blossom-end rot
A physiological disorder of tomatoes and other fruits, where the blossom end darkens and rots, typically caused by calcium deficiency or irregular watering.
If blossom-end rot appears on your tomatoes, suspect a calcium deficiency.
Compound of 尻 (shiri, 'buttocks, bottom') and 腐れ (kusare, 'rot, decay'), literally 'bottom rot', describing the location of the disorder on the fruit.