Blossom End Rot on Tomatoes: Causes, Prevention and What to Do
Blossom end rot is one of the most common tomato fruit problems. It usually appears as a pale, water-soaked area at the blossom end of a developing tomato—the end opposite the stalk. The patch then becomes brown or black, sunken and leathery.
Although it can look like a disease, blossom end rot is a physiological disorder. It is not caused by a fungus, bacterium or pest, and it cannot spread from one plant or fruit to another.
The disorder occurs when a developing fruit does not receive enough calcium at a critical stage of its growth. However, this does not automatically mean that the soil lacks calcium. In many gardens, the underlying problem is an interruption in the plant’s ability to absorb and transport calcium.
How to Recognise Blossom End Rot
The first sign is usually a small, pale or water-soaked patch at the blossom end of a young tomato.
As the fruit grows, the affected area may:
Turn tan, brown or black
Become sunken
Develop a dry, leathery texture
Expand across a significant part of the fruit
Become vulnerable to secondary decay
The damage normally begins at the end opposite the stalk, although the exact position may vary slightly according to the shape of the fruit.
A secondary fungal or bacterial infection may later develop in the damaged tissue. This can make the affected area soft, mouldy or rotten, but the original cause remains blossom end rot.
Is Blossom End Rot a Disease?
Blossom end rot is not an infectious disease.
It does not spread:
Between tomatoes on the same plant
From one plant to another
Through contact
Through wind or splashing water
There is no need to apply a fungicide or insecticide to treat blossom end rot. These products cannot correct the disrupted calcium supply that caused the damage.
Several fruits may develop the disorder at the same time because they are experiencing the same growing conditions.
What Causes Blossom End Rot?
Calcium is needed to build and maintain strong plant cell walls. Developing tomatoes require a continuous supply as their cells divide and enlarge.
Calcium moves from the soil into the roots with water. It is then transported through the plant. If water uptake becomes irregular or the roots cannot function properly, an adequate amount of calcium may fail to reach the developing fruit.
This means blossom end rot can occur even when a soil test shows that calcium is present.
The most common contributing factors are:
Soil repeatedly changing from very dry to very wet
Inadequate watering
Heavy rain or watering after drought
Root damage
Waterlogged soil
Restricted roots
Excessive fertiliser
High salt levels in soil or container compost
Rapid, soft growth caused by too much nitrogen
Very rapid fruit growth
Cold soil limiting root activity
Extreme heat increasing water stress
A genuinely low supply of available calcium
Soil pH that restricts nutrient availability
Several of these factors may affect a plant at the same time.
Why Irregular Watering Causes Blossom End Rot
Calcium is not moved around the plant as freely as some other nutrients. Its delivery to developing fruit depends heavily on a steady flow of water through healthy roots.
When soil becomes too dry, water and calcium uptake slow down. If the soil is then flooded, the fruit may begin expanding rapidly, but the disrupted calcium supply may already have damaged the developing cells.
This is why blossom end rot often appears after:
A dry period followed by heavy rain
Containers being allowed to dry out completely
Small, frequent surface watering
Sudden changes in irrigation
Hot weather increasing the plant’s water use
The damage may not become visible immediately. A dark patch appearing today may have resulted from stress that occurred several days earlier.
Can Overwatering Cause Blossom End Rot?
Yes. Although blossom end rot is commonly associated with dry soil, excessive water can also contribute.
Waterlogged soil contains less oxygen. Tomato roots need oxygen to function, and prolonged saturation can damage them or prevent normal nutrient uptake.
A plant in wet soil may therefore be unable to absorb sufficient calcium even though both water and calcium are present.
Good moisture management means keeping the root zone reasonably consistent, not keeping it constantly saturated.
How Root Damage Contributes
Tomato roots may be damaged by:
Deep cultivation near the plant
Hoeing too close to the stem
Transplanting
Waterlogging
Root disease
Pests feeding below ground
Severe drying
A container becoming excessively hot
Roots becoming tightly restricted
Once roots are damaged, their ability to absorb water and nutrients is reduced.
Avoid digging deeply around established tomato plants. Remove weeds carefully and install supports early so they do not have to be pushed through the root zone later.
Can Too Much Fertiliser Cause Blossom End Rot?
Excessive fertiliser can increase the risk in several ways.
High concentrations of fertiliser salts can make it more difficult for roots to take up water. Too much nitrogen can also encourage rapid leafy growth, increasing the plant’s demand for water and calcium.
