"Adventures" in apple thinning in 2019

Is it just me or is it every time I think I chemically thinned apples adequately, come July, it’s like where the heck did all those apples come from? In the ideal world, we would have a hoard — I mean literally thousands — of people, out there hand thinning in June. Obviously not going to happen. Therefore, chemical fruit thinning remains one of the most challenging AND most important spray(s) of the year. Some of my “adventures” in apple chemical thinning in 2019 follow.

The nibble fruit thinning approach as espoused by Dr. Duane Greene at UMass was advisable. This includes using NAA (Fruitone, PoMaxa), carbaryl (Sevin), and 6-BA (Maxcel, Exilis) at the appropriate timing (beginning at bloom and continuing through 10-12 mm fruitlet size) and during good weather (warm, partly cloudy, neither of which occurred together at a particularly good time). Still, this approach generally resulted in inadequate thinning. Apple trees were rarely under considerable carbohydrate stress during most of the chemical thinning window for chemical thinners to be particularly effective. But, it (nibble approach) definitely did some thinning. Some might argue the results were acceptable. But I am tired of too many small, clustered-up apples, particularly when it comes to crop-load sensitive varieties like Honeycrisp wherein fruit quality (size, red color, and flavor) suffers.

NEWA output, apple carbohydrate model, UMass Orchard, Belchertown, MA

The Pollen Tube Growth Model. New this year, I followed it (the PTGM, https://ptgm.newa.cornell.edu/) closely, fully intending to apply lime sulfur to a block of Honeycrisp. Which I did. The result, it smoked the flower petals at a high rate! I was pleased. I was so pleased — and a bit scared! — that I did not follow-up with another application of lime sulfur, which is advised per the PTGM to get that last cohort of flowers, including lateral bloom, that was pollinated. Kind of a mistake, as although the lime sulfur spray at bloom definitely resulted in king fruit set only (mostly?), at the end there was still too many apples on these trees! Hand thinning followed in the summer. Note to self, don’t be gun shy, follow the recommendation of the PTGM. Of course if I do it again next year, and apply lime sulfur twice, I will probably strip the trees! (Would not be the first time, see below.) So, who out there is willing to give bloom thinning with caustic thinners a go in 2020?

Honeycrips flowers after 4% lime sulfur solution application

The result on most clusters after lime sulfur application. Sweet!

Malusim app and the fruitlet growth rate model. I used the Malusim app (https://malusim.org/) in its first year of general release to help measure apple fruitlets and predict fruit set (using the fruitlet growth rate model.) Four varieties — Pazazz, Gala, Fuji, and Honeycrisp. Two sets of trees — five trees per variety, five (only) flower clusters per tree. Only 25 flower clusters per variety. Suppose to do 75. Trying to see how little I can get away with, yup, I’m lazy, I’ll admit it. The result, well, interesting. Seems like things were pretty much on track. With the exception of the lime sulfur application, all other trees received the standard UMass chemical thinner application(s), whatever that was. I won’t bore you with all the details, you will have to wait for an upcoming jmcextman blog or Fruit Notes article, but suffice it to say, in the end, still too many apples at harvest. Too damn many.

Malusim app output predicting fruit set of Gala apple trees in 2019 at the UMass Orchard

Yes you can, strip trees of apples that is. Using ethephon. And 6-BA. And Vydate. Yup, I did it, Golden Delicious, really sick of hand thinning in the past, so a tank mix of above did it. Really did it! And fruits were about one inch diameter! Bottom two-thirds of trees, all apples fell off beginning about a week after application. Interestingly, top one-third of trees had a nicely thinned crop. Shows you where the spray hits and where spurs are weaker (more shaded). Also, there was a pretty good carbohydrate deficit around application. Good thing I don’t make a living doing this.

One more quick note, multiple applications of ReTain, again using Duane Greene’s recommendation, did a nice job of holding Honeycrisp on trees and they took on real nice color in October. A major PYO orchard in easter MA confirms this approach. For more information: http://umassfruitnotes.com/v83n3/a1.pdf.

Honeycrisp apples at Tougas Family Farm on 10/18/19 that were treated with ReTain