Crop Load Management Update (5/13/20): Rescue Thinning

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Red Delicious apples

Excessive fruit set on ‘Red Delicious’. With large fruit (>18 mm) options for chemical thinning become more limited and volatile.

Challenging… Frustrating… Atypical…

I’ve used all of these words – and some colorful language – to describe the 2020 chemical thinning window. Unseasonably cool temperatures have been a major limitation in post-bloom thinner efficacy. When temperatures have been acceptable, we had very short periods of opportunity for chemical thinner applications, largely due to near constant, Nebraska-like winds.

To complicate matters, most sites in the southeast observed several frost/freeze events. There is widespread variation in fruit set due to these cold events. Some blocks are exhibiting sub-lethal cold injury, such as frost rings, russet, and seed damage. Blocks that have evidence of cold injury are generally more sensitive to chemical thinner applications – rates may need to be reduced to avoid over-thinning. Please check for seed/fruit damage before making additional thinning applications.

Another challenging factor is that cool weather has delayed the visible response of previous chemical thinner applications. In orchards that received post-bloom thinner applications 2 weeks ago, very few fruit have fallen from the canopy. With minor agitation, some abscising fruit can be detached from the tree. In some cases however, there are some fruit that appear weak/stunted that seem to be firmly attached and have healthy seeds. It is very challenging to determine if these fruit are abscising or persisting without detailed monitoring of fruit growth and development.


With several frost/freeze events in 2020, some fruit/blocks are exhibiting sub lethal cold injury, such as frost rings, russet, and seed damage.

Seed damage

Seed damage (note discolored/aborted seeds in fruit at center and right) has been observed in several orchards. Be sure to check for seed/fruit damage before making additional chemical thinner applications.

Per the forecast, one of the “better” opportunities for post bloom thinner activity in 2020 will occur over the next few days. While ‘Rome’ is in the post bloom thinning window (8-14 mm diameter), most varieties are at a stage of development (>18 mm) that key post bloom chemical thinners (6-BA and NAA) have poor/limited activity. With large fruit (>18 mm) options for chemical thinning become more limited and volatile. If more than a minor reduction of crop load is needed, then thinning programs including ethephon are generally required. Ethephon has the reputation of being an erratic thinner – some of my colleagues from more northern growing regions refer to this as the “elephant gun” of chemical thinners. For better or worse, many growers will need to rely on this tool to re-thin significant acreage in the southeast.

A few comments about thinning with ethephon:

  • This chemistry is very temperature dependent. Mild/poor thinning activity is observed in the 60’s; good thinning activity is observed in the 70’s to low 80’s; and aggressive responses are observed in the mid to upper 80’s. Ethephon should not be applied if daily high temperatures exceed 90 degrees F.
  • The thinning response from ethephon is very localized – sections of the canopy that do not receive thinner will not show a response. Good spray coverage is essential for a consistent response. Since fruit set is generally stronger in the upper 2/3’s of the canopy, directing the majority of the spray material to the upper canopy is generally advisable.
  • Some varieties are far more sensitive (Golden Delicious and Rome) to ethephon when compared to others (Red Delicious and Fuji). Rate selection is dependent on cultivar sensitivity and current fruit set.
  • The pH of the spray solution should be between 3 to 5 for optimal product performance. If you have alkaline (high pH) water, an acidifying surfactant or buffering agent is recommended.

See guidelines from the 2020 Integrated Orchard Management Guide for Commercial Apples in the Southeast (below). While developed for another region, another nice resource for rescue thinning was developed by researchers at University of Massachusetts and Rutgers University.

Thinning chart

Running the Carbon Balance Model

Output from the Carbon Balance Model for four sites in western NC (Apple Wedge, Flat Rock, Sugarloaf, and Morganton) and two sites in North Georgia (Blue Ridge and Ellijay) is below.

If you have access to NEWA, I highly recommend calculating the carbon balance for your own site. Predicted weather data is used in these estimates and the forecast can change throughout the course of a day. If you’d like to run the model on your own, the link at the NEWA site is on the drop-down menu under “Crop Management”. Choose a site nearest your orchard location and enter your Green Tip and Full Bloom date. If using the 2019 version of the model, select the % of flowering spurs that is most appropriate for the block(s) you plan to thin. Then select the green “Calculate” button.

It is very likely that if you run the model on your own, you will observe different and more accurate output. Potential reasons for this deviation in output includes:

1) Use of different model versions. I am reporting data from the 2019 version of the model and selected 50-75% flowering spurs.

2) Different green tip/bloom dates used.

3) Model output is based on forecast data, so the output will change as the forecast changes (a key incentive to run this on your own, if possible).

