Water
Calcium uptake disruption
Blossom end rot is usually caused by inconsistent watering interrupting how calcium moves into developing fruit, not a true shortage of calcium in the soil.
How to confirm it
- Check whether watering has swung between very dry and very wet recently.
- Confirm the dark, sunken spot is on the blossom end of the fruit rather than damage from an insect or the stem end.
- Rule out an actual soil calcium deficiency with a soil test before adding calcium products.
Next Steps
Fixes connected to this cause
Water consistently during fruiting
Blossom end rot usually responds to steadier watering rather than added calcium, since the underlying problem is delivery, not supply.