Apple thinning pointers for 2013

Phil Schwallier<schwalli@msu.edu>, and Amy Irish-Brown, Michigan State University Extension - May 17, 2013

Guidelines to help you make good apple thinning decisions.

This 2013, apple thinning season factors are unique, as always, and will need consideration before performing thinning. Michigan State University Extension educators suggest growers plan on making multiple thinning treatments this year to achieve your target crop load. We currently have the potential for a very large apple crop and starting your thinning early at petal fall is highly recommended for 2013.

The extremely low crop set in 2012 has triggered the biennial bearing cycle that apples are notorious for (on 2013, off 2014, on 2015 and on). We need to break this potential biennial cycle by thinning this crop down and then use summer NAA to promote return bloom for 2014.

We had no crop last year, which has produced a “green” snowball bloom this year and is expected to strengthen trees and fruit set for the 2013 apple crop. The “green” bloom refers to the abundant amount of leaves that are present on the trees before and during bloom. Last year, hot and very droughty weather has caused some minor reduction in bloom on limbs, trees and on a few varieties such as Red Delicious and Empire. This is the exception and only in minor areas.

Frost occurred on May 13, 2013, with minimum temperatures at 24 degrees Fahrenheit which caused some damage to flowers. Growers will have to carefully assess frost damaged sites and adjust their thinning program accordingly. Excellent sites and frost-protected areas have almost no frost damage and need to be thinned aggressively. Bee activity, pollination and even fertilization appear to be good. Temperatures for the next seven to 10 days are forecasted to be warm; ideal for excellent fruit set.

Thinning factors:

No crop 2012 (more resistant to thinning 2013).
Hot, dry 2012 summer (can weaken 2013 bloom, we don’t expect weak 2012 flowers).
Biennial bearing cycle triggered, we must break the cycle.
Strong snowball bloom, many large showy flowers.
Abundant leaves (“green” snowball bloom, strengthens set).
Good bee activity, predicted good pollination and fertilization.
Frost on May 13, near 24F in unprotected areas (access your own bud strength).
Good sites and frost protection (helicopters, wind machines, sprinklers and bale burning) have almost no damage and will need aggressive thinning approach.
Some minor areas of reduced bloom (insignificant).
Weather forecasts are for warm temperatures over the next several days and colder during the 10 millimeter stage.
Leave CHECK trees.
“Nibble” thinning, thin early and often to gradually reduce the crop.
Promalin treatments.
Multiple thinning should be considered to get the job done for 2013.

Thinning approach: nibble thinning

“Nibble” 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 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 at the petal fall timing – perhaps perfect thinning for some sites.

This first thinning will nibble off perhaps up to half of the target thinning of this excessive crop load. Usually, additional thinning will be required at the 10 millimeter stage. The next chance to thin will occur at 10 to 12 millimeter 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.

Table 1. 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, Idared, 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 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, Paulared	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).