Heavy feeding does not guarantee that more calcium will reach the fruit. It may instead create a nutrient imbalance or damage the roots.
Follow the recommended application rate for any fertiliser. Do not respond to blossom end rot by applying several different feeds in quick succession.
Are Certain Tomatoes More Susceptible?
Any tomato can develop blossom end rot, but some types are more vulnerable.
It is often seen more frequently in:
Large-fruited tomatoes
Plum and paste tomatoes
Elongated varieties
Fast-growing fruit
Container-grown plants
Plants with restricted roots
Different varieties may respond differently even when they are growing in the same bed.
The first fruits of the season are sometimes affected while later tomatoes develop normally. Young plants may have relatively limited root systems when their first fruit begins its rapid growth. Once the roots expand and watering becomes more consistent, the problem may disappear.
How to Prevent Blossom End Rot
Prevention focuses on maintaining healthy roots and a reliable supply of moisture.
Keep Soil Moisture Consistent
Check the soil regularly rather than watering according to a fixed timetable.
Feel approximately 5–8 centimetres below the surface. If the soil remains moist, additional water may not be needed. If it feels dry at that depth, water thoroughly.
The aim is to moisten the root zone. Repeatedly applying a small amount to the surface can encourage shallow roots and may leave deeper soil dry.
Water Deeply
Apply water slowly enough to soak into the soil rather than running away from the plant.
Deep watering encourages roots to explore a larger volume of soil. A larger root system gives the plant more stable access to both moisture and nutrients.
Use Mulch
Organic mulch can reduce evaporation and slow rapid changes in soil moisture.
Suitable materials include:
Mature compost
Leaf mould
Straw
Other clean organic mulches
Apply mulch after the soil has been watered and warmed. Keep thick material slightly away from the main stem.
Protect the Roots
Avoid deep cultivation around established plants. Pull or cut weeds carefully and do not repeatedly disturb the soil near the stem.
Install stakes, cages and other supports before planting or while plants are still small.
Feed Moderately
Use fertiliser according to the needs of the plant and the fertility of the soil.
Avoid excessive applications of high-nitrogen feeds. Once flowering and fruiting are established, a balanced feed intended for fruiting crops may be more appropriate than one designed primarily to promote leafy growth.
Maintain Suitable Drainage
Improve poorly drained soil before planting. Containers must have clear drainage holes and should not remain in stagnant water.
If the root zone stays wet for long periods, reducing watering alone may not be enough. Soil structure, container drainage or the growing position may also need attention.
Provide Enough Root Space
Tomatoes grown in undersized containers are more vulnerable because their compost can dry very quickly and the roots have little access to reserve moisture.
Choose a container suitable for the mature size of the variety. Large indeterminate plants need considerably more root space than compact patio tomatoes.
Preventing Blossom End Rot in Containers
Container-grown tomatoes require particularly careful watering.
During hot or windy weather, a mature plant may use water quickly enough to require daily attention. The goal is not necessarily to water every container every day, but to check them often enough that they do not dry out severely.
Helpful measures include:
Using a large container
Ensuring the drainage holes remain open
Watering the entire volume of compost
Applying water slowly
Mulching the surface
Protecting the container from intense reflected heat
Feeding at the recommended concentration
Avoiding long periods in standing water
A dry surface does not always mean the whole container is dry. Equally, a wet surface does not prove that moisture has reached the lower roots. Check below the surface whenever possible.
Should Calcium Be Added to the Soil?
Calcium should only be added when there is a reason to believe it is genuinely deficient or unavailable because of unsuitable soil conditions.
A soil test is the most reliable way to assess calcium levels and pH.
Repeatedly adding calcium without testing may:
Alter the soil’s nutrient balance
Change the pH
Interfere with the availability of other nutrients
Fail to address the actual watering or root problem
Many soils already contain enough calcium for tomato plants. Improving moisture consistency is often more effective than adding another product.
Do Eggshells Prevent Blossom End Rot?
Eggshells contain calcium, but they break down slowly. Large pieces placed around an established tomato plant will not release calcium quickly enough to repair fruit that is already affected.
Finely crushed eggshells can contribute small amounts of calcium to soil over a long period as part of normal composting and soil improvement. They should not be regarded as an immediate treatment for blossom end rot.
Adding eggshells also cannot correct irregular watering, damaged roots or waterlogged soil.
