What Causes Blossom End Rot on Tomatoes? A Complete Guide to Understanding and Fixing the Problem
Introduction: The Frustration of Blossom End Rot
You’ve waited patiently for your tomato plants to produce beautiful fruits — only to find black, sunken spots forming on the bottom of the tomatoes.
This heartbreaking issue is called blossom end rot, and while it looks serious, it’s not caused by disease — it’s a physiological disorder that can be corrected.
In this guide, you’ll learn what causes blossom end rot, how to fix it, and simple ways to prevent it so you can enjoy a healthy, abundant tomato harvest.
What Is Blossom End Rot?
Blossom end rot appears as:
- Black or brown sunken patches at the blossom (bottom) end of the fruit
- Dry, leathery, and sunken texture
- Spots that expand as the fruit grows
- Often affecting the first few fruits of the season
Although it looks bad, blossom end rot is not contagious and doesn’t spread from fruit to fruit — but it signals an important problem in the plant’s health.
What Causes Blossom End Rot?
1. Calcium Deficiency in the Fruit
Primary Cause:
Blossom end rot happens because the developing tomato fruit doesn’t receive enough calcium.
Important to know:
- The soil may have enough calcium, but if the plant can’t move it effectively to the fruit, problems occur.
- It’s a calcium uptake issue, not always a soil deficiency.
Factors That Lead to Calcium Deficiency in Tomatoes
1. Inconsistent Watering
Explanation:
Tomatoes rely on a steady moisture supply to absorb calcium properly.
Fluctuations between dry and wet conditions disrupt calcium transport.
Solution:
- Water deeply and consistently, about 1–2 inches per week.
- Mulch around plants to retain soil moisture.
2. Rapid Growth Spurts
Explanation:
In early fruiting stages, rapid plant growth can outpace calcium delivery to new fruits.
Solution:
- Maintain even moisture.
- Avoid over-fertilizing with nitrogen early in the season, which can encourage fast, leafy growth.
3. Excessive Fertilizer Use
Explanation:
Too much nitrogen fertilizer promotes leafy growth at the expense of fruit health and disrupts calcium absorption.
Solution:
- Use a balanced fertilizer designed for tomatoes, lower in nitrogen and higher in phosphorus and potassium.
4. Poor Soil Structure
Explanation:
Compacted or poorly draining soil makes it harder for plants to access nutrients.
Solution:
- Amend soil with compost or organic matter.
- Ensure good drainage and aeration.
5. Low Calcium in Soil
Explanation:
In rare cases, the soil itself is deficient in calcium.
Solution:
- Test your soil.
- If needed, add amendments like lime (before planting) or gypsum (can be applied during the season).
How to Prevent Blossom End Rot
1. Water Consistently
- Water tomatoes deeply once or twice a week depending on weather.
- Keep soil evenly moist, especially during fruit set and early growth stages.
2. Mulch Your Plants
- Apply 2–3 inches of straw, shredded leaves, or compost around plants.
- Mulching regulates soil moisture and temperature.
3. Use Tomato-Specific Fertilizers
- Choose fertilizers with moderate nitrogen levels.
- Look for products labeled for tomatoes, often with N-P-K ratios like 5-10-10 or 10-10-10.
4. Avoid Root Disturbance
- Be gentle when cultivating around plants to avoid damaging roots that transport water and nutrients.
5. Add Calcium if Necessary
- Apply calcium sprays (calcium nitrate or calcium chloride) to developing fruits if needed.
- These sprays can help but work best as a preventive measure rather than a cure.
How to Treat Tomatoes Already Affected by Blossom End Rot
- Remove affected fruits so the plant can focus on producing healthy new ones.
- Adjust watering immediately to even out moisture levels.
- Continue monitoring and maintaining steady moisture and nutrition.
New fruits can develop normally once the underlying issue is corrected.
Tomatoes Most Susceptible to Blossom End Rot
- Large-fruited varieties like beefsteaks
- Early season varieties that grow quickly
- Roma and paste-type tomatoes
These tomatoes have high water and calcium demands and are more sensitive to environmental stress.
Other Crops That Can Get Blossom End Rot
- Peppers
- Eggplants
- Squash
Similar symptoms and prevention methods apply to these crops as well.
Conclusion: Stop Blossom End Rot Before It Starts
While blossom end rot can be disappointing, it’s not a death sentence for your tomato plants.
By focusing on consistent watering, good soil health, balanced fertilization, and, if needed, calcium supplementation, you can prevent this common problem and enjoy a season full of delicious, beautiful tomatoes.
Catch the signs early, make a few adjustments, and you’ll be back on track for a bountiful harvest!
Top 10 Questions About Blossom End Rot
1. What is the main cause of blossom end rot in tomatoes?
Inconsistent watering that disrupts calcium uptake is the primary cause.
2. Can I still eat tomatoes with blossom end rot?
Yes — you can cut away the affected part and use the rest if the fruit is otherwise healthy.
3. Will blossom end rot spread to other fruits?
No — each fruit is affected individually based on calcium availability during its growth.
4. Does adding calcium to the soil fix blossom end rot immediately?
Not instantly — it takes time for calcium amendments to affect plant health.
5. Should I water tomatoes daily to prevent blossom end rot?
Water deeply 2–3 times a week rather than shallow daily watering to maintain even moisture.
6. Is blossom end rot caused by a fungus or disease?
No — it’s a physiological issue, not a fungal, bacterial, or viral disease.
7. Can I spray tomatoes with calcium to prevent blossom end rot?
Yes — calcium sprays can help if used preventively on developing fruits.
8. Why do only my first tomatoes have blossom end rot?
Early fruits often suffer the most because plants grow rapidly at that stage, stressing calcium movement.
9. Will using mulch really help prevent blossom end rot?
Yes — mulch is crucial for maintaining steady soil moisture.
10. Do cherry tomatoes get blossom end rot?
They can, but it’s much less common compared to large-fruited varieties.