Managing Cropload with the Pollen Tube Growth Model In Organic Apple Orchards

Project Overview

Project Type: Farmer/Rancher
Funds awarded in 2022: $14,756.00
Projected End Date: 01/15/2024
Grant Recipient: Two Onion Farm
Region: North Central
State: Wisconsin
Project Coordinator:
Christopher McGuire
Blue Roof Orchard (formerly Two Onion Farm)

Information Products


  • Fruits: apples


  • Crop Production: cropload management
  • Production Systems: organic agriculture

    Proposal summary:

    Apple trees often bear too many fruit, resulting in small, poor quality fruit and reduced bloom in the following year.  Therefore it is important to "thin" flowers or small fruitlets and reduce the cropload.  Without access to synthetic chemical thinning agents, organic apple growers often thin by hand, which is labor-intensive and ineffective.  A promising alternative is to spray Regalia (a non-toxic plant extract allowed under organic standards and commonly sprayed for disease control) during bloom and to use the Pollen Tube Growth Model (PTGM), a free computer model which guides the timing of thinning sprays based on weather.  However, organic orchardists have been slow to adopt these techniques because they are relatively untested, the PTGM is currently limited to only seven apple varieties, the PTGM's guidance for timing the initial thinning spray is questionable, and the PTGM may be time-consuming to implement.  In our commercial organic apple orchard, we propose (i) to measure the effectiveness of Regalia and the PTGM for crop thinning in varieties not explicitly included in the PTGM and using three variations on initial spray timing, (ii) to measure the time required to implement the model, and (iii) to share results with other growers. 

    Project objectives from proposal:

    1. Evaluate the effectiveness of Regalia sprayed as a crop thinning agent during bloom according to the recommendations of the Pollen Tube Growth Model in seven varieties which are not explicitly included in the model and using three variations on the model's recommendation for timing of the initial spray. 
    2. Compare the time required to implement the Pollen Tube Growth Model and perform thinning sprays to the time required for hand-thinning unsprayed trees.
    3. Share results via conference presentations, written reports, and an online video.
    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.