Blossom end rot is not a disease, but a physiological disorder – produced by incorrect growing conditions. It is caused by a lack of the nutrient calcium in the fruit. This causes the cell membranes to break down and the affected cells to die. New cells don’t grow and this results in the formation of the sunken areas.
It is highly unlikely that soils, potting composts and growing-bags don’t contain enough calcium, but its movement within the plant needs a good, regular supply of water to the plant. A lack of water at the roots, or an irregular, non-constant supply of water, prevents calcium movement. The fruit, being the furthest away from the stem, is the first part of the plant to suffer if a lack of water prevents calcium getting to them.
Plants growing in growing-bags and, to a lesser extent large pots, that have limited space for the roots to grow are most at risk. But plants growing in the ground can also be affected.
Blossom end rot can also occur when fertiliser is added to dry soil or compost, because the concentration of nutrients restricts water uptake by the roots. Excess applications of nitrogen-rich fertilisers makes the problem worse.