MARKUSIM - Precision chemical apple fruit thinning for the rest of us?

Chemical thinning of apples is one of the most important sprays growers make all season. Final fruit set affects profitability greatly. Thus, being able to predict fruit set while making chemical thinning sprays is an important decision-making step. Although there are precision tools, including the apple fruitlet growth rate model, to help here, they are admittedly time consuming and onerous, particularly in orchards with many different blocks and varieties which may respond differently to chemical thinning sprays. Here, I will propose using a relatively simple method, “MARKUSIM,” for helping growers predict apple fruit set and determine need for chemical thinning sprays across their orchard blocks.

First, I have to give credit 100% to Mark Rusell and Jill Mackenzie, Two of Clubs Orchard, LLC, Appleton NY for “MARKUSIM” – get it, a play on “Malusim” – to come up with this method for predicting apple fruit set. I have, however, as described here, abbreviated their procedure somewhat. I have only done this one year and really need to do it for another year or two to get a better feel for its “accuracy.” My initial impression, however, is that it forces you at the very least to look at a defined sample of fruitlet clusters and give you great intuition quickly as to what is going on and how the chemical thinning sprays are working (or not).

The PROCEDURE, in as simple terms as I can describe it:

1. FLAG (with flagging tape, don't use white, D’oh!) ten representative trees per block/variety at the pink to bloom bud stage.
2. Attach numbered CLOTHESPINS to five flowering/fruiting clusters per tree at the pink to bloom bud stage. Colored (fluorescent orange spray painted) and numbered (Sharpie permanent marker) clothespins are easier to find. Or you can use fluorescent orange flagging tape alone or in conjunction with the clothespins. Bottom line is the clusters need to be easy to find over the next few weeks. And they should be in the TOPS of trees too! Figure 1 for example.
3. Beginning when FRUITLETS exceed 5 to 6 mm in size, go out and RECORD the NUMBER of apple fruitlets you THINK are growing and persisting. At first it will appear most are doing this. No need to measure! Just count the number of fruitlets that appear to be staying! RECORD! I used a simple Gsheet as in Figure 2.
4. After the first CHEMICAL thinning spray, 3 or 4 days later, again RECORD the NUMBER of fruitlets that appear to be GROWING. It will start to become quite obvious which are which, GROWING or NOT GROWING and likely to abscise. RECORD!
5. REPEAT two or three or four times to PREDICT fruit set and determine need (or not) for more thinning sprays. TYPICALLY, you want one apple remaining per two fruit clusters = 10% fruit set assuming bloom is good (no frost, etc.) and you have pruned trees to roughly a desired number of flowering/fruit spurs (precision pruning). 

OK, I did this (roughly) at the UMass Orchard in Belchertown in 2024. In a Honeycrisp and Macoun block, both grown to tall-spindle. I had some interesting results, mostly on account of the fact MARKUSIM was predicting a very low fruit set, far below 10% by the final date I counted. In fact, more like 2% for both Honeycrisp and McIntosh, as can be seen in Figure 2 for Honeycrisp. Also note Figure 3, depicting a final fruit set predicted to be less than 0.5 apples per cluster which would be 10% set. Needless to say, I was concerned. There was a frost/freeze event here in late April when trees were late tight cluster to pink bud stage, so I kind of figured that might be an issue. I don’t have chemical thinning details, but the presumption was petal fall and approximately 10 millimeter chemical thinning sprays went on. I did count the final fruit set (number of apples on tree) just pre-harvest, and in reality, fruit set was approximately 28 apples per tree in the Honeycrisp, assuming my target was about 60 apples per tree, that is 5% fruit set (assuming about 120 flower clustes per tree at bloom). Whoaa, what went slightly wrong with MARKUSIM? Not sure? Maybe I did not clothespin enough spurs in the tops of trees? (Note to self, ladder needed next year.) Maybe I picked errant behaving spurs? Maybe I was not a good judge of what appeared to be growing (or not) fruitlets? See Figures 4-6 for examples. I will say, however, and this may be fodder for another Fruit Notes article, but the other predicting fruit set tool I used (Malusim) were more on-target compared to what I ended up with MARKUSIM? Although Malusim predicted pretty low fruit set on Macoun. Anecdotally, here at the Orchard, we all kind of felt/agreed that the final fruit set at harvest was much better than expected in late May where we kind of thought excess thinning might have occurred. Crops in bottom of trees were definitely affected by the frost, whereas tops were still over-cropped. Damn weather!

So, I did tell you MARKUSIM needs a couple more seasons of evaluation before I can wholeheartedly recommend, but it was kind of fun and a heck of a lot easier than measuring fruitlets. Maybe it’s just “semi-precision?” Which is better than “no precision,” aka seat-of -the-pants? You decide.

Figure 1 - Example spur marked with numbered clothespin. Darn, thought I had a better pic but you get the idea!

Figure 2 - Gsheet used to record MARKUSIM data. Rows 2-51 are number of apples on each cluster determined to be growing (vs. starting potential of 5 apples). Sum is a calculation; % set is a calculation based on sum and potential (250 = 50 clusters times 5 apples per cluster); avg. no. apples is a calculation.

Figure 3 - Honeycrisp average number of apples per cluster. By 6/2 it should be about 0.5 apples per cluster, not still the nose bleed dive seen here.

Figure 4 - How many fruitlets growing/staying here? I'd say 4?

Figure 5 - How many fruitlets growing/staying here? 6? 5? 3? It's not always clear cut is it?

Figure 6 - How many fruitlets growing/staying here? Easy-peasy, ONE!