Plant Problem Diagnostic
Blossom-End Rot: Identification and Prevention Across Vegetable Crops
The short answer
Blossom-end rot is not a disease and not caused by an insect or fungus. It is a physiological disorder that shows up as a water-soaked spot at the blossom (bottom) end of the fruit that turns sunken, leathery, and dark brown to black. It affects tomato, pepper, eggplant, squash and other cucurbits, and watermelon. The underlying cause is a shortage of calcium reaching the developing fruit, which is usually a delivery problem driven by uneven soil moisture rather than a lack of calcium in the soil itself.
Because it is a calcium-delivery and watering issue, the fix is cultural: keep soil moisture even with deep, regular watering and mulch, avoid over-fertilizing (especially with high-nitrogen or ammonium fertilizers), and don't damage roots. Affected fruit will not recover, but later fruit on the same plant can be fine once conditions stabilize. If it persists despite steady watering, a soil test through your local Extension can rule out an actual calcium or pH problem.
How to identify it
- A water-soaked spot at the blossom end of the fruit that enlarges into a sunken, leathery, dark brown to black patch
- Appears on the bottom of the fruit, not on leaves or stems
- Often shows on the first fruit of the season or during hot, dry spells
- Sometimes affects only some fruit while nearby fruit stays healthy
Conditions that favor it
- Fluctuating soil moisture, especially drought stress alternating with heavy watering or rain
- Rapid early growth that outpaces calcium movement into fruit
- Excess nitrogen, particularly ammonium forms, or high salinity in soil
- Root damage from cultivation or overly dry, then saturated, soil
Prevention steps
- 1Water deeply and evenly — Use infrequent, deep irrigation to keep soil uniformly moist rather than swinging between bone-dry and soaked.
- 2Mulch the root zone — Apply mulch around plants to slow evaporation and buffer soil moisture.
- 3Go easy on nitrogen — Avoid over-fertilizing and avoid heavy ammonium-based nitrogen, which can worsen the disorder; follow soil-test guidance.
- 4Test soil before adding calcium — If it recurs, get a soil test through Extension; add lime only if the test shows low calcium or low pH, following its recommendation.
When to escalate
Blossom-end rot usually resolves once watering is steady. If most fruit is affected across several plantings despite even moisture, have your soil tested through your local Extension office to check calcium levels, pH, and salinity before applying amendments.
✔ Foliar calcium sprays and home remedies are unreliable fixes; the most dependable control is consistent soil moisture. Do not assume you need to add calcium without a soil test.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- NC State Extension — Blossom-End Rot of Tomato, Pepper, and Watermelon — Blossom-end rot is a calcium disorder driven by uneven moisture and low calcium ratios; affects tomato, pepper, watermelon
- University of Minnesota Extension — Blossom End Rot in Tomatoes and other Vegetable Crops — Cause is lack of calcium in the fruit, often from moisture fluctuation not soil deficiency; prevention via even watering and mulch
- University of Wisconsin-Madison Extension — Blossom End Rot — Affects peppers, eggplant, and cucurbits as well as tomato; managed by consistent moisture and avoiding over-fertilizing