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

View the project final report

Commodities

• Fruits: apples

Practices

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

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 trees during bloom with bloom-thinning materials.

We evaluated one bloom-thinning product, Regalia (a non-toxic plant extract allowed under organic standards and commonly sprayed for disease control), sprayed in conjunction with spray oil.  In ten experimental blocks, we timed bloom thinning sprays using several variations of the Pollen Tube Growth Model (PTGM), a free computer model which guides the timing of thinning sprays based on weather.  In general, the thinning sprays in our study did not affect cropload, time required for subsequent hand-thinning, fruit yield, or return bloom relative to unsprayed control plots.

Unfortunately, the project did not reveal any effective or promising techniques for managing cropload.  The combination of Regalia and oil did not prove effective as a thinning spray.  The pollen tube growth model may be accurate when combined with other, more effective, blossom thinning products, but we found the model difficult to utilize in a diverse apple orchard with many uncommon varieties because the model is time-consuming to implement, it will likely dictate multiple sprays some of which may not be feasible to apply because of weather or logistics, and it has only been developed for use with seven common commercial apple varieties. 

We have shared and will continue to share project results with other growers through an online report and video, a field day, articles in grower newsletters, and posts on grower discussion groups and listservs.

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 a field day, articles in grower newsletters, posts on online growers forums and listservs, an online written report, and an online video.