There are three options for using plant growth regulators for pre-harvest drop control. One is NAA, sold as several different products, another is AVG, sold as ReTain (Valent USA Agricultural Products), and the third is 1-methylcylopropene sold as HarvistaTM (Agrofresh). The three differ in mode of action and timing of application. NAA is typically applied just once before the onset of fruit drop and lasts for about one week before its effect is gone. ReTain is typically applied 2-3 weeks before anticipated harvest, has long lasting drop control if applied under favorable conditions, and repeat applications can be made to extend the period of time apples will remain on the tree. Harvista can be applied 3 to 21 days before harvest and requires specialized equipment (contact Agrofresh). Both ReTain and Harvista will slow fruit ripening. For an excellent summary/discussion of uisng apple PGR's for drop control, see this Penn State University article: Apple PGRs - Prevention of Preharvest Drop in Apple Orchards.
Application details for NAA (Fruitone-L, Refine 3.5 WSG, Refine 3.5L)
- rates of 10 ppm are usually effective, however, up to 20 ppm can be used
- good coverage at dilute application is preferred
- apply just as apples starts to loosen; apply too early and drop control will be limited; apply too late, and drop will have already started
- drop control can last for 7-10 days; sometimes two applications (at 5-7 days apart) can extend that to 14 days
- NAA application can result in accelerated fruit maturity and fruit softening
Application details for ReTain
!Note that the ReTain label gives very specific instructions depending on intended use -- read it!
General use guidelines for ReTain include:
- apply 21 to 7 days before anticipated harvest (7 day PHI)
- use one pouch (or less) per application; two applications/two pouches are allowed
- on Honeycrisp and Gala, consider using reduced rates -- early application of ReTain can inhibit color development on these varieties
- use an organosilicone surfactant at a concentration of 0.05 to 0.1% (6.4 to 12.8 oz surfactant per 100 gallons water) with ReTain, do not apply when temperature exceeds 85 degrees F.
- dilute applications are recommended, up to 2 X concentration
- avoid application when rain is expected within 12 hours
- do not apply to stressed trees
Application details for HarvistaTM
- applied at a rate of 48 to 242 fl. oz.
- Do not add any water to the container
- Do not allow product to contact any copper
- drop control can last for 7-10 days