Thinning Alert: Apple Thinning Responses May Be Aggressive

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Strons Initial Set

Post-bloom thinning update

The majority of post-bloom chemical thinning decisions will likely occur this week. The forecast calls for rather warm temperatures (70’s to low 80’s) to coincide with the next several days of the post-bloom thinning window.

All post-bloom chemical thinners have good thinning activity with warm temperatures (>68 F) and all chemistries and all have potential to over-thin at excessively warm temperatures (mid-high 80’s). Additionally, warm temperatures can increase the susceptibility of trees to chemical thinners.

This year, thinning decisions will be further complicated by several factors, including: a protracted bloom period, some blossom/bud mortality due to cold weather, relatively poor conditions for pollinator activity during bloom, and sub-lethal damage to spur leaves. All of these factors should be considered on a block-by-block basis.

Carbohydrate Balance Model

The Apple Carbohydrate Balance Model is a useful tool in making chemical thinning decisions. This model was not developed to account for all factors that influence the efficacy of a thinner application; it was developed to estimate the carbohydrate status of the tree. In short, the carbohydrate balance model is a valuable tool, but other factors need to be considered when making chemical thinning decision (see above).

A positive carbohydrate balance makes it more difficult to thin whereas increasing carbohydrate deficits are predicted to increase the response to chemical thinning sprays. For example, you might respond to a predicted carbohydrate surplus or mild deficit by either adopting a more aggressive approach to chemical thinning, or by waiting until more ideal conditions for thinning develop. Alternatively, if the model is predicting a severe carbohydrate deficit then you might consider reducing the rate of chemical thinner or perhaps not applying a thinner at that time for risk of over-thinning.

Output from the Carbon Balance Model for three sites in WNC (Flat Rock, Sugarloaf, and Moravian Falls) and a thinning index is below. Based this output and the thinning index, thinning responses to normal rates of thinner may be aggressive or excessive. Please use caution!

Flat Rock (Richmind) Apple Carbon Balance Model Estimates: (4/30/18)
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Status (g/day) Thinning Rec
(°F) (°F) (MJ/
Prod Demand Bal 4-Day Ave Bal
4/30 70 33 23.0 89.16 69.76 19.40 -25.43 Decrease chemical thinner rate by 15%
5/1 75 46 24.5 89.63 111.18 -21.55 -45.21 Decrease chemical thinner rate by 30%
5/2 76 53 23.9 87.57 128.55 -40.98 -55.37 Decrease chemical thinner rate by 30%
5/3 80 57 23.8 85.59 144.16 -58.58 -53.42 Decrease chemical thinner rate by 30%
5/4 79 58 20.7 80.67 140.38 -59.72
5/5 74 60 15.2 66.48 128.70 -62.22
5/6 73 56 16.9 77.12 110.27 -33.15
Edneyville (Sugarloaf Mtn.) Apple Carbon Balance Model Estimates: (4/30/18)
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Status (g/day) Thinning Rec
(°F) (°F) (MJ/
Prod Demand Bal 4-Day Ave Balance
4/30 68 36 22.5 73.78 73.38 0.40 42.87 Decrease chemical thinner rate by 30%
5/1 76 47 24.5 76.43 119.24 -42.81 -63.59 Decrease chemical thinner rate by 50%
5/2 76 53 23.9 76.88 134.88 -58.01 -73.3 Decrease chemical thinner rate by 50%
5/3 78 57 23.8 77.81 148.86 -71.05 -72.28 Decrease chemical thinner rate by 50%
5/4 80 58 20.8 73.31 155.81 -82.50
5/5 74 60 15.3 62.42 144.07 -81.65
5/6 72 56 17.0 73.16 127.11 -53.94
Moravian Falls Apple Carbon Balance Model Estimates: (4/30/18)
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Status (g/day) Thinning Rec
(°F) (°F) (MJ/
Prod Demand Bal 4-Day Ave Balance
4/30 70 45 24.6 69.13 96.72 -27.59 -57.99 Decrease chemical thinner rate by 30%
5/1 78 43 24.5 71.75 111.88 -40.12 -75.72 Decrease chemical thinner rate by 50%
5/2 79 54 24.2 70.71 143.63 -72.91 -90.32 Do not thin (many fruits will fall off naturally)
5/3 82 57 24.1 72.00 163.35 -91.35 -88.23 Do not thin (many fruits will fall off naturally)
5/4 82 59 21.1 69.32 167.80 -98.49
5/5 78 61 16.7 62.73 161.28 -98.55
5/6 76 57 20.6 79.79 144.31 -64.52

Thinning index

Crop removal on young trees

Complete crop removal on new/young plantings is an important step in orchard establishment. If fruit size is ~8-12 mm, the anticipated warm temperatures could provide an excellent window for de-fruiting young orchards using chemical thinners. There are several ways that this can be accomplished:

1) Post-bloom thinning. The current recommendation in the Integrated Orchard Management Guide for Commercial Apples in the Southeast is as follows:

2 oz. NAA (Fruitone L, Pomaxa, Refine 3.5 WSG) +1 lb carbaryl + 2 pint ethephon  per 100 gallons. Applied at ~ 8 to 10 mm fruit diameter. Caution: Application can suppress vegetative growth for 3 to 6 weeks.

While the above combination is an effective method in crop removal, a potential disadvantage of this treatment is that it can stunt vegetative growth.

2) Post-bloom thinning. In other apple producing regions, de-fruiting recommendations have shifted to use of materials that do not have negative effects on vegetative growth and development. If you are interested in trying an alternative method of crop load removal on young trees this year, please consider the following:

150 ppm 6-BA (MaxCel; Exilis Plus; Exilis 9.5 SC) + 1 quart carbaryl + 1 quart non-ionic surfactant per 100 gallons.

While some fruit may persist after treatment, this combination will not retard vegetative growth. Daytime highs must be greater than 70 F for this treatment to be effective.

3) Hand thinning. Physical removal of blossom or fruit by hand or with pruning shears/scissors. While effective, this maintenance activity is labor intensive and may not be practical with large plantings. Try to limit removal of spurs and tender shoots. Avoid manual crop removal on days where environmental conditions are conducive to fire blight infection.