Precision Thinning: Target Fruit per Tree Achieved on ‘Gala’ at HVRL

Our last set of fruitlet measurement done on June 6th, indicates that the precision thinning on ‘Gala’ has achieved the set target of fruit per tree and with that we will cease thinning applications for the season. To assess the efficacy of each round of PGRs applied for the chemical thinning, we used Malusim application. The picture below presents a graph generated by ‘Fruit Growth Rate Model’ in Malusim app using the measurements of 15 fruit clusters per tree of 5 representative ‘Gala’ trees.

Based on the average flower cluster count per tree and the fact that an average flower cluster contains 5 flowers, the model calculates potential fruit-set of 100%, which was in our case 1018 fruit per tree. The model also indicates with a pink area our target fruit per tree, which I set to 120 and it was presented in the graph as 12% fruit-set.

Three vertical blue bars represent calculated fruit per tree after each fruitlet measurement, which were recorded on May 28th, June 2nd, and June 6th.  Our chemical thinner application dates are indicated by gray vertical lines. Using the last set of measurements, the model constructed the third blue bar, describing the current fruit set per tree. Since the calculated fruit-set level corresponds almost perfectly to the target fruit-set level, we concluded that out precision thinning with PGRs reached its goal.

‘Fruit Growth Rate Model’ in Malusim app is quite simple to use and with the NEWA’s ‘Apple CHO Thinning v2019 Model’, presents valuable tool to help you decide when to spray chemical thinners and it calculates for you reached level of fruit-set after each spray.

However, over the last couple of weeks, we received a few reports about distrust of ‘Fruit Growth Rate Model’ usage and confusions about the way the data should be entered. For that reason, I will describe how and why you should enter your data, so the model makes a correct calculation and gives you the best estimation of current fruit-set.

Malusim app can be install on a phone or tablet and data entry for ‘Fruit Growth Rate Model’ can be done in the field by using the voice option. If you are not comfortable with the voice option, you may type in data in Excel spreadsheet in the way described in the model and as shown in the photo below, and copy it from Excel file and pasted it into the ‘import sheet’.


It is important to type the date in the format mm/dd/year. In ‘fruitlet’ column you must enter a reading for #1, #2, #3, #4, #5 within every observed flower cluster. Only for the first set of measurements (3-4 days after petal fall), you must enter 0.1, if the fruit is missing. In that way, the model recognizes that the total number of flowers per cluster is 5, but since the reading is small, it affects the average fruit size as little as possible. The model does not require 0 or 0.1 for the consequent measuring set if the fruit is missing. Instead, you do not record it or keep the Excel cell blank. Example shown in the photo below.

After entering the data, hit ‘import’ and the model will generate a graph. If you want to see the vertical gray lines with your spray dates, you can enter spray information in the model ‘Spray Records’ (see the photo below).


At the end, I want to share a protocol for precision thinning data collection developed by Terence Robinson and Poliana Francescatto with additional instruction on how to make sure you record the data correctly and avoid common pitfalls.

Many of you would conclude that this sort of data collection is tedious and time consuming and therefore would rather avoid it. However, I have to highlight that this is the only way which allows you to correctly apply chemical thinners without a fear of under or over-thinning, which can lead to a huge loss in yield or reduced fruit size.

From my experience, 2 people can complete step #2 and #4 (tree and flower cluster selection and tagging) in a couple of hours. First reading is always the longest and should not take more than 2-3 hours. Each following reading should not take more than 1-2 hrs. On average, that means you will have to invest 16 work hours for each orchard block in which you want to apply chemical thinners.


The Precision Thinning Protocol by T. Robinson and P. Francescatto

1 – Select a mature orchard.

2 – From pink to petal fall, mark and tag 5 representative trees (Tree#1 – Tree#2 – Tree#3 – Tree#4 – Tree#5) and count ALL flower clusters (flower buds) on each tree (the earlier you count the better to see the clusters):

3 – Calculate target crop load for a high yield = desired number of fruit per tree.

4 – Tag 15 spurs (flower clusters) per tree on each of 5 representative trees (75 total spurs) (preferable at pink, otherwise at bloom). Make sure you do not mark clusters on terminal or axillary buds on 1-year wood. Try to choose the 15 clusters according to the cluster distribution on the tree. For instance, if you have more flower clusters on the top part, mark more clusters there and so on.

5 – It is not necessary to number the individual fruitlets in each cluster (it is optional), however each fruitlet has to be measured. IF you opt for not numbering the fruitlets you HAVE to be careful when taking the measurements not to measure twice the same fruitlet within the cluster. Each cluster has to be numbered (1 to 15) and the measurements (fruit diameter) from that cluster have to be correspondent to that cluster. We recommend you to buy a caliper with a dial read-out in millimeters to take the measurements.

6 – Apply one of two spray protocols of thinning sprays from Terence’s recommendation list or follow your own thinning program.

7 – Use the carbohydrate model to adjust rates up or down based on model recommendations and the amount of thinning to be done (

IF you decide not to apply a bloom and/or a petal fall thinning spray you still can follow the protocol and measure the fruitlets, however it is opt to you. In this case, the model will tell you the potential number of fruit per tree and how much thinning needs to be done at the later stages. Take the first measurement when fruitlets reach 5-6mm (usually the king fruitlet). Then 3-4 days after the first measurement take a second measurement.

8 – Measure fruit diameters (3-4 and again 7-8 days after petal fall spray, and/or 3-4 and 7-8 days after 10-12mm spray and/or 3-4 and 7-8 days after 18 mm spray). The number of times to measure will depend on when you reach the target number.


Things you HAVE to pay attention at every time you take the measurements

– Take data “precisely”:

– Make sure you are ALWAYS taking measurements from the right cluster. You might get confused if there are two clusters too close or if you used a long ribbon (flag) that can twist around nearby clusters. Try to avoid marking those too close clusters and do not use a too long ribbon.

– Fruit are not round so ALWAYS pick the largest OR the narrowest size of the fruit to measure. If you choose to measure the largest side, then all the subsequent measurements have to be taken from the largest side as well – at day 1, day 2 day 3 and so on.

– If you opt for not marking the fruitlets, please make sure you will not measure the same fruitlet twice at the same day.

–  Make sure you are writing the measurements in the right position (tree and cluster) in the datasheet provided. However, fruitlets within each cluster do not need to be in order.

– If you find more than 5 fruitlets within the cluster just remove the weakest fruitlet. It will come off later anyways.

– If you break a cluster please remove all the previous measurements.

About Dana Acimovic

Mrs. Dana Acimovic has recently been appointed to the position of Extension Associate in Horticulture at the Hudson Valley Research Lab. Dana graduated with an MS in Horticulture from Michigan State University. She originally joined the Hudson Valley Research Lab in 2016 as a research support specialist in Jentsch’s lab. During the last four seasons, she has closely collaborated with post-docs in Dr. Robinson’s Lab in conducting evaluations of apple and cherry planting systems and rootstocks, as well as studies on precision thinning, pruning and irrigation. She has also led a new Cabernet Franc clonal rootstock trial at the lab. Her recent work also includes testing different strategies for minimizing sunburn damage on apple fruit. She plans to continue her work in many of these areas in her new position.
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