Blossom end rot is the dark, sunken patch that forms on the bottom (flower end) of tomatoes, peppers, eggplants and sometimes squash and watermelon. It is not a disease. It happens when not enough calcium reaches the developing fruit, and the usual reason is uneven watering, not a lack of calcium in the soil. The fix is steady soil moisture, mulch, moderate nitrogen and a soil test; calcium sprays do not work.
This guide explains why it happens, how to prevent it, and what to do with affected fruit. It follows guidance from the Royal Horticultural Society, the University of Kentucky and Clemson University, listed at the end.
Blossom End Rot at a Glance
| Question | Answer |
|---|---|
| What it looks like | A water-soaked spot at the flower end that turns brown to black, sunken and leathery |
| Crops | Mainly tomatoes and peppers; also eggplant, squash and watermelon |
| Cause | Too little calcium reaching the fruit, usually because water supply to the plant is uneven |
| Is it a disease? | No. It is a physiological disorder; fungicides and pesticides will not help |
| Main fix | Keep soil evenly moist; mulch; avoid excess nitrogen; protect roots |
| Calcium sprays | Not effective |
| Affected fruit | Will not recover; remove it |
Why It Happens
Calcium travels from the roots to the rest of the plant in water, through the xylem. Leaves lose a lot of water, so they pull in most of the calcium; fruit, which loses little water, gets less. When soil moisture swings between wet and dry, the supply to the fruit drops, and the cells at the blossom end, the last part of the fruit to receive calcium, collapse.
The RHS notes that it is very rare for garden soils, growing bags or potting mixes to actually lack calcium. The problem is delivery, not supply. The University of Kentucky lists the conditions that make it worse:
- Uneven moisture. Soil that is too wet or too dry slows root growth and water uptake.
- Too much nitrogen. Lush leaves draw more water, and the calcium goes with it to the leaves instead of the fruit.
- Competing nutrients. High levels of ammonium, magnesium, potassium or sodium compete with calcium for uptake.
- Root damage. Hoeing or cultivating close to plants cuts roots.
- Acidic soil. Calcium is less available below pH 6.0.
- Fast early growth. The first fruits of the season are often the most affected.
How to Prevent Blossom End Rot
- Water evenly. Clemson calls consistent soil moisture the most effective prevention. Do not let soil dry out between waterings; a drip line on a timer helps.
- Water containers more often, not more heavily. Plants in pots and growing bags are most at risk. The RHS advises that in hot weather it is better to water twice a day than once with double the volume.
- Mulch. A layer of organic mulch or straw slows moisture loss.
- Test the soil before planting. Kentucky suggests a pH of about 6.5 to 7.0 for most vegetables, with lime applied several months before planting if the soil is acidic.
- Go easy on nitrogen. Prefer nitrate forms, such as calcium nitrate, over ammonium-based fertilizers, and avoid heavy feeding that produces soft, leafy growth.
- Protect the roots. Do not dig or hoe close to established plants.
- Choose varieties carefully. In Kentucky’s experience, Whopper, Wonder Boy and Big Boy show high incidence, Jet Star, Early Girl and Better Boy lower, and cherry tomatoes are not known to develop it.
Why Calcium Sprays and Eggshells Do Not Fix It
Clemson explains that calcium sprayed on leaves cannot move through the phloem into the fruit, and fruit skin cannot absorb it. Kentucky reports that calcium chloride foliar sprays have not been found effective. The RHS adds that the nitrate in some calcium products can push more leafy growth, which draws calcium away from the fruit. Adding calcium to the soil only helps when a soil test shows it is low or the pH is too acidic; eggshells release calcium far too slowly to affect this season’s fruit.
What to Do With Affected Fruit
Affected fruit will not recover, so pick it off to let the plant put its energy into healthy fruit. The damaged area can be invaded by fungi and bacteria. Later fruits often develop normally once watering evens out.
Blossom End Rot or Something Else?
| Symptom | Likely cause |
|---|---|
| Sunken dark patch at the bottom of the fruit | Blossom end rot |
| Pale, papery patch on the side facing the sun | Sunscald |
| Cracks radiating from or circling the stem end | Fruit cracking, often after heavy rain or watering following dry spells |
| Concentric dark rings on fruit near the stem | Possible fungal disease such as early blight |
Frequently Asked Questions
Can you eat a tomato with blossom end rot?
Our sources focus on prevention and on removing affected fruit, and do not address eating it. Many gardeners cut away a small, dry spot on an otherwise firm fruit, but discard any fruit that is soft, moldy or rotting, since microbes can invade the damaged tissue.
Does Epsom salt help blossom end rot?
No. Epsom salt is magnesium sulfate, and extra magnesium competes with calcium for uptake.
Will blossom end rot spread to other plants?
No. It is not infectious. It reflects growing conditions, so plants grown the same way may show it at the same time.
Why do only the first tomatoes have it?
Early fruit forms while the plant is growing fastest and roots are still establishing, so calcium supply to the fruit falls behind. Later fruit is often fine.
Sources
- Royal Horticultural Society: Blossom End Rot
- University of Kentucky Center for Crop Diversification: Blossom End Rot
- Clemson HGIC: Gardening Myths, Fix Blossom End Rot with Calcium Sprays
Related: tomato suckers and our pest and plant problem guides.
