Three chemicals are available for reducing pre-harvest drop, extending harvest timing, and increasing storage life:
1) Aminoethoxyvinylglycine, AVG (ReTain)
- Delays maturity and thus fruit drop by inhibiting ethylene production.
2) Napthaleneacetic acid, NAA (Fruitone N, Fruitone L, PoMaxa)
- Decreases abscission of the fruit stem from the bud but can advance maturity especially when used at high rates or with repeated applications.
3) 1-Methylcyclopropene, 1-MCP (SmartFresh, Harvista)
- Blocks the ethylene receptors, thereby preventing the autocatalytic climacteric process of fruit ripening.
- SmartFresh is used in storage rooms on pre-climacteric fruit.
- Harvista, the sprayable formulation of 1-MCP, is not yet commercially available for growers in Virginia.
Below are some suggested strategies for using ReTain and or NAA in your orchards this year. Applications should be based upon the predicted harvest date for each cultivar.
- 1) ReTain (AVG) applied alone
- Used at 1 pouch/acre [333 g (50 g a.i.) or 11.7 oz]. Apply to full coverage (~100 gal/acre). Use with an organosilicone surfactant (e.g. Sylgard 309 or Silwet L-77). Allow at least six hours of drying time. ReTain has a 7 day PHI.
- For single pick cultivars, apply ReTain four weeks ahead of anticipated harvest date.
- Standard approach for single pick varieties—reduces fruit drop and delays the harvest period by reducing ethylene production. Can also allow more time for the fruit to color on the tree.
- For multi-pick varieties (e.g., Gala and Honeycrisp), apply ReTain one to two weeks ahead of the first harvest date.
- Will not delay maturity, of the first harvest, but will do so for the later picks.
- Ethylene suppression effect can last for up to 4 months in regular air storage.
2) NAA applied alone
- Use at 10-20 ppm. High rates (>= 20 ppm) may advance maturity and reduce storage life. See the 2014 Spray Guide for cultivar specific recommendations.
- Do not make more than two applications per year.
- Do not apply more than at 3X concentration.
- Organosilicone or nonionic surfactants are recommended.
- Don’t tank mix NAA with calcium (a water conditioner might be needed if you have hard water).
- Use maturity indices (starch rating) and limb tapping to determine timing.
- Takes 2-4 days for the material to become active in the tree.
- Apply single applications at 14 to 5 days before harvest; or two applications between 10-14 and 5-7 days before harvest.
- Or use the branch tap method to decide when to apply NAA
- As you get close to harvest, firmly strike a 3-4” diameter branch with the palm of your hand.
- Do this on a daily basis.
- If more than two fruit per limb fall, then apply NAA or consider harvesting within the next few days.
- Single applications last about 7 days; split applications (two 10 ppm applications about five days apart) last about 12 days. Fruitone L has a 2 day PHI; PoMaxa has a 7 day PHI.
- For apples that produce a lot of ethylene, NAA alone may not provide sufficient drop control. This is especially true in hot, dry years.
3) Combined sprays of ReTain and NAA
For the past several years, I have been evaluating the combination of ReTain with NAA at various timings and rates on Red Delicious, York, and Rome. Analysis of these experiments is still underway, but my initial assessment is that the combined sprays are especially effective at reducing pre-harvest drop while not reducing storage potential especially in hot, dry years.
When ReTain (AVG) was tank mixed with NAA, I found decreased ethylene production in stored fruit, and increased fruit quality (firmness–though differences were often 1 lb pressure or less). Additionally, the closer ReTain is applied to harvest, the better the fruit quality in storage. By using both materials (AVG and NAA), growers have the option of improving drop control without losing storage potential.
Some of my treatments included:
- ReTain (full-rate) at 4 weeks before harvest, followed by NAA (10 ppm) at two weeks before harvest. (Provided good drop control and storage life.)
- ReTain (half-rate) at 4 week before harvest, followed by NAA (10 ppm) at two weeks before harvest. (Provided good drop control and storage life, but did not extend harvest window or storage life as long as using the full rate.)
- ReTain (full-rate) at 2 weeks before harvest, plus NAA (10 ppm) at 2 weeks before harvest. (This provided the best drop control and storage life.)
- ReTain (half-rate) at 2 weeks before harvest, plus NAA (10 ppm) at 2 weeks before harvest. (This was not as effective at reducing pre-harvest drop compared to the full rate, but is a cost effective option when treatment is needed on a large acreage.)
I suggest growers consider the market destination for their fruit prior to applying either ReTain or NAA. With the multiple combinations and permutations of chemistries, rates, and timings, growers can manage harvest date, fruit drop, and storage quality with a fair amount of precision. While combined ReTain and NAA applications are effective at decreasing pre-harvest drop, this approach is not necessary for all cultivars, storage plans, or market destinations.
Here are some examples: For apples that are going to be sold through direct markets, NAA alone may be sufficient to prevent drop. However, PYO operations may want to consider ReTain to delay the harvest of early ripening cultivars until the peak apple picking crowds arrive in September. For apples that are going to be treated with SmartFresh, the additional ethylene suppression from ReTain may not be necessary, and NAA alone might be sufficient to reduce pre-harvest drops. For growers who have significant acreage of a single cultivar, or who have multiple cultivars that overlap in harvest timing (or otherwise expect to have limited labor available for harvest), applying ReTain to part of the orchard can help delay harvest and stretch out harvest window.
For additional reading, Dr. Duane Greene (University of Massachusetts) recently wrote this article about his experiences with pre-harvest drop materials.