May 12, 2016
Carbon balance model used for apple chemical thinning

Chemical thinning is a complex process that annually challenges the professional apple grower. In recent years, many growers have started using the MaluSim Carbon Balance Model to assist them in managing the crop load of their trees.

This useful tool is found on the NEWA weather website. It uses current and forecasted sunlight and temperature inputs to indicate the sensitivity of the developing apple fruits to chemical thinning. The model predicts whether there will be either a deficit or surplus of carbohydrate, making the trees in turn, either easier or harder to thin.

When the forecast is accurate for temperature and sunlight, the predictive power of the model is very good. We note the accuracy of the model in the off-season, when reviewing the results of completed chemical thinner trials and using the actual recorded temperatures and light data for the thinning window. The carbon balance model does a good job of evaluating the effects of the weather on fruitlet susceptibility. But there is more to chemical thinning process than that.

Figure 1. The Chemical Thinning Triangle. Photo: Penn State
Figure 1. The Chemical Thinning Triangle. Photo: Penn State

Figure 1 illustrates the interactions between the chemical thinner, the environment, and the tree to obtain successful fruit thinning. Carbon balance accounts for the effect of the environment on the susceptibility of the tree. Environment also affects how the chemical will perform in a number of ways, which are listed across the base of the triangle. The environment, chemical thinner and the tree interact in ways not described by the carbon balance, as shown on the other two sides of the triangle.

Other key interactions affect chemical thinning activity. In order for a plant growth regulator such as NAA or 6BA to be effective, it must be absorbed by the leaf, transported to the site of activity, where it regulates some metabolic activity of the tree or the fruit with enough strength to influence fruit set. Temperature can affect each of these steps, such that thinners are much less effective during cold temperatures and act more strongly in hot weather.

It is good to have the carbon balance model to help describe how the environment may affect the trees, but the other interactions must also be understood well in order to have successful thinning.

For more information visit the Penn State Tree Fruit Production website.

— James Schupp and Robert Crassweller, Penn State University

Source: Penn State Extension

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