Apple Thinning Update (04/25/17): Blue Ridge, GA

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Apple Carbohydrate Balance Model for Blue Ridge, GA: April 25, 2017

The Apple Carbohydrate Balance Model was developed by researchers at Cornell University, uses temperature and light data to predict the daily carbohydrate balance of a model apple tree, and has been used to help inform thinning decisions. The model was run using model was run using observed weather data in Blue Ridge, GA ( up to April 24, 2017. The predicted daily carbohydrate balance was calculated for this week using forecasted weather data obtained from Model output is below.

2017 Carbon Balance Model Output

Apple trees are running on stored carbohydrate reserves up until around bloom, so the daily carbohydrate balance up to this time is probably not important. Bloom is when stored reserves run out, and current photosynthesis starts to play an increasingly important role in supplying the carbohydrates required for growth of spur leaves, shoots and flowers/fruit. The carbohydrate balance is not going to have a significant effect on the activity of thinning sprays at bloom or petal fall. However, from around 8 mm fruit diameter on, the carbohydrate balance will influence thinning activity.

Running MaluSim in Predictive Mode

Using the weather forecasts for Blue Ridge for the next 7 days to run the model gives the daily carbohydrate balances in the table below.

Carbon balance and Thinning index at X days after bud break

Using the information from the Apple 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 only developed to estimate the carbohydrate status of the tree. In short, the carbon balance model is a valuable tool, but other factors need to be considered when making chemical thinning decision. The following article from Penn State Extension provides some additional commentary on the use of the carbon balance model for thinning decisions:

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.