Apple carbohydrate model thinning predictions for 2013

Publish time: 10th March, 2014      Source: Michigan State University Extension
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A summary evaluation of how well the Malusim Model predicted thinning windows for 2013.


Posted on February 26, 2014 by Phil Schwallier, Michigan State University Extension


Chemical thinning is a very critical annual apple orchard practice, and the most stressful and difficult practice to implement. Over time, new approaches to crop load management have been developed. The most recent approach involves the use of the Malusim Model (carbohydrate model). Michigan State University Extension educators have been using this model informally in Michigan for several years. This article is a look back at the predictions and results for the 2013 season.

Alan Lasko and Terence Robinson of Cornell University developed the Malusim Model. It can be used to help predict apple fruit set and thinning sensitivity. The theory behind fruit set is that fruitlets need energy (carbohydrates) to grow and remain on the tree. If energy is short, fruitlets will thin easily and drop. If energy is plentiful, fruitlets will resist thinning and set well. Malusim predicts the daily energy supply and demand and thus predicts fruitlet-thinning sensitivity. The Malusim output prediction for the 2013 Sparta Enviro-weather station is indicated in Figure 1. Daily negative grams of carbon (carbohydrate balance) indicate tree and fruitlet stress, which means fruitlets will thin easily. The greater the negative daily balance, then the greater the fruitlet thinning or drop. Positive daily balance indicates “no stress” and thus fruitlets will resist thinning and set.

Figure 1. Malusim 2013 for Sparta Enviro-weather station

Malusim 2013 table

A thinning trial on Honeycrisp was conducted to evaluate the model predictions and when actual thinning occurred. Treatments were applied every 3.5 days from petal fall until 28 days after full bloom. There were three periods of predicted serious stress, indicated in Figure 1 as circles 1, 2 and 3, with four-day average carbohydrate balances of -53, -37 and -3 grams of carbon. Thinning during these periods resulted in greatest response to thinners. Thinning treatments on May 21, 2013, May 27, 2013, and June 10, 2013, had 72, 73 and 22 percent thinning, respectively, compared to the UTC (Figure 2).

Figure 2. Honeycrisp thinning trial 2013

Honeycrisp thinning trial table

During other periods, thinning was less (Table 1), but at sometimes significant. During other significant thinning timings (May 24, 2013, and May 30, 2013), thinning was 62 and 33 percent, respectively; carbohydrate levels were predicted to be quite adequate, yet significant thinning occurred.

Table 1. Honeycrisp percent thinning as a percent of the UTC

Application date

% Crop thinning

4-day average carbohydrate balance

May  21



May 24



May 28



May 30



June 3



June 7



June 10



June 14



June 17




Honeycrisp were quite sensitive to thinning early in the traditional thinning window. Malusim predicted significant stress early at petal fall and at 10 millimeters. These times corresponded with the greatest thinning as well. The model was correct in its predictions for both of these timings. Malusim predicted some stress at the 25-millimetere stage and good thinning did occur at that time as well.

The model is proving to be a good guide, but not an absolutely precise guide. It is another tool to use to help apple producers make good thinning decisions and should be used in conjunction with past thinning records, grower experience and weather forecasts.


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