Blossom End Rot is a Physiological Problem of Tomatoes & Related Crops. It’s Due to Calcium Shortage But Isn’t Cured By Adding Calcium. Here’s The Solution.
Blossom End Rot Symptoms
Typically Blossom End Rot shows itself as as dark spots on the end of fruits where the flower (blossom) grew. The fruit often continues to grow, but the black part is dead and will usually rot. Sometimes it gets secondary fungal infections. The size of the dead spot varies in size and, where the fruit does continue to grow, will often form a flat depressed lesion.
There is no cure for the fruit that have BEM but it is possible to prevent new fruit from getting it.
Which Plants Get Blossom End Rot?
Tomatoes are the plant I get most questions about, but it can also affect peppers, chillis, aubergines, courgettes, marrows and squashes.
What Causes Blossom End Rot?
BEM is caused by a lack of calcium in the fruit, it’s not a disease. Knowing this many gardeners advise spraying or feeding with Epsom Salts. More calcium isn’t the answer for various reasons. Firstly the problem isn’t a lack of calcium in the plant, it’s a shortage in the developing fruit. The problem is one of calcium mobility, the plant is struggling to move the calcium into the fast developing fruit.
Secondly, the soil or compost is rarely short of calcium. Most soils have plenty of calcium. Adding more doesn’t help.
The answer is to make the calcium available to the fruit.
Interestingly, there is some evidence that fruit growth rate is a factor in BEM incidence. Faster growing varieties are more susceptible.
How Does Calcium Shortage Cause Blossom End Rot?
Calcium is needed to strengthen the cell walls in the fruit. If it is absent or in short supply the cells walls rupture and the cell dies. Once this happens there is no cure for that fruit. But we can prevent it happening to other fruit on the plant.
Epsom Salts Works For Me!
I often get told that Epsom salt foliar feeds work for some growers, but I doubt it actually does. It’s true to say that if you spray the fruit themselves they will absorb a minute amount of calcium. But even then it has to be mobile enough to move to the blossom end. There is no conclusive research to prove it works. It’s just anecdotal.
So why do people claim Epsom salts works? Often the reason is that conditions have changed and the condition is no longer exhibited. For example, watering may have increased or the weather may have become cooler. Both of these will help stop BEM.
It’s important not to confuse a cure with the cause of the cure!
Blossom End Rot Prevention
The simplest way to prevent BER is to give the plant sufficient water. It really is that simple.
When the plant has enough water it can mobilise the calcium it has and send it to the parts of the plant that are lacking it. That’s usually the fruit …especially the end where the blossom was. It’s the most distant part of the fruit so this makes sense.
Other Causes Of Blossom End Rot
If you are giving the plant enough water but then add a top dressing of fertiliser to the soil or compost it can often be the tipping point that sends the plant over the edge. That’s because the fertiliser will make it harder for the plant to take up the soil moisture quite as easily as before. So if you add fertiliser as a top dressing I suggest you ensure you water enough to prevent this. Just keep the soil moist for a few days.
The second cause of BER is tomato bags. They don’t contain much compost and, even if given a lot of water, tend to dry out too much between waterings. That limits calcium uptake in to the plant and precipitates BER. Plants grown in the soil, or in very large containers are less likely to get BER, provided the watering is sufficient.
When there is insufficient water availability there is an increased concentration of salts in the sap and this limits fruit transpiration and hence calcium mobility.
High humidity is also likely to cause BER. When humidity is high, such as in tunnels, the plant doesn’t need to take up so much water and hence the uptake of calcium is limited.
The cure is to increase ventilation. This drops the humidity and causes the plant to take up more water. The result is the plant getting more calcium and it being mobile enough to get into the fruit.
High nitrogen feeds can also lead to BEM. The nitrogen stimulates leaf growth which takes all the available calcium, with the fruit being left without the calcium it needs.
Does Overwatering Cause Blossom End Rot?
I see frequent references to overwatering causing BER. But these are all on gardening websites with lots of advertising, so I question if this is more about poorly researched content than reality. I suspect it is as these sites are often more about being click bait than accuracy.
Certainly I can find no research indicating that overwatering causes BME. The only references I see to BEM is where it says that even if there is some overwatering, after water shortage, it doesn’t cure Blossom End Rot.
More On Tomato Growing
There’s more on growing tomatoes via this link.