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Why do apples, pears, oranges, lemons, and grapefruit sometimes drop before harvest?

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lime processing — fruit processing Limes being prepared for processing into juice, Tecoman, Mexico.

Apples, pears, oranges, lemons, and grapefruit may drop from the tree during the final stage of their maturation period. This phenomenon, known as preharvest drop, is a natural occurrence that growers often manage to protect their yields.1

The process of fruit drop is biologically linked to abscission, a mechanism controlled by plant hormones. Hormones such as ethylene and abscisic acid promote the synthesis of enzymes at the base of the fruit stem, which catalyze the breakdown of cell walls, eventually causing the fruit to detach.2

To mitigate this loss, farmers often apply dilute sprays of growth-regulating substances, such as naphthaleneacetic acid (NAA), to delay the abscission process. Additionally, chemicals like Alar have been used on apples to reduce preharvest drop while simultaneously improving fruit quality, including firmness and color.1