affects
Signs that lead here
Each of these lists the other things that produce the same sign, and how to tell them apart.
On sprays and treatments
GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.
Plant-specific guides
Where this is one of several causes that look alike on a particular plant, GrowingHQ publishes the differential rather than the general answer.