North Carolina

Edneyville, NC (Apple Wedge)

5.13.20: Edneyville (Apple Wedge) Apple Carbohydrate Thinning Model Results
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Balance (g/day) Accum Degree Days
(°F) (°F) (MJ/m2) Daily 7-Day Ave (since bloom)
13-May 64 46 11.6 9.39 23.9 344
14-May 73 52 22.6 25.09 12.09 356.9
15-May 76 56 21.8 9.27 6.23 371.8
16-May 81 58 23.1 -1.51 388.6
17-May 82 61 22.7 -11.88 406.6
18-May 78 55 18.6 -1.65 421.7
19-May 77 56 23.6 14.9 436.9

Edneyville, NC (Sugarloaf Mtn)

5.13.20: Edneyville (Sugarloaf Mtn) Apple Carbohydrate Thinning Model Results
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Balance (g/day) Accum Degree Days
(°F) (°F) (MJ/m2) Daily 7-Day Ave (since bloom)
13-May 61 46 11.2 10.53 25.66 315.2
14-May 73 51 23 26.74 14.24 327.9
15-May 76 55 21.8 11.03 8.47 342.5
16-May 80 57 23.6 4.42 358.8
17-May 82 61 22.8 -11.17 376.7
18-May 79 54 19.1 0.63 391.9
19-May 76 56 23.6 17.12 406.8

Flat Rock, NC

5.13.20: Flat Rock Apple Carbohydrate Thinning Model Results
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Balance (g/day) Accum Degree Days
(°F) (°F) (MJ/m2) Daily 7-Day Ave (since bloom)
13-May 63 45 11.4 11.77 25.11 331.1
14-May 74 52 23.1 24.31 14.42 344.3
15-May 76 55 21.5 10.34 8.62 358.9
16-May 80 57 23.3 3.88 375.2
17-May 82 61 22.9 -10.87 393.2
18-May 79 54 19.3 1.28 408.3
19-May 76 55 23.8 19.6 422.9

Morganton, NC

5.13.20: Morganton (Apple Hill Orchard) Apple Carbohydrate Thinning Model Results
Date Max Temp Min Temp Solar Rad Tree Carbohydrate Balance (g/day) Accum Degree Days
(°F) (°F) (MJ/m2) Daily 7-Day Ave (since bloom)
13-May 64 46 10.6 -4.09 5.32 388.2
14-May 79 52 24 6.01 -6.34 402.8
15-May 81 57 21.9 -13.04 -15.63 419.4
16-May 86 60 23.3 -27.48 438.2
17-May 87 63 22.7 -38.14 458.1
18-May 85 56 20.2 -23.52 475.4
19-May 82 57 23.7 -9.13 492.3


Blue Ridge, GA

Predicted Thinning Index: 5.13.20 Blue Ridge, GA
Date Days after bud break High Temperature (F) Carbon Balance (g/day) Thinning Index
13-May 55 73 -30.1 -21.54
14-May 56 78 -26.0 -21.41
15-May 57 79 -8.5 -27.52
16-May 58 81 -29.7 -30.64
17-May 59 83 -44.4 -14.37
18-May 60 77 -17.8 8.54
19-May 61 71 19.1
20-May 62 74 24.3

Ellijay, GA

Predicted Thinning Index: 5.13.20 Ellijay, GA
Date Days after budbreak High Temperature (F) Carbon Balance (g/day) Thinning Index
13-May 55 76 -25.5 -23.2
14-May 56 79 -30.5 -27.5
15-May 57 79 -13.6 -34.9
16-May 58 82 -38.2 -37.8
17-May 59 85 -52.8 -20.3
18-May 60 78 -22.5 3.7
19-May 61 73 14.3
20-May 62 75 19.1

Thinning Index (For GA sites)

Thinning index

Notes on Using Output from the Carbon Balance Model

The Apple Carbohydrate Balance Model is a useful tool in making chemical thinning decisions. This model was not developed to account for all factors that influence the efficacy of a thinner application; it was developed to estimate the carbohydrate status of the tree. In short, the carbohydrate balance model is a valuable tool, but other factors need to be considered when making a chemical thinning decision.

A positive carbohydrate balance makes it more difficult to thin whereas increasing carbohydrate deficits are predicted to increase the response to chemical thinning sprays. For example, you might respond to a predicted carbohydrate surplus or mild deficit by either adopting a more aggressive approach to chemical thinning or by waiting until more ideal conditions for thinning develop. Alternatively, if the model is predicting a severe carbohydrate deficit then you might consider reducing the rate of chemical thinner or perhaps not applying a thinner at that time for risk of over-thinning.