Thinning Windows  and Recommendations
The following information has been adapted from the Apple Thinning Guide by P. G. Schwallier from Michigan State University.  This is an excellent, easy to use thinning guide, and it is available from:
The Great Lakes Publishing Company.
343 South Union St.
Sparta, MI 49345
Phone: 616/887-9008
Fax: 616/887-2666
E-mail: mac82nd@aol.com

P. G. Schwallier divides the period in which the fruitlets are receptive to thinning into 5 subwindows opening at bloom and closing at about 30 days DAFB (days after petal fall), and include Bloom, Petal Fall, Early Fruit Set, Late Fruit Set, and Closing. Each subwindow is approximately 5 days wide depending on weather conditions.

The best general chemical thinning generally occurs at petal fall; however, if the weather is not favorable for thinning during that window, spays should be applied as soon as possible. If you need to thin aggressively, then early applications at good rates should be used. Moderately and mild thinning occur at later dates using lower rates of the thinning materials.

For a chart on thinning windows, please the April 99 issue of this newsletter.

Recommended materials according to fruitlet size

Bloom Thinners: 
Wilthin® is a urea sulfuric acid complex, which is not yet registered in Vermont. It is a desiccant, not a hormone or a plant regulator.  It only affects blooms that are open at the time of application.  Preliminary research conducted last year at UVM  indicates no significant difference between Wilthin and NAA + Sevin.  An explanation for this may be that last year we had an extended bloom time due to the cooler temperatures during bloom.  Wilthin should not be applied within seven days of prebloom sprays containing metals such as zinc or copper

Petal Fall (up to 8 mm)
Thinning at this time will result in greater fruit size, but it usually must be followed by a later thinner application to decrease the fruit number.
Labeled thinning materials include NAA, NAD, and Sevin. ACCEL (6-BA and GA4+7) is a mild thinner. It has a wide thinning window, from bloom to 30 DAFB). It is not compatible with NAA or NAD on the same tree within the same year. ACCEL may reduce fruit size at higher rates. For more information on specific materials, refer to your New England Apple Management Guide and the LABEL.

Early Fruit Set (8 mm to 12 mm)
Same materials recommended for during petal fall. However, usually higher rates on some materials are recommended, and combination of materials are generally more effective than single materials.

Late Fruit Set (15 mm to 20 mm)
Use same materials as in PF, but higher rates must be used at this time.

Recommendations: 
Table 2 was developed by Duane W. Greene and Wesley R. Autio of UMass. There are differences in the recommendations of this table and what is in the New England Pest Management Guide  The materials that are recommended in this table are more up to date to what is now available for you to use.   Table 3 (not seen in this web publication) comes from the Pennsylvania Tree Fruit Production Guide (http://tfpg.cas.psu.edu/ Table 17) where the various growth regulators available in the market are discussed. Please remember that the recommendations are for Pennsylvania and some recommendations may no apply for Vermont.  It is a very good list that you can use as a guide
                                         

Determining the size of the fruitlets

A general estimate of fruitlet size can be obtained by measuring the diameter of the king blossoms. Measure as many as possible to get an average of the fruitlet diameter. If frost damage has occurred and some of the king blossoms are damaged, then your measurements should consist of half king and the other half side blossoms. As the number of king blossoms decreases, so will the average diameter of the fruitlet decrease. You will have to use a smaller diameter size to start your thinning (i.e., instead of thinning at 8 mm king blooms, thin at 6 mm).

Estimating Fruit Set

Early fruit set is the window the first chance to evaluate fruit set. Characteristics of fruitlets that are setting include:

  1. Fruitlet stems that curve upwards towards the sun
  2. Fruitlets that are steadily increasing in diameter
  3. Fruitlets and stems that retain a green color
  4. The sepals fold in and close over the calyx end

Characteristics of fruitlets that are not setting include:

  1. Fruitlets stems that remain straight
  2. Fruitlets that stop growing
  3. Fruitlets and stems that have yellow or red color
  4. The sepals remain folded open

Greene and Krupa (1999) have been conducting research at U Mass to determine the earliest that fruit set can accurately be assessed.  At six days after petal fall, the task to visually distinguished between fruit that would continue to grow and fruit that would abscise was not possible  It was not until 8 days after petal fall that this task was possible.  However, according to the thinning windows, the best time to thin is between petal fall and eight days.  So, in practical terms, it is difficult to determine fruit set at the peak time to do get the best results from your thinning. 

I have included in this newsletter two charts that appear in the Apple Thinning Guide called the Fruit Set and Thinning Planner and the Fruit Set Evaluation Checkoff  to help you on your thinning decisions, one of the most difficult orchard management practices.

Literature cited 
Black, B. L. , P.D. Petracek and  M. J. Bukovac. 1995.  The effect of temperature on uptake of NAA by Redchief 'Delicious' apple leaves.  J. Am. Soc. Hort. Sci. 120(3):441-445.
Greene and Krupa. 1999.  Predicting fruit set and thinning response on apples.  New England Fruit Meetings.  105:22-28.
Iungerman, K. and D. Rosenberger.  2000. Northeast Tree Fruit Newsletter.  4(3)
Schwallier, P. G.  1996.  Apple Thinning Guide.  Great Lakes Publishing Co.  Sparta, MI.
1998-1999 Pennsylvania Tree Fruit Production Guide.  Penn State University (http://tfpg.cas.psu.edu/)

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