In general, the earliest signs of Blossom-end rot are areas of white or brown tissue cells. If the condition is internal, symptoms will appear in the placenta or in the Blossom-end pericarp.

External Disorders
External Blossom-end rot begins as a small, water-saturated spot at or near the blossom scar of green tomatoes. As the spot gets larger in diameter, covering up 1/2 of the fruit's surface, the affected tissue becomes dry and turns a light to dark brown and leathery.

It's widely known that Blossom-end rot occurs when calcium and/or water levels are low in the root zone. According to Adams and Ho (1993) BER generally develops due to a "lack of coordination between the transport of assimilates by the phloem and of calcium by the xylem during the rapid cell enlargement in the distal placenta tissue, i.e., an interaction between the rates of fruit growth and of calcium acquisition at the distal end of the fruit." Genetics and the environment also make plants susceptible to BER

Anatomical Level
The lack of calcium at the anatomical level is the immediate cause of tissue damage that leads to the development of BER. Even when calcium is abundant in the root zone, the localized deficiency in the distal (Blossom-end) locular tissue of the fruit may cause the development of Blossom-end rot.

There are various reasons for the low levels of calcium. Deposition into the calcium pectate and calcium phosphate fractions in the distal pulp tissue is low under all conditions (Minamide & Ho, 1993)

BER may also occur because external factors reduce the import of calcium, interfering with and reducing the requirements of the cell walls and cell membranes. Semi-permeability of the cell membrane or weakened cell walls causes leakage of the cell content and could possibly be the direct cause of Blossom-end rot symptoms.