Apple Preharvest Drop-Control Sprays

As apples mature they begin to produce large amounts of the ripening hormone, ethylene. One of the ripening processes stimulated by ethylene is stem loosening. Ethylene stimulates the production of enzymes that break down the cell walls in the abscission zone of the stem, leaving the fruit connected to the tree by only the vascular strands, which are easily broken. Once this natural process is complete, susceptible varieties begin to drop. It should also be noted that enzymatic cell wall breakdown is an irreversible process. Once the fruit stems loosen, there is no way to strengthen them up!

Stop drops are plant growth regulators that interfere with the enzymatic breakdown of the cell walls in the abscission zone. Two plant growth regulators are currently registered for control of preharvest drop in apples. Napthaleneacetic acid (NAA) and aminoethoxyvinylglycine (AVG) are both effective, but they are very different compounds with respect to the modes of action, optimal timing, and effect on the fruit. This section offers a brush up on stop drops—how the two work and how to optimize control of preharvest drop with each of them.