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. Three plant growth regulators are currently registered for control of preharvest drop in apples. Harvista, napthaleneacetic acid (NAA) and aminoethoxyvinylglycine (AVG) are all effective, but they are very different compounds with respect to the modes of action, optimal timing, and effect on the fruit. This article offers an overview of each. Additional details are in the fact sheet titled "Apple Preharvest-Drop-Control Sprays."
Harvista is a 1-mcp product designed for preharvest application to apples. 1-mcp blocks the ripening effects of ethylene by binding up the ethylene receptors on the plant cell membranes, making them unresponsive to ethylene action. Treatment with Harvista slows starch disappearance, fruit softening, red color development, and preharvest drop, and can delay the onset of watercore. Because Harvista can be applied close to anticipated harvest date (to within 3 days prior to harvest), the desired characteristics of fruit maturity can develop normally, and treatment can be applied just before deleterious effects begin. Do not use Harvista on stressed trees. The effects of using Harvista with other stop drops has not been fully evaluated. Harvista is applied by a proprietary in-line injector system. Contact AgroFresh for additional information.
The active ingredient in ReTain is aminoethoxyvinylglycine (AVG), a "look-alike" for one of the chemical precursors to ethylene. When absorbed into plant tissues, AVG binds irreversibly with a key enzyme. This prevents the ethylene precursor from binding, thus blocking the production of ethylene. Natural ripening processes are slowed, including stem loosening, fruit flesh softening, starch disappearance, and red color formation.
In order for AVG to be effective it must be applied well in advance of the climacteric rise in ethylene production that signals the onset of fruit maturity. The label recommends applying ReTain 4 weeks before anticipated harvest for cultivars that are normally harvested in a single picking. Varieties differ in the production of ethylene and consequently differ in their response to ReTain. Low ethylene producers such as Gala are strongly influenced, while ethylene production is much harder to control for high ethylene varieties such as McIntosh.
In 2015 the ReTain label was expanded to allow use of up to two pouches per acre, either as a single spray or as a split application for apple. ReTain use on pear is limited to a single pouch per spray, with up to two sprays allowed per season.
Synthetic auxins interfere directly with the enzymes that create the abscission zone. Today, the one auxin that is registered for this use is NAA.(Fruitone L, PoMaxa). Unlike ReTain, fruit maturity of apples treated with NAA is not delayed; in some cases, it may be accelerated.
Since the optimal application time for NAA is just before the onset of drop, NAA offers a "rescue" treatment, should the threat of preharvest drop be increased due to unforeseen circumstances.
A single spray of NAA can provide about 7 days of drop control. Since it is less expensive than ReTain, it may be more cost effective to use NAA when only a few days of drop control are needed to conduct an orderly harvest. For example, when using ethephon to promote fruit coloring, growers may also use NAA to prevent excessive fruit drop resulting from accelerated fruit maturation.
Timing an NAA stop-drop spray requires monitoring of fruit maturity. The label says to apply NAA seven to 14 days before harvest. A single spray of 10 to 20 ppm NAA can control drop for 7 to 10 days from the date of application, but it takes 2 or 3 days to "kick in." If NAA is applied too early, then effective drop control may wear off too soon. If NAA is applied a few days too late, a significant portion of the crop may drop before it takes effect. Predictive degree-day models and the pattern of starch disappearance, as gauged by the starch index test, can provide a general indication of whether the potential for drop is earlier or later than normal, but more direct monitoring is desirable for the actual timing of the sprays.
A Monitoring Strategy for Determining the Start of Preharvest Drop
Varieties that are susceptible to preharvest drop should be monitored to determine when fruit drop is beginning. Limb tapping is one method that can be used to determine the onset of drop as fruit near maturity. Bump several scaffold limbs or leaders of 3 or 4 inches in diameter throughout the block on a daily basis. Use the palm of your hand with a short firm stroke, striking the limb at its mid-point (just like golf, this skill improves with practice and experience).
If zero to one apple per limb drops on average, it is too soon to apply NAA. If the average is about two, check again later the same day or the next morning. When several apples drop in response to limb bumping, it is time to harvest within 2 days or apply NAA.
A concentration of 10 ppm NAA is usually adequate for an effective stop drop. To obtain the maximum drop control, use a split application of 10 ppm in the first spray, followed by a second spray of 10 ppm five days after the first. Split applications can provide some drop control for 10 to 14 days from the date of the first application.
NAA must be applied with good coverage and plenty of water. Concentrating beyond 4X (less than 75 gallons of water per acre for 300 gallon TRV) may diminish the effectiveness. The use of alternate row spraying is discouraged. Use of a nonionic or organosilicone surfactant is recommended to enhance uptake. Weather conditions following the application also impact efficacy.
Rewetting within 1 to 2 days of the spray application and spraying under slow drying conditions (high humidity) will increase the uptake of NAA. Temperatures in the mid-70s produce a better response than cooler temperatures, while excessively hot weather immediately following an NAA spray will likely result in accelerated fruit ripening.
The use of NAA on trees previously treated with ReTain results in drop control that is superior to that obtained by using either product alone. Fruit treated in this manner, then left for an extended time on the tree, may have limited storage potential. Half rates of ReTain are not sufficient to prevent the advanced ripening sometimes caused by NAA. If the fruit is destined for storage, then a full rate of ReTain should be used when tank-mixing ReTain and NAA.
Record Keeping for the Application of Plant Growth Regulators
Evaluating your application of plant growth regulators to your orchards is an important but often overlooked aspect of record keeping. The response of most PGRs is highly influenced by the weather conditions before, during, and after they are applied to the trees. Therefore, it is important that you have good weather records so you can learn how weather influences your use of PGRs in your orchard. The Penn State Spray Record Keeping Spreadsheet can be downloaded and used to track conditions under which you applied your preharvest drop materials.