Apple Thinning 2020

by: Amanda Green, Tree Fruit Specialist, OMAFRA

Thinning with Frost Damage

The majority of apple growing regions in Ontario experienced critical temperatures for frost damage this spring during bud development. Severe frost damage to blossoms was evident in the worst hit blocks by presenting a lack of bloom. Frost damage at the bloom stage can also be quite subtle; looking in an orchard that had a full bloom, I found flowers with absent styles (Figure 1) or flowers with damaged receptacles and ovaries and heard similar reports from consultants as well. With these subtle signs of frost damage, there may be some uncertainty around the level of frost damage and how to approach thinning this year.

Three pictures of apple blooms, the first apple blossom is healthy showing both styles and anthers, the second picture shows a blossom with no styles and the third picture is a close up photo showing black styles
Figure 1. A) An apple blossom with healthy styles. B) An apple blossom with no styles. C) A closer view of the apple blossom in B showing black, shriveled styles.

In the past Orchard Network Newsletter I wrote an article on Working with a Reduced Labour Force in Apples which I had advised that if you have a reduced crew at thinning, this would be the year to thin aggressively and to aim to hit your target and overshoot it. I had also advised to try the “nibble thinning” approach to take some crop load off at blossom thinning and petal fall/ early fruitlet thinning. If you are concerned with the level of frost damage in your orchard, it may be prudent to wait to apply those early fruitlet thinners until you see the post bloom fruitlets start to swell at fruit set. In an orchard that had frost damage, I am seeing fruitlets not swell and are abscising and falling off as I lightly run my hands over the clusters while other fruitlets are showing fruit set. For advice on thinning with a frost affected orchard, here is an excerpt from the OMAFRA Thinning of Tree Fruit webpage:

Thinning after a badly frosted bloom

Carefully evaluate damage to the bloom. Petals, stamens, pistils and ovaries can exhibit a significant difference in their response to freezing temperatures. See Figure 2, for further details. Although flower petals are browned off by frost, the process of fruit set can still take place from pollination that occurred 1-2 days before the frost event.

Frost around bloom usually produces a variety of frost damage symptoms: bands of russeting around the fruitlet, called frost rings; russeting that runs lengthways from stem to calyx; reduced seed count and misshapen fruit (blocky or ridged)

A light amount of frost damage should not prevent the use of thinning treatments that worked in the past for any given cultivar. However, use extreme caution when thinning after a significant frost. Leaves damaged by frost absorb more of the thinner and fruitlets with reduced seed count abscise more easily. Consider the application of lower rates of thinner for many varieties under these circumstances, or on a badly damaged block, do not apply a thinner but rely on hand thinning if necessary.

Depending on the severity of the damage, growers may thin areas of the orchard that escaped the worst of the frost damage. Differences in elevation and airflow in the orchard can lead to great variations in frost damage. Trees on lower ground can be exposed to air 2-3˚C colder than trees on higher ground in the same frost event.

When deciding to apply thinners, the grower may plan to spray some of the block because of what appears to be a fairly normal set on trees on higher ground. Nozzles can be shut off in areas of the block that show significant potential losses in crop load from frost. Likewise, nozzles that direct thinning sprays into the lower canopy can be shut off.

Figure 2. Anatomy of an Apple Bloom
Figure 2. Anatomy of an apple bloom.

Thinning when the king bloom is lost to frost

Chemical thinners usually work best when the king fruitlet is larger, stronger and more dominant than side bloom fruitlets. The king will normally hold on when weaker, smaller fruitlets drop in response to treatments.

If the king bloom or king fruitlet is lost to frost, size and strength differences among remaining fruitlets can be minimal, which makes them equally weak and vulnerable to thinners. The best approach in this case may be to wait until size differences appear in the cluster, provided the largest fruitlet does not exceed the size range for fruitlet thinning. Size differences may become more apparent several days after full bloom. A few hours’ difference in pollination timing can show up in size differences of side bloom fruitlets as the postbloom period progresses.

