Local research is the best evidence to inform our decisions, and we have a rich history here in Nova Scotia! I have recently explored the local studies on crop load management to summarize and share. I do not recommend changing your thinning program if it already works. Your on-farm experiences are just as valuable. However, newer growers or growers looking to change their practices need a point of reference to make decisions.
Local testing of chemical thinning products is essential because thinning activity is sensitive to weather and tree factors that are unique to production regions. Much of our local research on crop load management as far back as 1951 is no longer relevant because products and varieties have changed. Thankfully we can still draw from studies done from 2002 to 2012.
I have compiled the results for Honeycrisp and Gala using registered products only. The summaries are described and shown below but for easy access to the large tables, download the document, "Summary of Nova Scotia Crop Load Management Studies".
Honeycrisp
Honeycrisp - Blossom Thinners
Blossom thinners are used with the intention of reducing fruit set early in development. All of the materials used for blossom thinning work by preventing pollen germination and flower fertilization. The timeframe is very narrow for using blossom thinners to successfully keep king fruitlets while removing side fruitlets.
There is debate about whether blossom thinning is practical, especially in regions like ours that have inconsistent weather conditions at bloom. Blossom thinning is likely useful in only some years.
It is apparent in several local studies on Honeycrisp that blossom thinners do reduce fruit set (Table 1). However, measurements later in the season show inconsistent effects on crop load, harvest fruit size, and red colour coverage. What is clear is that secondary thinning is necessary after blossom thinning. Likely the main role of blossom thinning in our region is to reduce fruit set early to promote annual bearing.
Please note that lime sulphur is more likely to cause foliar damage than ATS, but it might be considered an organic option.
Table 1: Summary of Nova Scotia crop load management studies on Honeycrisp with blossom thinners at full bloom (FB).
Note: Click on the image for larger view.
Honeycrisp - Petal Fall Thinners
The caustic materials used for blossom thinning are no longer expected to work after flowers are successfully fertilized, and it was shown in the local study that ATS applied at early petal fall did not reduce fruit set or have any other influence (Table 2).
The materials used for fruitlet thinning are plant growth regulators, also known as chemical thinners. Thinners used during the petal fall window are expected to have significant opportunity to improve fruit size and quality because fruit development is still early. Local studies show that conservative petal fall treatments did not improve fruit quality but that more aggressive treatments did (Table 2). However, there may be hesitancy to thin aggressively during petal fall because the fruit set cannot be predicted yet and early thinning is a risk.
Although the conservative treatments like Fruitone alone, MaxCel alone, and Sevin alone did not improve fruit quality, they did generally reduce fruit set (Table 2). They could be considered an alternative approach to blossom thinning to promote annual bearing.
The product combinations of MaxCel + Fruitone or MaxCel + Sevin resulted in more large and extra large fruit that is undesirable for Honeycrisp.
Table 2: Summary of Nova Scotia crop load management studies on Honeycrisp with petal fall (PF; 4-6 mm) thinners. Please note that most of this petal fall thinning data is supplied by one trial in 2011 and that conclusions would be more robust from multiple years of study.
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Honeycrisp - Fruitlet Thinners (7-14 mm)
The fruitlet stage is the traditional time to apply thinners. By this time, it is possible to observe how many fruitlets are growing after fertilization and those growing fruitlets have the potential to set. Fruit set is still not guaranteed at this point and will be affected by the weather and the strength of the competing fruitlets. However, thinning action must be taken by now if it has not already been done.
In almost all of the local studies, product combinations reduced fruit set and crop load and increased harvest size and colour (Table 3). Fruitone + Sevin is our industry standard and showed fairly consistent local results. MaxCel 100 PPM + Sevin resulted in more large and extra large fruit that is not desirable for Honeycrisp.
Table 3: Summary of Nova Scotia crop load management studies on Honeycrisp with fruitlet (7-14 mm) thinners.
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Honeycrisp - Rescue Thinners and Nibble Thinning
By late fruit set, thinning is considered a rescue operation and unfortunately there are no reliable options yet. Local research found that MaxCel applied late actually increased fruit retention (one year of study)(Table 4).
Finally, perhaps a nibble thinning approach is taken by thinning at multiple stages of fruitlet development. Local research showed that ATS applied at full bloom then Sevin at petal fall thinned and increased fruit size. Adding to this ATS + Sevin program MaxCel at 10 mm caused overthinning.
Table 4: Summary of Nova Scotia crop load management studies on Honeycrisp with rescue (15-30 mm) thinners and the nibble thinning approach (multiple stages).
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Gala
Gala - Blossom and Fruitlet Thinners
Fewer studies have been done with Gala and in only two years of study. The blossom thinner ATS decreased crop load and resulted in less small fruit in the fruit grading distribution (Table 5). For fruitlet thinning, Fruitone alone and Fruitone + Sevin decreased fruit set and crop load, usually without increasing fruit size (Table 5). The combination of MaxCel + Sevin thinned AND increased fruit size (Table 5).
Using the blossom thinner ATS and then the fruitlet thinner MaxCel + Sevin was more aggressive than either alone and did increase fruit size at harvest (Table 5). The 50% reduction in crop load was significant but may be desired on a small fruited variety like Gala.
Table 5: Summary of Nova Scotia crop load management studies on Gala with blossom thinners (FB; full bloom), fruitlet development (7-14 mm), and the nibble thinning approach (multiple stages).
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