Apple Thinning Revisited
Dan Kelly, Canton, MO

Thinning is a project I really did not relish, whether it was by mixing up the caustic lime-sulfur and fish oil to spray the flowers, spraying lime sulfur after fruit-set, or hand picking off the little fruitlets. The last time I sprayed the lime fish oil solution, my tree’s foliage was so badly burned, I resolved to never use it again. And for me, hand apple thinning on MM 106 and MM 111 trees was daunting and there was no way to cover the acreage in a timely manner to make a difference.


So I decided on the French method: Laissez-faire, letting things take their own course. Having a dozen varieties on 4.5 acres on larger rootstocks has been an ebb and flow. Plenty of one variety and not so much of other varieties. In the next year, they would trade their bi-annual habit.


If you are new to apples, you might not think about what most growers know: too may blooms, too many little apples. Too many little apples, too many insect entries. This also means too much stress for the tree, lower sales of good grading fruit, and too much stress for the grower.


In the winter issue of our OFGA newsletter, Just Picked! (Vol. 16, Issue 1), we included a report about Greg Peck’s researchManaging Apple Crop Load and Diseases with Bloom Thinning Applications in an Organically Managed ‘Honeycrisp’/‘MM.111’ Orchard (Peck et al., 2016; Yoder et al., 2015).


This research changed my attitude regarding apple thinning.


Peck said, “We saw an opportunity to target two of the key barriers for organic apple production in the mid-Atlantic, namely reliable crop load and disease management.”


Luckily for my peace of mind, two of the products tested in the trial were ones with which I had been familiar, Regalia from Marrone Bio Innovations, a product for disease control, and JMS Stylet-Oil from JMS Flower Farms, a dual purpose fungicide and insecticide. Neither had previously negative effects in my apple trees.


This approach to apple thinning is based on a concept called the pollen tube growth model (PTGM), and it was developed at Virginia Tech. The PTGM begins when the desired number of king bloom flowers are in full bloom, that is when the petals no longer cover the reproductive organs thus allowing for cross-pollination.

Finally digesting what I could of the research and translating it to metric, I applied 2 quarts per acre of Regalia with 2 quarts of JMS Stylet-Oil in 100 gallons of water. I did not have a fancy calibrator to determine the style length, but the ruler was in millimeters.


Note on the May 27 image (see Gallery) that there is no leaf burn. And on the July 8 image, the thinning to me was satisfactory without laying a hand on the fruit. I had been hoping that the Regalia would hold the Cedar Apple Rust (CAR) in check, but the sporulation time lasted longer than the Regalia I had ordered. So the control of CAR on susceptible varieties was not so good. However, this year I received a SARE grant to control deer and cedar apple rust by removing the cedar trees and installing a new cedar tree deer-proof fence, and I am hoping to see positive results of that in 2021. With this new thinning spray regime, I am confident to have greater resilience in annual apple production.


For more information on “Implementing the Pollen Tube Growth Model on NEWA” (Network for Environment and Weather Applications*), explore this link:

PDF article on the New York State Horticultural Society’s website

*NEWA gives end users free access to 40 IPM, crop management, and degree day tools driven by weather data from weather stations primarily located on farms (click here for website access). Find them on the main menu under Weather Data, Pest Forecasts, and Crop Management. Created in 1995, NEWA has over 400 weather stations in 11 partner states with individual growers connecting in from five other states in the Northeast, mid-Atlantic, Southeast, and North Central regions of the US and continues to grow.



About the Author

Dan Kelly owns and operates Blue Heron Orchard in Canton, MO. His orchard features a line of on-farm processed certified organic apple butter, apple sauce, apple syrup, apple cider, apple cider vinegars, apple fruit leather, and applewood-smoked, fire-roasted jalapenos. Dan and his wife Cherie Sampson also have an acre of certified organic vegetables. Check out their website for more information:
Blue Heron Orchard.



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