Publication 360, Guide to Fruit Production: Chemical Control of Preharvest Drop of Apples

Pub 360: Guie to Fruit Production > Apples > Thinning and Plant Growth Regulators > Chemical Control of Preharvest Drop of Apples

Excerpt from Ch. 4, Publication 360, Guide to Fruit Production 2012-2013,

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Cover of Publication 360, Guide to Fruit ProductionTable of Contents

  1. Other Information on Plant Growth Regulators and Thinning
  1. Other Information on Apples
  2. Chapter 4: Apple Calendar - PDF 357 kb

Chemical Control of Preharvest Drop of Apples

Cultivars differ in their tendency toward preharvest drop. In this respect, McIntosh gives the greatest concern in Ontario.

Conditions associated with premature fruit drop include:

  • trees with high nitrogen content in late summer
  • trees that carry a heavy crop
  • hot days and warm nights just before and during harvest
  • trees deficient in either moisture or the nutrients boron or magnesium
  • trees heavily damaged by pests

Products to control preharvest drop


Products such as Fruitone-N and Fruit Fix Concentrate contain NAA (naphthaleneacetic acid) and are registered to control drop on apples. These contain the same active ingredient, NAA, used at different concentrations than for thinning.

  • Apply NAA as soon as the first undamaged apples begin to drop, at the rate of 10 ppm or single strength rate. Timing is important. Do not confuse the normal preharvest drop of sound apples with those that have been pushed off or are dropping because of insects, disease or nutritional deficiency.
  • NAA is effective for 7-10 days after it is sprayed. For stop-drop prevention beyond this, repeat the application of a single strength rate every 7 days. It takes 1-2 days for NAA to take effect if applied before any harvest has taken place. If applied after the orchard has been spot picked, it may take longer to take effect. Therefore, apply NAA as soon as the spot pick is finished. Do not apply NAA closer than 5 days before harvest. Refer to the label.
  • Apply NAA in a dilute spray to achieve more uniform coverage. This is important because NAA is only locally systemic. High volumes of water take longer to dry and allow increased absorption of NAA. The optimum conditions for maximum absorption are at or near 21-24ºC and high humidity. Absorption is less on foliage injured by insects, diseases or frost and at temperatures below 16ºC.
  • The addition of a non-ionic type spreader sticker like Agral 90 improves absorption of NAA under less than ideal weather conditions.
  • NAA inhibits fruit abscission but the fruit continues to mature at an accelerated rate. The higher the concentration used and the greater the number of applications, the greater the ripening effect. One application of single strength (10 ppm) has little effect on direct ripening.
  • Apples treated with NAA for preharvest drop will not keep well in long term storage, especially in McIntosh. Apples treated twice with NAA should be sold immediately.


ReTain is a plant growth regulator and an effective preharvest drop control agent for apples, such as Honeycrisp, McIntosh, Red Delicious and Northern Spy. It reduces the production of ethylene in maturing apples.

  • Apply this material 4-5 weeks before anticipated harvest date. ReTain may delay maturity of apples on the tree and allow a longer harvest window for a particular variety. Although ripening in ReTain-treated fruit is temporarily delayed, treatment with ReTain may increase soluble solids, colour, fruit size, fruit firmness and reduce the incidence of water core.
  • Pick fruit treated with ReTain at the same internal maturity, based on starch-iodine tests, as untreated fruit. There may be a 7-10 day delay from predicted harvest date of untreated fruit to the start of the harvest of ReTain-treated fruit.

Recommended use 4 weeks before anticipated harvest

  • Use one 333 g pouch (50 g active ingredient) of ReTain per 0.4 ha (1 acre). Use with Sylgard 309 (organosilicone surfactant) at 0.5-1.0 L of Sylgard 309 per 1,000 L of water.
  • Apply 4 weeks before anticipated harvest, adjusted for seasonal differences that have advanced or delayed the "normal" harvest date. It is better to be a week early than a week late. Late applications will not perform well because ethylene production within the plant may have already begun and this is difficult to suppress once initiated. To help determine the start of the "normal" harvest period, refer to historical trends for harvest dates for each cultivar in your area.


  • When you mix ReTain with the surfactant, keep agitation to a minimum because Sylgard 309 has a tendency to foam.
  • To minimize foam, add Sylgard 309 last, when the tank is full. Compatibility and performance data for ReTain with antifoam products are not available.
  • Do not use surfactants other than Sylgard 309 because they are not registered with ReTain in Canada. If the rate of ReTain is reduced, maintain the Sylgard rate at a 0.05%-0.1% v/v ratio.
  • Discard any unused spray material at the end of each day.
  • Do not tank-mix with other products except Dipel or Foray. Combination effects and potential interactions between ReTain and NAA (napthaleneacetic acid) or ethephon products have not been thoroughly evaluated. Use of these products on ReTain-treated blocks may negate or reduce the benefits of ReTain.
  • Do not use overhead irrigation for 8 hours after application.
  • Adjust spray water volumes based on tree size and spacing. Do not apply to the point of runoff but wet the tree under slow drying conditions. Use tree-row volume (TRV) reductions cautiously and test on a few rows or small blocks to gain a level of comfort with this technique. For many orchards, 1,000 L water per ha is adequate for size-controlling rootstocks.
  • For best results, have spray water at a pH between 6 and 8.

To maximize benefits of using ReTain, segregate threated fruit from untreated fruit in storage to reduce exposure to ethylene.


Additional considerations when using ReTain

  • Spray only trees that are healthy and not under severe drought, insect, disease or nutritional stress.
  • Apply treatments to Gala strains, Golden Delicious types and Honeycrisp with caution. These cultivars appear to be more responsive to ReTain compared to McIntosh. Using ReTain on these sensitive cultivars can significantly impede crop maturity and development.
  • For multiple-pick cultivars such as Royal Gala, time application four weeks from the second harvest pick date for best results.
  • The harvest window for treated cultivars is shorter but delayed and results in fewer pickings. This can increase harvest efficiency.
  • ReTain is not likely to compensate for the effects of an aggressive calcium spray program to control bitter pit. Calcium sprays can advance apple maturity by accelerating ethylene production in maturing fruit.

For more information:
Toll Free: 1-877-424-1300
Author: OMAFRA Staff
Creation Date: 05 July 2007
Last Reviewed: 01 November 2010