ReviewA review of apple preharvest fruit drop and practices for horticultural management

- Apple fruit abscission is influenced by development, genetics, and environment.
- Abscising fruit are advanced in maturity compared with non-abscised fruit.
- Regulatory changes promote cell wall degradation, involving hydrolytic enzymes.
- Ethylene is associated with abscission promotion.
- Plant bioregulators influence a plant's hormone system to manage fruit drop.

Preharvest fruit drop is a challenge to apple production, in which fruit abscise from the tree prior to horticultural maturity. Depending on the growing season and the cultivar, yield losses of up to 30% are common by the beginning of harvest and worsen with any delay in harvest. This apple fruit abscission is influenced by developmental and environmental cues. Perception of metabolic changes leads to regulatory changes that promote cell wall degradation, involving the hydrolytic enzymes polygalacturonase and cellulase. Ethylene has a clear association with abscission promotion; whereas, the role of other plant hormones is unclear. The study of plant bioregulators has identified that apple abscission is managed by the ethylene biosynthesis inhibitor, aminoethoxyvinylglycine, and the competitive antagonist for ethylene receptors, 1-methylcyclopropene. This review examines recent progress in understanding preharvest fruit drop of apples, discusses horticultural practices that may alleviate preharvest fruit drop, and provides suggestions for future research.