Apples sometimes drop to the ground before they reach the desired harvest maturity. This pre-harvest fruit drop can cause significant losses in apple orchards. As apples mature, an abscission layer of specialized cells naturally forms between the apple stem and the fruit spur to which it is attached. The abscission layer allows the apple to be released from the tree without causing damage to the tree. However, apples may fall off the tree before the ideal harvest maturity has been obtained.
There are many potential reasons why apples may drop prematurely, and some cultivars are more prone to drop than others. McIntosh apples are prone to falling off the tree before harvest. Apples with short stems may physically “push” themselves off of the branch they are attached to, or other apples that share the same fruiting spur as fruit size increases towards harvest. Both biotic and abiotic stressors can cause fruit to drop prematurely. Drought stress and strong winds are common causes of pre-harvest drop, as well as heavily cropped trees. Apples with severe disease or insect damage will also be more prone to fall off the tree before harvest.
Two plant growth regulators are commonly applied to apple orchards to minimize pre-harvest drop, aminoaminoethoxyvinylglycine (AVG, ReTain®) and 1-naphthaleneacetic acid (NAA). AVG retards maturity and thus fruit drop by inhibiting ethylene production, while NAA decreases abscission of the fruit stem from the bud but can advance maturity after repeated applications.
Greg Peck, Virginia Tech