Apple Thinning

EarlyFruit Set Window (8mm up to 12mm).
This is the traditional time for chemically thinning
apples. All labeled thinning materials are effective at this
time including NAA, NAD, ACCEL, Vydate, Sevin.
Slightly higher rates are needed on some materials,
combination treatments have generally been more effective
than single materials.

Late Fruit Set Window (15-20MM).All labeled
thinning materials are effectiveat this time including
NAA, ACCEL, Vydate, Sevin. The high rates of these
materials must be used at this timing. In general early
applications of good rates thin aggressively. Moderate and
mild thinning occurs at lower rates and/or at later
timings.
Rates of individual thinners and/or combinations
should be based on past grower experience with
individual cultivars in each fruit block or use variety and
rate.

Climatic conditions cannot be controlled but can
greatly effect the strength of fruit set and the effectiveness
of chemical thinning materials. Dr. Rich Marini, VPI
reports that a combination of temperature, humidity, wind
and elevation will all effect chemical activity. Thinners
when applied during poor drying conditions will general
increase activity. Dew or light rain following treatment
may resuspend the chemical and cause additional uptake.
Cloudy conditions cause shading and reduce
carbohydrate levels in young fruits, causing poor fruit
retention. Marini reports applying thinners just before,
during, or just after a three-day cloudy period, especially
when temperatures are above 65F would likely increase
the thinning response.
Michigan information indicates that thinning activity
is related to temperature with more activity when
materials are applied in a warming trend.
What does the above mean to you the grower? It
means that good records and daily observation are
essential when working with chemical thinners. Rates,
materials and timing must be adjusted based on the
seasons current weather conditions.

Chemical Choice

NAAis one of our oldest and most reliable thinners.
It can be applied from petal fall to 20MM fruit size at rates
of 5ppm to 20ppm per 100 gallons.
Caution: on red delicious do not apply
concentrations more than 5ppm to avoid pygmy
fruits. Do not use NAA or NAD on any trees that are
to be treated with Accel, Promalin or Provide this
year! Pygmy fruits may result.

NADis a mild form of NAA and is used at PF and
early fruit set only! It is very affective on summer varieties
such as Paulared, Jerseymac, Macintosh cultivars and
Macoun. It is usually applied at 40-50 ppm per 100

Chemical thinning involves some risk, but in general
has been a satisfactory practice in improving fruit size and
consistent annual fruit production. No single practice in
apple production will have a greater impact on the bottom
line than the utilization of plant growth regulators
(PGR's) for chemical thinning.
Chemical thinning stabilizes annual crop production
and improves size, color and quality of fruit. Research
has shown that fruit size is directly related to how early
fruits are thinned. Thinning that reduces the clustering of
fruit will improve fruit color and quality. Adequate
chemical thinning will promote or guarantee return
bloom, and promote consistent annual production of
crops.
No single thinning program is applicable to all
orchards because of the many variables. Past experience
combined with detailed records of materials, rates, crop
performance, crop management practices, yield and
weather conditions are you best guide.
It is essential to understand which thinning materials
are available, how they work, and the different windows of
opportunity available for their application. Knowing the
cultivar response to these different materials will greatly
increase the success of your thinning program.
If you have specific thinning questions do not hesitate
to call your state or area county extension fruit agent.

Windows of application for thinning
apples

Bloom thinners. The only labeled material in NJ is
Wilthin. It is a new material that has been written up in
many popular articles. Apply thinners at 70-80% full
bloom, at 2-3 pints per 100 gallons of water, at a
minimum of 150 gallons of water per acre. It has been
effective primarily on the west coast on Red Delicious
cultivars. No significant difference was seen on thinning or
on return bloom on Macouns, in last years trials in
Hunterdon County, NJ.
Not enough information is available yet on this
product on east coast varieties to gauge its effectiveness.
Indications are that much higher rates are needed.
Additional research this year should provide better
information.
Bloom thinning is a good idea in apple production
theoretically, as the earlier we thin the better fruit size that
can be obtained. Thinning early lets us come back for
repeat applications with other materials if necessary at
petal fall and later.

Petal Fall Window (PF up to 8mm size).All
labeled thinning materials are effective at this time
including NAA, NAD, ACCEL, Vydate, Sevin.
Thinning early gives us greater fruit size, but usually at
PF we do not take enough fruit off and must come back
again with another application.

1998 New Jersey Commercial Tree Fruit Production Guide

77