Do Calcium Sprays Work?
Spraying calcium onto tomato leaves or fruit is unlikely to correct the underlying problem reliably.
Calcium is not readily redistributed from leaves to developing fruit. The fruit needs a continuous supply delivered through the plant’s roots and water-transport system.
A foliar spray cannot repair cells that have already been damaged. Maintaining healthy roots and steady soil moisture remains the more useful response.
Will Epsom Salt Help?
Epsom salt contains magnesium sulphate, not calcium.
It does not provide the calcium associated with blossom end rot and should not be applied as a routine treatment. Excess magnesium can compete with calcium and may make an existing nutrient imbalance worse.
Use magnesium only when there is a sound reason to believe it is deficient.
Can Antacids or Calcium Tablets Be Used?
Products intended for human consumption are not designed to manage soil fertility. Their use does not correct the most common causes of blossom end rot and may introduce unnecessary ingredients into the soil.
If a genuine calcium deficiency is confirmed, use a soil amendment suitable for horticultural use and apply it according to the soil test and product instructions.
What to Do When Blossom End Rot Appears
Once blossom end rot is visible, the damaged part of that tomato will not recover.
Take the following steps:
Check the soil moisture below the surface.
Correct severe dryness or waterlogging.
Review how frequently and deeply the plants are being watered.
Check container size and drainage.
Stop any excessive feeding.
Avoid disturbing the root zone.
Apply mulch if the soil is bare and drying rapidly.
Continue monitoring new fruit.
Do not make several drastic changes at once. Sudden overwatering or heavy feeding may create further stress.
Should Affected Tomatoes Be Removed?
Severely affected young tomatoes can be removed. This prevents the plant from continuing to invest energy in fruit that will not develop normally.
Slightly affected fruit may continue growing. If the damaged area remains dry and secondary decay has not developed, the unaffected portion may still be usable after the damaged tissue has been removed.
Discard fruit that is:
Soft
Mouldy
Leaking
Foul-smelling
Extensively rotten
Damaged internally
Blossom end rot itself is not infectious, but damaged tissue can provide an entry point for decay organisms.
Will New Tomatoes Also Be Affected?
Not necessarily.
Blossom end rot often affects one group of fruit while later tomatoes remain healthy. Once moisture becomes more consistent and the root system develops, calcium delivery may improve.
Continue checking newly forming fruit. If the disorder keeps appearing, reassess:
Watering depth and frequency
Drainage
Root health
Container size
Fert Fertiliser concentration
Salt accumulation
Soil pH
The possibility of a genuine calcium shortage
Why Only One Plant May Be Affected
Tomatoes growing next to one another do not always experience identical root conditions.
One plant may have:
A smaller root system
More fruit
Greater exposure to wind or sun
A damaged stem or root
Less access to irrigation
A more susceptible variety
A smaller or faster-drying container
Check the affected plant individually instead of assuming that the whole bed needs additional water or fertiliser.
Blossom End Rot or Something Else?
Not every dark mark on a tomato is blossom end rot.
Blossom End Rot
Usually begins at the end opposite the stalk and becomes dark, sunken and leathery.
Sunscald
Usually appears on the side of the fruit exposed to intense sunlight. The affected area becomes pale, yellow-white or papery.
Fruit Disease
May create spots, soft rotting tissue, mould, cracking or lesions elsewhere on the fruit. Related symptoms may also appear on leaves or stems.
Pest Damage
May produce holes, feeding scars or irregular wounds. Larvae, droppings or other signs of feeding may be present.
Soil Contact Damage
Fruit resting against wet soil may develop damaged or rotten areas where it touches the ground.
Correct identification matters because each problem requires a different response.
Blossom End Rot Is Usually a Water-Management Problem
Calcium is essential to healthy tomato development, but blossom end rot is rarely solved simply by adding more calcium.
In most cases, the priorities are:
Consistent moisture
Deep watering
Healthy, undisturbed roots
Good drainage
Adequate root space
Moderate feeding
Protection from rapid drying
The damaged fruit cannot be repaired, but the plant itself can remain healthy and productive. Correct the growing conditions calmly, monitor the next developing tomatoes and avoid unnecessary treatments.
With steadier moisture and a stronger root system, later fruit will often develop normally.
For complete advice on growing healthy tomato plants from sowing through to harvest, return to How to Grow Tomatoes: Complete Guide.
