Thinning early lets us come back for repeat applications with other materials if necessary at petal fall
In New York, NAA is recommended as a bloom thinner for Empire and other smaller fruited varieties.
When fruit set conditions warrant a bloom application on these cultivars an application of NAA at
Bloom can do some limited thinning. NY recommends 10ppm. I would try on a trial basis 3-5 parts
per million NAA. Additional thinning applications will be needed in most years. NAA does not have a
memory and therefore repeat applications are not cumulative.
Petal Fall Window (PF up to 8MM size)
All labeled thinning materials are effective at this time including NAA, NAD, ACCELL, 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.
Early Fruit 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, ACCELL, Vydate, Sevin, ethephon. Slightly higher rates are need on
some materials, combination treatments have generally been more effective than single materials.
Late Fruit Set Window (15-20MM)
All labeled thinning materials are effective at this time including NAA, ACCELL, Vydate, Sevin,
ethephon. The high rates of these materials must be used at this timing along with combinations of
During this time of thinning the receptivity and response to thinners is declining. the use of
combinations is necessaryat this time. Ethephon 2 or Ethrel can has also been effective on some
varieties at this time. Dr. Ross Byers of VPI has done extensive work on using Ethrel for apple
Some Notes and Suggestions for Apple Thinning in 1999
- 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 guidelines as outlined in our 1998 NJ
Commercial Tree Fruit Manual.
New Thoughts on NAA:many researchers have been indicating that while NAA thins fruit and
helps with return bloom, it does not increase fruit size. In addition, in up to a third of the cases, NAA
may reduce fruit size. Hence the move toward Sevin XLR or Sevin and NAA combinations. Mostof
us are recommending against NAA for spur red deliciousor at the very least only in a combination and
not more than 5ppm on Reds.
The NAA base rate depends on the variety, the harder to thin cultivars require the higher NAA
concentration. The exception would beSpur Delicious. I would not use more than 5ppm NAA on
Reds and I would use it only in combination with Sevin or Vydate at Petal Fall.