Apple thinning points for 2014

Phil Schwallier<schwalli@msu.edu>, and Amy Irish-Brown, Michigan State University Extension - June 04, 2014

The 2014 apple thinning season factors are unique, as always, and will need consideration before performing thinning.

It is particularly noted that this long, extreme cold winter may cause some winter damage. Michigan State University Extension recommends making multiple thinning treatments this year to achieve your target crop load, and plan on starting your thinning early at petal fall. We have a good bloom this year, better than expected after the large crop last year. Some varieties have no or only light return bloom, such as Goldens, Fuji and Honeycrisp.

Trees appeared weak up until the warm weather moved in which greatly improved the appearance of the bloom and the foliage. Some trees are showing signs of a lighter set, such as Empire and Reds. Frost damage is not present this year and Kings are all alive. Bee activity, pollination and even fertilization were wonderful.

Thinning factors:

Heavy crop in 2013 means easier to thin in 2014.
Good bloom, many large, showy flowers.
Abundant leaves; “green” snowball bloom, strengthens set.
Good bee activity, predicted good pollination and fertilization.
No frost damage, but considerable tree winter damage.
Some varieties and trees with areas of reduced bloom.
Leave CHECK trees.
Use precision crop load management or “nibble” thinning, thin early and often to gradually reduce the crop.
Multiple thinning.

Precision crop load thinning or nibble thinning

Precision crop load management thinning is a strategy to chemically thin often and multiple times throughout the bloom and fruit set window. Technically, nibble thinning begins with blossom thinning (lime-sulfur and oil or ATS). This treatment seems to be less successful in Michigan and thus is not practiced very much. The real first thinning period occurs at petal fall to 6 millimeters. At petal fall, trees are not very sensitive to thinning, and most years no significant thinning occurs.

This year, with warm temperatures forecasted for the next several days, some good thinning is predicted, perhaps perfect thinning. This first thinning will remove off perhaps up to half of the target thinning crop load. Usually additional thinning will be required at the 10-millimeter stage. The next chance to thin will occur at the 10- to 12-millimeter stage and the last will be at 18 millimeters. This process of reducing the crop gradually will result in a better consistent thinning with a reduce risk of over-thinning or under-thinning.

Other considerations:

Oil can be added to thinners to increase the thinning by 10 percent.
Oil is not compatible with Captan and Sulfur. Where this is a concern, use a surfactant instead of oil.
Agri-Mec and oil can cause additional thinning when mixed with thinners.
Cloudy, warm weather will increase fruit drop.
Sunny, cold weather will increase set.

Thinning materials and recommendation for multiple thinning

Variety		Petal fall to 6 mm	10 to 12 mm stage		Variety comment
Variety		Petal fall to 6 mm	If needed	If more aggressive thinning is needed	Variety comment
Frost damage	Kings dead	Sevin or NAA 10 ppm	Standard Rates
Frost damage	Significant damage	Wait to access set.	Tops only, mild rates	Standard Rates
Easy to thin varieties	Cortland, Gingergold, Ida Red, Jonathan, Jonagold, McIntosh	Use Standard Rates Sevin or NAA 10 ppm	NAA 10 ppm	Sevin + NAA 5 ppm	Easy to thin
Easy to thin varieties	Jonathan with MaxCel	Sevin	Sevin	Sevin + MaxCel 50 ppm	Small fruited and easy to thin
Intermediate to thin varieties	Empire	Sevin + MaxCel 100 ppm	Sevin + MaxCel 100 ppm	Sevin + MaxCel 150 ppm	Small fruited
	Honeycrisp	Sevin + NAA 10 ppm	Sevin + NAA 10 ppm	Sevin + NAA 15 ppm	Tend to set multiple fruits/cluster, biennial
	Reds	Sevin + MaxCel 100 ppm	Sevin + MaxCel 100 ppm	Sevin + MaxCel 150 ppm	Biennial, sensitive to NAA
Difficult to thin varieties	Gala	Sevin + MaxCel 100 ppm	Sevin + MaxCel 100 ppm	Sevin + MaxCel 150 ppm
	Goldens, Paula Rred	Sevin + NAA 10 ppm	Sevin + NAA 15 ppm	Sevin + NAA 15 ppm	Biennial
	Rome	Sevin + NAA 10 ppm	Sevin + NAA 15 ppm	Sevin + NAA 20 ppm + 1 qt Oil	Tend to set multiple fruits/cluster
	Fuji	Sevin + MaxCel 100 ppm	Sevin + MaxCel 150 ppm	Sevin + MaxCel 150 ppm + 1 qt Oil	Biennial, sensitive to NAA

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).