Newsletter Articles
Blossom End Rot: What Causes It and How to Fix It
Written by David S.
A dark spot spreading across the bottom of your tomatoes is blossom end rot, and it is easy to mistake for a disease or a simple calcium shortage. It is actually a physiological disorder: uneven watering keeps calcium from reaching the fruit, so the fix is steady, consistent soil moisture and mulch, not piling on calcium, eggshells, or antacids.
Quick answer: Blossom end rot is a watering problem, not a disease, so treat the watering, not the calcium.
- It is a physiological disorder, not a disease or pest, so fungicides and insecticides do nothing.
- It usually happens even when the soil has plenty of calcium; the trouble is calcium reaching the fruit.
- Keep soil evenly moist: water deeply and regularly, about an inch a week, and mulch to even out moisture.
- Avoid drought-then-flood swings and heavy high-nitrogen feeding, which make it worse.
- Add calcium or lime only if a soil test shows a real shortage; foliar calcium sprays do not help much.
| Question | Answer |
|---|---|
| What is it? | A physiological disorder, not a disease or pest. |
| What causes it? | Too little calcium reaching the fruit, usually from uneven watering rather than low soil calcium. |
| What does it look like? | A water-soaked spot at the blossom (bottom) end that turns tan to black and flattened. |
| Which plants get it? | Tomatoes most often, plus peppers, eggplant, squash, and melons. |
| How do you fix it? | Keep soil evenly moist with deep watering and mulch, and avoid heavy nitrogen feeding. |
| Do calcium sprays or eggshells help? | Usually not; foliar calcium is poorly absorbed and the soil typically has enough calcium already. |
What Blossom End Rot Is (and Is Not)
Blossom end rot is a physiological disorder, not a disease or a pest, so there is nothing to spray for it. University of Illinois Extension states plainly that it is not caused by a disease or insects and is actually a physiological disorder.[1] NC State Extension describes it as an abiotic disorder, meaning it is caused by a non-living factor rather than a pathogen.[2] Clemson Cooperative Extension likewise classes it as a physiological disorder of tomato.[3] That is why fungicides and insecticides do nothing for it. For other tomato troubles that are actual pests or diseases, see our guide to common tomato plant problems.
What Actually Causes It
The real cause is too little calcium reaching the fruit, and that is almost always a watering problem rather than a lack of calcium in the soil. University of Illinois Extension explains that most of the time there is sufficient calcium in the soil, but the plant’s ability to take it up can be the problem, since calcium uptake depends on active transpiration and moves with water into the fruit.[1] University of Minnesota Extension makes the same point, noting that blossom end rot does not mean there is a lack of calcium in the soil, only that the developing fruit did not get enough.[4] NC State Extension adds that calcium is mobile and must travel to the fruit with water, so extreme fluctuations in water supply bring it on.[2] Drought stress, swinging soil moisture, and rapid growth from heavy nitrogen all make it worse. Our guide to water-wise watering covers how to keep moisture steady.
How to Prevent and Stop It
You prevent blossom end rot by keeping soil moisture steady, not by feeding the plant more calcium. University of Illinois Extension advises watering deeply rather than shallowly and often, aiming for about one inch of water per week from rain or irrigation, and using mulch to conserve and even out soil moisture.[1] It also cautions against excess fertilizer, especially ammonium-based nitrogen, and says to run a soil test before assuming you need to add calcium.[1] Clemson Cooperative Extension similarly recommends a uniform supply of moisture through irrigation and mulch, and liming only according to a soil-test report.[3] Steady soil, in short, matters more than any additive. Building that kind of soil is covered in our guide to creating healthy soil, and our overview of growing tomatoes in Southern California rounds out the care basics.
Do Eggshells, Tums, or Calcium Sprays Work?
Usually not, because the problem is getting existing calcium to the fruit, not adding more calcium to the soil. University of Illinois Extension notes that foliar applications of calcium do not help much, because the calcium is poorly absorbed and poorly moved to the fruit.[1] Since the soil usually already has enough calcium, as both University of Illinois Extension and University of Minnesota Extension point out, piling on more calcium through sprays, eggshells, or antacids does not address the real issue.[4] Fix the watering first, and the next flush of fruit usually comes in clean.
Seeing it on your tomatoes? Stop by any Green Thumb Nursery and our team can help you set up steady watering and mulch so your next harvest stays blemish-free.
Blossom End Rot FAQ
What is blossom end rot?
A physiological disorder, not a disease. University of Illinois Extension states it is not caused by a disease or insects but is a physiological disorder, so there is nothing to spray for it.[1]
What causes blossom end rot?
Too little calcium reaching the fruit, usually from uneven watering. University of Illinois Extension explains the soil usually has enough calcium, but inconsistent moisture limits the plant’s ability to move it into the fruit.[1]
Do eggshells or Tums fix blossom end rot?
Generally no. University of Illinois Extension notes foliar calcium is poorly absorbed and does not help much, and since the soil usually already has enough calcium, adding more is not the fix; steady watering is.[1]
What plants get blossom end rot?
Tomatoes most often, but not only tomatoes. University of Illinois Extension notes it also affects peppers, eggplant, squash, and watermelons.[1]
How do you prevent blossom end rot?
Keep soil moisture even. University of Minnesota Extension recommends regular watering to keep the soil moist, mulch to retain moisture between waterings, and fertilizing according to a soil test.[4]
Sources
- [1] University of Illinois Extension, Blossom End Rot (Good Growing): extension.illinois.edu
- [2] NC State Extension, Blossom-End Rot of Tomato, Pepper, and Watermelon: content.ces.ncsu.edu
- [3] Clemson Cooperative Extension, Tomato Diseases and Disorders: hgic.clemson.edu
- [4] University of Minnesota Extension, Tomato Disorders: extension.umn.edu
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