What is tomato blossom end rot?

Rotting plants are usually associated with diseases caused by bacteria or fungi -- but not this one! Blossom end rot, which most commonly affects tomatoes, is actually due to a nutrient deficiency where the tomatoes don't get enough calcium. Low calcium levels can still occur in the tomato plant even if there is sufficient calcium in the soil. In this case, rather than amending the soil with unecessary fertilizer, affected plants can be treated using a spray such as this one: Bonide Rot-Stop
 
Here is an article with more details about treating and preventing blossom end rot: https://www.canr.msu.edu/resources/blossom_end_rot_tip_sheet