BER is caused by a calcium deficiency, usually induced by fluctuations in a plant’s water supply. Because calcium is not a “mobile” element in plants, even brief changes in the water supply can cause BER.

Droughty soil or damage to the roots from excessive or improper cultivation (severe root pruning) can restrict water intake, thus preventing plants from getting the calcium they need. Plants also can develop calcium deficiency and BER if they are growing in highly acidic soil or are getting too much water from heavy rain, overirrigation, or high relative humidity.

[–] DrinkBoba@lemmy.world 6 points 2 years ago

This is the correct answer

[–] grayman@lemmy.world 3 points 2 years ago

Yep. Mine did that like 15 years ago. It was freaky.

[–] SpermKiller@kbin.social 3 points 2 years ago

To add further, most people with blossom end rot are watering incorrectly, ie a little every day, when you should water a lot but less often (about every 3 days, depending on your climate). Mulching will also help you manage better, and testing the humidity of the soil before watering goes a long way.

Remember, tomatoes are plants from warm regions, they can handle a little drought but they do much worse if they're over watered. Watering deeply and less often also encourages them to develop deeper, stronger roots.