Publication 360, Guide
to Fruit Production: Chemical Control of Preharvest Drop of Apples
Excerpt from Ch. 4, Publication 360, Guide
to Fruit Production 2012-2013,
Other Information on Plant Growth Regulators
Chapter 4: Apple
Calendar - PDF 357 kb
Other Information on Apples
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
trees that carry a heavy crop
hot days and warm nights just before and
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
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
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
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 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
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
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
Local: (519) 826-4047