Comparing the Malusim app to the ‘Schwallier’ and ‘Ferri’ XLS spreadsheet versions of the fruitlet growth rate model in 2020 to predict fruit set in Gala, Honeycrisp, and Pazazz® apples

INTRODUCTION

Chemical thinning sprays are the most trying and most important decisions an apple orchardist can make. Factors that influence chemical thinner application include weather, carbohydrate balance, and fruitlet growth rate. The Malusim app (malusim.org) uses the fruitlet growth rate and carbohydrate balance models to better inform chemical thinning decisions. Two XLS (Microsoft Excel) spreadsheets are also available for inputting fruit measurements and predicting fruit set based on the fruitlet growth rate model.

METHOD

Five tall-spindle apple trees in each of three varieties – Gala, Honeycrisp, and Pazazz® –  were selected at the UMass Orchard in Belchertown, MA. In May 2020, bloom (total number of flower clusters) in each of the five trees was counted to get an estimate of potential fruit set, and fourteen flower/fruit clusters were selected and tagged for fruitlet growth measurements. Fruitlet measurements were started on 27-May, and then made on 31-May, 4-June, and 12-June. Fruitlet measurements were entered using the Malusim app smartphone (iPhone) voice recognition feature and results calculated in Malusim (malusim.org) to get predicted fruit set. From Malusim the same data was exported and used in the Schwallier and Ferri XLS spreadsheets/apps to get predicted fruit set. The Ferri XLS spreadsheet is a modification of the Schwallier sheet by Tom and Joe Ferri, T&K Orchard, Clarksburg, Ontario, Canada and not publicly available, but available on request. The fruitlet growth rate model output included percent fruit predicted to set and fruit numbers per tree on each measurement date so that the need for a chemical thinning spray could be better assessed.

Honeycrisp trees selected for counting bloom and measuring fruitlets

Cluster selected and tagged for subsequent measuring apple fruitlets

Digital caliper used for measuring fruitlets

Sample Malusim app output

Sample Schwallier XLS spreadsheet output

Sample Ferri (modification of Schwallier) XLS spreadsheet output

RESULTS

For each variety, all three predictions of fruit set were similar within variety. Therefore, any of the three “apps” could be used to predict fruit set. In the end, however, final fruit set, as counted by the number of apples left on each tree, was less than predicted by the apps except for Pazazz®. And actual fruit number per tree counted at harvest was less than the target number of fruit per tree. (Ugh.) A severe carbohydrate deficit at the time of chemical thinner application – as indicated by the Carbohydrate Balance in Malusim – is the likely culprit.

Gala predicted fruit set. Target was 80 fruit per tree, actual at harvest was 45 apples.

Honeycrisp predicted fruit set. Target was 70 fruit per tree, actual at harvest was 26 apples.

Pazazz® predicted fruit set. Target was 70 fruit per tree, actual at harvest was 12 apples.

Significant carboydrate deficit in the Malusim app during the chemical thinning window

DISCUSSION

Although the fruitlet growth rate model is a useful tool to help guide thinning decisions, setting it up and measuring fruits is an onerous process and has not been widely adopted by growers. What’s needed is a faster and simpler method of assessing fruit growth rate during the chemical thinning window. To that end we are investigating, and in collaboration with Carnegie Mellon University, computer imaging and learning to visualize and calculate fruit growth rate. Early results are promising.