Apple Thinning Update: 05/11/18

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The purpose of this post is to provide a brief update on projected temperatures and carbohydrate balance estimates. This may be of value to those who plan to thin over the weekend. We plan to provide another update on May 15th.

High temperatures (> 85 °F) will be observed at some locations over the weekend. If thinners are applied when daytime temperatures exceed 85 °F, thinning may be excessive. When temperatures rise above 85 °F a few days after a thinning application, a greater thinning response can occur. If increased thinning is not desired, consider reducing rates or delaying application.

While temperature is an important factor in estimating apple tree carbohydrate status, the interaction of thinning chemistry and temperature is not considered in this model.


Carbon Balance Model

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.

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 normal to slightly aggressive. In addition the model output, please consider the interaction between high temperatures and thinning chemistry. 

Edneyville (Sugarloaf Mtn.) Apple Carbon Balance Model Estimates: (5/11/18)
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Status (g/day) Thinning Rec.
(°F) (°F) (MJ/m2) Prod. Demand Bal. 4-Day Ave Balance
5/11 82 55 15.7 73.76 121.16 -47.40 -34.89 Decrease chemical thinner rate by 15%
5/12 84 62 23.1 91.60 125.57 -33.98 -28.61 Decrease chemical thinner rate by 15%
5/13 86 64 24.1 92.19 121.85 -29.65 -26.91 Decrease chemical thinner rate by 15%
5/14 86 65 22.0 87.33 115.85 -28.53 -25.26 Decrease chemical thinner rate by 15%
5/15 77 65 15.6 74.83 97.10 -22.28  
5/16 75 62 12.1 63.03 90.20 -27.17  
5/17 76 62 13.0 68.04 91.11 -23.07  

Flat Rock (Richmind) Apple Carbon Balance Model Estimates: (5/11/18)
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Status (g/day) Thinning Rec.
(°F) (°F) (MJ/m2) Prod. Demand Bal. 4-Day Ave Balance
5/11 79 53 16.1 79.17 90.94 -11.78 -16.37 Apply standard chemical thinner rate
5/12 84 61 23.2 93.02 105.68 -12.66 -19.47 Apply standard chemical thinner rate
5/13 86 64 24.2 92.11 109.21 -17.11 -23.57 Decrease chemical thinner rate by 15%
5/14 87 64 22.0 86.25 110.18 -23.93 -25.86 Decrease chemical thinner rate by 15%
5/15 78 64 15.5 73.72 97.91 -24.19  
5/16 75 62 12.0 61.91 90.96 -29.05  
5/17 77 62 13.0 66.93 93.19 -26.27  

Moravian Falls Apple Carbon Balance Model Estimates: (5/11/18)
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Status (g/day) Thinning Rec.
(°F) (°F) (MJ/m2) Prod. Demand Bal. 4-Day Ave Balance
5/11 83 65 17.6 71.46 145.33 -73.87 -40.25 Decrease chemical thinner rate by 30%
5/12 86 59 24.3 94.46 121.97 -27.51 -27.52 Decrease chemical thinner rate by 15%
5/13 88 61 24.7 93.22 119.68 -26.46 -26.84 Decrease chemical thinner rate by 15%
5/14 89 64 22.4 84.99 118.15 -33.16 -26.55 Decrease chemical thinner rate by 15%
5/15 82 63 17.1 78.12 101.08 -22.96  
5/16 78 63 14.3 70.63 95.44 -24.80  
5/17 79 62 13.8 69.75 95.03 -25.28  

Thinning index image

Written By

Chris ClavetResearch Specialist (828) 687-0570 (Office) chris_clavet@ncsu.eduHorticultural Science - NC State University

Contributing Specialist

Photo of Tom Kon, N.C. Cooperative ExtensionDr. Tom KonAssistant Professor: Southeastern Apple Research Specialist (828) 687-0570 (Office) tom_kon@ncsu.eduHorticultural Science - NC State University
Updated on May 14, 2018
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