Blossom end rot in tomatoes and peppers: it's not a disease

My neighbor Jim has grown tomatoes in his backyard on Long Island for fifteen years. Every summer, at least a portion of his crop develops that familiar dark, leathery rot on the bottom of the fruit.

My neighbor Jim has grown tomatoes in his backyard on Long Island for fifteen years. Every summer, at least a portion of his crop develops that familiar dark, leathery rot on the bottom of the fruit. He's tried calcium sprays. He's added lime. The rot keeps coming back. The reason it keeps coming back is that he never addressed the actual cause: his tomatoes dry out between waterings.

Blossom end rot looks like a disease. It isn't one. No pathogen is involved. Per Missouri Botanical Garden, blossom end rot is "a physiological disorder" — a disturbance in the normal physiology of the plant. You cannot spray it away.

What blossom end rot actually is

Calcium is a structural component of plant cell walls, and it is immobile in the plant — meaning it cannot move from older tissue to newer tissue once deposited. When a tomato or pepper fruit is developing rapidly, it relies on a constant stream of calcium delivered through the xylem (the plant's water-transport tissue) from the roots.

When calcium supply to the developing fruit tip is interrupted — even briefly — the cells at the blossom end break down. This breakdown manifests as the characteristic dark, sunken, leathery spot at the base of the fruit.

Per Missouri Botanical Garden, the early symptom is "a light tan patch on the blossom end of the green fruit." Over time the area turns dark brown or black and may become sunken or leathery. Sometimes internal black rot develops in the center of the fruit with little or no external symptoms. The fruit is not entirely lost — affected areas can be cut away and the rest eaten.