Blossom end rot disorder in tomato has been found to be caused by the deficiency of

Blossom end rot of tomato is a physiological disease and is caused by the deficiency of Calcium in the plant. It can occur in both green and ripe fruits and is identified by water-soaked areas that gradually widen and mature into sunken, brown, leathery spots on the bottom end.
This disease can be eradicated by adding high levels of calcium like bone meal, oyster shell or gypsum.