Many apple cultivars a susceptible to pre-harvest drop, which refers to the process where fruit falls from the tree prior to harvest. The production of ethylene by the ripening fruits stimulates the production of an enzyme that disintegrates cell walls and forms an abscission zone in the peduncle (fruit stem). Once the abscission zone has been formed, the fruit is connected to the tree only by the vascular elements, which can break easily and drop the fruit. Due to the irreversible nature of this process the best strategy to control pre-harvest fruit drop is to applied plant growth regulators (PGR) that will prevent the formation of the abscission zone.
There are 2 products available to growers that can be used to prevent pre-harvest fruit drop and are commonly refer to “stop-drop” or “sticker” materials:
1) ReTain: This product blocks the formation of ethylene, delaying apple maturity. ReTain must be applied 4 weeks before anticipated harvest for cultivars that are normally harvested in a single picking. In the case of cultivars that require several picking days (e.g., Gala or Honeycrisp), apply ReTain 7 to 14 days before the anticipated beginning of the first pick. This application will not delay the first harvest but will help control fruit drop for the following picking dates. There are differences among cultivars on ethelyne production, and thus response to ReTain applications. Low ethylene producing cultivars (i.e., Gala, Honeycrisp, Jonagold) will be more sensitive to ReTain and a half dose of the product (1/2 pouch or 333 g per acre) is recommended. Most varieties, however, require a full pouch of ReTain per acre in order to obtain satisfactory results.
2) NAA: Is a synthetic auxin that affect the activity of the enzymes that breakdown cell walls and form the abscission zone. Because NAA will target the enzyme and not ethylene, NAA applications will prevent fruit drop but they will not delay fruit maturity, as ReTain does, as it does not affect ethylene production. In fact, on hot weather days application of NAA might increase fruit ripening. There are several NAA commercial products (Fruitone L, Fruitone N, PoMaxa, Refine WSG, Refine 6.25L, Refine 24.4L), with a recommended application window of 7 to 14 days before harvest, and application rates of 10 to 20 ppm which will stop fruit drop for a period of 7 to 10 days. Optimal timing of NAA application will require growers to closely monitor fruit maturity. If NAA is applied too early the effect of the application will wear off before harvest has ended. However, if NAA is applied too late, there will be significant fruit drop. Wait until you start to see a few apples drop, and perhaps assist this by bumping a few branches and seeing if any apples drop. Then it’s time to apply the NAA. Longer stop-drop control can be obtained with a split application, 10 ppm applied 7-14 days apart.
In addition to ReTain and NAA there is also Harvista, a product from AgroFresh. The active ingredient in Harvista is 1-mcp, which blocks ethylene receptors so the fruit cannot respond to the ethylene it is producing. This product will stop fruit drop and delay fruit maturity. Harvista can be applied closer to harvest than ReTain, 3-14 days before the anticipated beginning of harvest. The only inconvenience of Harvista is that it needs to be applied with specialized equipment. Contact AgroFresh for additional information.
Always read and follow the label before spraying any products.This article was posted in Apples and tagged pre-harvest drop, stop-drop.