At a glance
- Blossom end rot is not a disease and not a fungus but a supply problem. The fruit lacks calcium because the water supply fluctuates.
- You recognise it by the dark, sunken, leathery patch always at the blossom end, that is at the bottom of the fruit, opposite the stalk.
- The most important lever is even watering. Swinging between drought and flood is the commonest cause, especially in pots.
- Mainly affected are tomato, pepper and courgette. The risk time is high summer, around CW 24 to 34, when the fruit grows fast.
The first ripe tomato hangs on the plant, but underneath sits an ugly black patch. Many think at once of rot or a fungus and reach for the spray. But no fungicide helps here, because blossom end rot is not a disease at all.
It is a cry for help from the plant. The fruit lacks calcium at a crucial spot, and the culprit is almost always the water balance. Understand this connection and you fix the cause with the watering can instead of chemicals, and the next fruits stay healthy.
Not a disease, a supply problem
Unlike a real fungal attack, behind blossom end rot there is no pathogen you could fight. The fruit grows fastest at the blossom end, and that is exactly where the plant must keep delivering calcium. But calcium moves through the plant only with the water stream. If the supply stalls, the fruit tip gets too little, the tissue dies and sinks in.
The tricky part: usually there is enough calcium in the soil. The problem is not the store but the transport. That is why it does no good simply to tip lime into the bed. The real key lies with the water.
How to recognise blossom end rot
Blossom end rot has a very clear appearance that sets it apart from real diseases.
The telltale features
- Always at the blossom end
The patch always sits at the bottom of the fruit, opposite the stalk. That is the surest sign. If the patch is elsewhere, it is something else.
- Leathery and sunken
The spot is dry, hard, leathery and brown to black, not soft and mushy. It feels firm, as if it had dried in.
- Grows with the fruit
First a small watery dot appears, which spreads into a large, flat patch as the fruit ripens.
- Do not confuse with late blight
Late blight is a fungus and makes patches anywhere on fruit and leaf. Blossom end rot, by contrast, sits only at the blossom end and is not an infectious disease.
Damage profile
Blossom end rot
Physiological disorder
- Typ
- Calcium shortage in the fruit, no pathogen
- Familie
- On tomato, pepper, courgette, aubergine
- Ernte
- Cause: fluctuating water supply
- Saison
- Risk time · CW 24 to 34
- Wasserbedarf
- Prevent: water evenly, mulch
Where it comes from: water and calcium
The chain of cause is always similar. If the soil dries out and is then watered through again, the calcium transport stalls while the fruit keeps growing. In pots this happens especially fast, because the small volume of soil dries out quickly.
Too much nitrogen plays a part too: it drives lush leaf growth that competes with the fruit for water and thus for calcium. And an excess of potassium or magnesium, for instance from over-feeding, blocks the calcium uptake as well. In the end it is always the same picture: too little calcium at the fruit tip.
How to prevent it
Water evenly
The be-all and end-all. Keep the soil evenly moist, without swinging between bone-dry and flooded. Better little and often than rarely and a lot.
Mulch
A layer of mulch from straw, grass clippings or leaves holds moisture in the soil and buffers fluctuations. It is the simplest insurance against blossom end rot.
Keep an extra eye on pots
Container plants dry out fastest. Use a big enough pot, check the soil daily and in high summer water twice if need be. More on it in Tomatoes in pots and on the balcony.
Feed moderately
Avoid a nitrogen surplus and do not over-feed with potash. A balanced tomato feed is enough. Too much of a good thing promotes the disorder rather than helping.
Remove affected fruit
Take off affected fruit; it will not recover. That way the plant puts its strength into the next, healthy fruit, once the water supply is right again.
Do not just tip lime into the bed
Because the word calcium shortage comes up, many scatter plenty of lime or eggshells into the bed. That usually does not help, because the soil normally holds enough calcium. The problem is the transport under fluctuating water, not the store. Fix the watering first; that works more reliably than any dose of lime.
The rest of the fruit stays edible
A tomato with a small blossom end spot need not be thrown away. Cut out the hard, brown spot generously, and the rest of the fruit is fine and tastes normal. Only if soft rot or mould has already formed at the spot does the whole fruit go on the compost.
No fungus, no spray. Blossom end rot is a water problem, and the solution is in the watering can.
The core rule against blossom end rot
Frequently asked questions
Is blossom end rot contagious?
No. It is not a disease and not a pathogen but a supply disorder of the single fruit. It does not spread to other plants. As soon as the water supply is even again, the new fruits stay healthy.
Can I still eat tomatoes with blossom end rot?
Yes, if you cut out the hard, brown spot generously. The rest of the fruit is fine. Only with soft secondary rot or mould does the whole fruit go.
Does adding lime or eggshells help?
Usually not. Most soils hold enough calcium; it just does not reach the fruit tip because of the fluctuating water. Instead of scattering lime, water more evenly and mulch.
Why does it hit my potted tomatoes harder?
Because pots dry out fast and are then often watered through all at once. That very swing triggers blossom end rot. A large pot, daily checks and mulch in the pot prevent it reliably.
Sources
- Julius Kühn Institute: blossom end rot on tomatoes and peppers
- Bavarian State Institute for Viticulture and Horticulture (LWG): preventing blossom end rot
- Related in the magazine: Courgette: from the variety to the harvest and The garden in July
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