Doing a crop load assessment at fruit set can help you assess how much thinning is required to reach your crop target. The Working with a Reduced Labour Force in Apples article covers two methods for setting your crop load target. As you begin thinning, the Fruitlet Growth Model can be a helpful tool to give you a more accurate and quicker prediction on the efficacy of your applied thinners than if you were to visually assess it.

Fruitlet Growth Model

The Fruitlet Growth Model is a very useful tool that can help to predict thinning efficacy 7-9 days after thinning application based on the growth rate of fruitlets. This would be much earlier than visual inspection and is also more accurate. This model was developed by Dr. Duane Greene from the University of Massachusetts and is based on the assumption that the fruitlets that will remain/persist will grow faster than the fruitlets that will abscise and drop. This model can be found on or in this Orchard Tools app developed by Perennia and does not require weather station data to be run. More information on this model can be found in the April 2018 issue of the Orchard Network Newsletter in the article titled Using the Fruitlet Growth Model in Your Orchard. More information on using the Perennia Orchard Tools App along with videos is available in this Perennia Orchard Outlook newsletter article.  When using this model, 100-130 growing degrees days, base 4.4°C, should accumulate between the time of application and the second measurement.

Sevin XLR (Carbaryl) Reminders

Sevin was re-evaluated in February 2017 which brought changes to the maximum application rate and the number of applications. See Table 1 for maximum application rates and restricted entry intervals (REI) for Sevin XLR.

Table 1: Maximum application rates and restricted entry intervals for Sevin XLR

Orchard System
High-Density, Trellised
Traditional Medium- to Low- Density, Not Trellised
Total Season Maximum Application Rate
1.5 kg a.i./ ha
1.0 kg a.i./ha
Total Season Maximum Product Rate
3.22 L/ ha
2.15 L/ha
Maximum Number of Applications
Restricted entry interval (REI) for hand thinning
14 days
10 days
REI for hand pruning, scouting, pinching, tying and training
4 days
0.5 days

Other Thinning Reminders

There are many factors to consider when applying thinners. Table 2 and Table 3 go over application timings, optimal temperatures for application and restricted entry intervals and pre-harvest intervals. More information on thinning can be found on the OMAFRA Thinning of Tree Fruit page, which covers descriptions of thinning products, cultivar specific information and factors that influence the response to chemical thinners.

Table 2: Restricted Entry Intervals (REI) and Pre-Harvest Intervals (PHI) for Apple Chemical Fruitlet Thinners

Fruitlet Thinning Product
Sevin XLR (Carbaryl)
See Table 1.
75 days
MaxCel (6-BA)
12 hours
86 days
Cilis Plus (6-BA)
12 hours
28 days
Fruitone-L (NAA)
When dry
5 days

Table 3: Optimum Timing and Temperatures to Apply Apple Chemical Fruitlet Thinners

Fruitlet Thinning Product
Application Window
Optimum Fruitlet Size
Optimum Temperature
Temperature Limits
Carbaryl (Sevin XLR)
6 mm fruitlet size to 25 days after full bloom
7-14 mm
21-25˚C during application and 3 days following application*
Application at below 17˚C will result in insufficient thinning. Application at temperatures above 27˚C can result in excessive thinning.
6-BA (Cilis Plus or MaxCel)
When king fruitlets are 5-20 mm
When king fruitlets are 5-15 mm
Greater than 20˚C, not exceeding 30 ˚C *
Avoid application when temperatures are above 30˚C
Applications will be more effective when temperatures are above 18˚ the day of application and the following 2-3 days.
NAA (Fruitone-L)
Between full bloom and 15 mm
When king fruitlets are 5-10 mm

Do not apply below 15.6 ˚C or above 26.7 ˚C.
*For 6-BA (Maxcel or Cilis Plus) and carbaryl, as temperatures rise, these products will become “hotter” and thin more. Above 25˚C you will want to want to reduce the spray concentration.

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