Blossom-end rot in tomatoes and peppers
Blossom-end rot is a common nuisance on tomatoes, peppers, chillies and courgettes: on the underside of the fruit – where the flower once sat – a dark, sunken, leathery patch forms. Many mistake it for a fungal disease, but it is in fact a deficiency symptom and is not contagious.
The cause is a calcium deficiency in the fruit – usually not because there is too little calcium in the soil, but because the plant cannot transport it into the fruits when the water supply is uneven. The good news: with the right care the problem can almost always be stopped.
In short
- A dark, sunken patch at the lower (blossom) end of the fruit.
- The cause is calcium deficiency due to irregular watering – not a contagious disease.
- Prevent: water evenly, mulch, fertilise moderately.
- Remove affected fruits – the plant itself stays healthy.
How to spot blossom-end rot
At first a small, watery-glassy spot appears at the lower end of the fruit. It quickly grows larger, sinks in, turns brown to black and looks leathery-dry. Mainly the first ripening fruits and fast-growing, elongated varieties are affected. The leaves and the rest of the plant stay healthy.
Why blossom-end rot occurs
Calcium is transported into the plant with the water flow. If the water supply falters – for example through dry spells followed by heavy watering – too little calcium reaches the fruits. It is encouraged by:
- Irregular watering and drought stress, especially in container plants.
- Too much nitrogen or potassium, which blocks calcium uptake.
- Acidic or low-lime soils.
- High evaporation on hot days with a small root ball.
Preventing blossom-end rot
- Water evenly: better regularly and thoroughly than seldom and a lot – avoid fluctuations at all costs.
- Mulch: a mulch layer keeps the soil moisture constant.
- Fertilise moderately: do not overdo the nitrogen; special tomato fertilisers are balanced.
- Lime supply: on low-lime soil add algal lime or garden lime and check the pH value.
- Enough root space: large pots (at least 10–15 l per tomato) buffer drought stress.
What to do in an acute case
If blossom-end rot is already there, you can counteract it:
- Remove affected fruits – they will not recover and only cost the plant energy.
- Even out the water supply immediately, ideally with mulch.
- Adjust fertilising: reduce nitrogen, ensure balanced nutrients.
Tip
Blossom-end rot is not contagious – so you do not need to remove the whole plant. Usually the next fruits ripen perfectly again once the water supply is balanced.
Caution
Do not react with even more fertiliser – additional nitrogen often makes the problem worse, because it further hinders calcium uptake.
Frequently asked questions
Can I still eat affected tomatoes?
You can use the healthy part of the fruit – cut away the rotten area generously.
Does adding eggshells to the soil help?
Only in the long term, as calcium is released from them very slowly. Even watering works immediately and is more decisive.
Well-rooted, strong young plants cope better with fluctuations. We raise our young plants to be robust – a good start to a healthy harvest.