Having reviewed A Growers Guide to Predicting the Response to a Chemical Thinner Application developed by Dr. Duane Greene, I decided that I did not have the time to complete the full measurement protocol, however, I was very interested in doing some fruit measurements and getting a 'visual' history of how a group (30) of 'Redmax' McIntosh apple trees trained to a super-spindle would behave during the fruit set and thinning period. (I also did not think I really needed to take all the measurements and use the accompanying predicting apple thinning spreadsheet in this small group of trees to get a handle on what was going on.)
So, I selected and tagged 5 spurs on 5 trees, and using digital calipers measured all the fruitlets on each spur (beginning 20-May) for a total of 125 spurs. I felt this was good representation of the fruit spurs on this group of trees. I made subsequent measurements on 24-May, 1-June, and 11-June for a total of 4 measurement dates.
I applied thinning sprays as follows: first, a petal-fall spray of Sevin XLR Plus (Bayer CropScience) on 17-May at 1.5 pt./acre, and then Maxcel (Valent BioSciences) on 25-May at 75 ppm. Note that the first thinning spray was applied 3 days before the first measurement date (20-May), and that Maxcel was applied between the 2nd (24-May) and 3rd (1-June) measurement dates. I should also mention that from June 7–11, temperatures exceeded 90 F. each day. (Temperature chart | Light chart)
Here are the visual results (click on a tree number below). They should be self-explanatory, but for each tree there are 5 spurs (rows), and 4 columns for each measurement date. The numbers under the picture(s) are the actual fruit measurements in mm.
Tree 1 | Tree 2 | Tree 3 | Tree 4 | Tree 5
So what? Well, by 24-June it was not clear to me if the petal fall spray of Sevin had done enough to thin the crop fully, although I was starting to see some pretty good size differentiation of (mostly) the king fruit. So I proceeded with the Maxcel spray at a moderate rate. By 1-June, however, it had become pretty clear that things were thinning down quite nicely and I decided that the two thinning applications were headed in the right direction. The heat wave described above cinched things for me.
Looking at the figures for final fruit set, it ranges from 8 to 20 %. I still have to make one final check to see how many fruit remain, and need to do an estimate of how many fruit I want per tree. My conclusion at this point, however, is that I have certainly thinned enough, and in fact, there may be less fruit per tree than I would like to reach my yield per acre goal of 600 to 700 boxes/acre.
Finally, my overall conclusion is that this process certainly helped me get a very good visual and numerical picture of what was going on. I do think I would have reached mostly the same conclusion without taking the measurements while using observation and experience to ascertain how my thinning sprays had worked and where final fruit set was ultimately headed.