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HRT-RECIPE - Predicting fruit set using the fruitlet growth rate model

These are the basic INGREDIENTS and DIRECTIONS for predicting apple fruit set using the fruitlet growth rate model. If using the recommended "Ferri" spreadsheet, download it using the link below. Also note "for more information." Any questions or comments, contact Jon Clements.



  1. 5 tall-spindle trees of same variety
  2. Flagging tape and permanent marker
  3. Flower cluster labels (
  4. 14 flower clusters per tree, times 5 trees = 70 flower clusters total
  5. Digital caliper
  6. Malusim app (Google Play or Apple Store)
  7. or Fruit Growth Model (iOS only, search App Store on your iPhone)
  8. or Ferri spreadsheet (Ferri spreadsheet 2023 Master v 2.1.2 for predicting fruit set w/ macros) and Perennia: Orchard Tools app


  1. Tag trees 1-5, count total number of blossom cluters per each tree, and determine desired crop load at harvest
  2. Tag clusters (1-14)
  3. Begin measuring fruitlets at app. 6-7 mm
  4. Measure fruitlets at 4-7 day intervals, entering measurements into Malusim app or Orchard Tools. Measurement interval will depend on temperature and/or chemical thinner application(s), both of which affect fruitlet growth rate
  5. Run fruitlet growth rate model in Malusim app
  6. Or export data from Orchard Tools and copy into Ferri spreadsheet and run fruitlet growth rate model. (A reminder, if using the Ferri spreadsheet, on each measurement date, the number of remaining clusters with fruitlets on each tree must be counted for the model run to be most accurate.)
  7. Use predicted fruit set to determine need for further chemical thinning sprays
  8. Enjoy!

For more background and directional information:

Introducing the Malusim App

How to use the Malusim app

How to use the predicting fruit set spreadsheet From Michigan State University, this is for the Scwallier version of the XLS spreadsheet, but has good background information on the fruitlet growth rate model. If you use the Schwallier spreadsheet, however, you have to mark and measure the same individual fruits in each cluster, which is not necessary if using the Ferri spreadsheet.

Development of a fruitlet growth model to predict thinner response on apples

Finally, click on first image below and cycle through my Predicting Fruit Set Recipe presentation...or watch the YouTube video!

Special thanks to Phil Schwallier and Tom and Joe Ferri for all the background information, predicting fruit set spreadsheets, and inspiration for all of us to get this done!


Jon Clements
Last Updated: 
March 2021