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Thoughts on Apple Thinning

Reprinted from the Rutgers Cooperative Extension Plant & Pest Advisory Fruit Edition
Vol. 4 No. 3, April 20, 1999, p. 2-3 and Vol. 4 No. 4, April 27, 1999, p. 4-5

Win Cowgill, County Agricultural Agent

With the 1999 growing season running earlier than usual again, it is time to think about apple thin.
Some growers have considered bloom thinning in the past. Bloom thinning is always risky in NJ
because of frost concerns, this may be even more true this year.

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 what thinning materials are available, how they work, and the different
windows of opportunity are available for their application. Knowing the cultivar response to these
different materials will greatly increase the success of your thinning program. Many factors can
influence the effectiveness of plant growth regulators used for chemical thinning of apples. Below are
some of the factors followed by a discussion of timing windows for application, the materials
available and some general recommendations.

  • 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 thata 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
    re-suspend 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 meant 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.

Windows of Application for Thinning Apples

Bloom Thinning Apple - 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.

1999 North Jersey Tree Fruit Annual Report