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Bloom end rot can be caused by several things but the main thing is an underdeveloped root system.
If the dirt is too cold, when you plant and the roots don't develop well that can cause it early on, but the plant will usually recover as the weather warms and the roots are able to develop later in the season as long as the plant is given plenty low nitrogen fertilizer.
If the dirt is too hard and compact making it impossible for the plants to develop a good root system that can also cause it; you can alleviate that problem by working a lot of organic material into the soil before you plant.
If you dig too close to the plants to weed or break the soil for better aeration and you damage the roots that can cause it as well.
If the plants are hit with a sudden drought and they can't get the nutrients they need this will also cause it.