Nibble Thin for a Quality Bin

Nibble Thin for a Quality Bin

Erika DeBrouwer, Tree Fruit Specialist, OMAFRA

There are a multitude of ways to reach your desirable crop load within your apple orchard, but feasibility, time management, and calculation need to be considered when making decisions. This article will go through precision crop load importance, the many ways to nibble thin your crop throughout the season, when to nibble thin and updates of ongoing research.

Precision Crop Load Management

By reducing crop load through precision management, an increase in fruit size is apparent (Table 1). Crop load management is not only important in growing larger apples, but also quality apples with desired colouration.

When managing crop load there are a few methods that growers can use

  1. Pruning
  2. Chemical thinning
  3. Hand thinning

Pruning is a common practice to adjust crop load but is utilized for practices other than crop load management, including training tree shape and removing unhealthy/dead wood. Chemical thinning, although tedious, is starting to become more popular due to a decrease expense in regard to labour. Hand thinning has become less popular in recent years due to the cost associated with labour, along with the energy utilized by the tree for fruit that will eventually be taken off. These three methods are used to enhance efficiency and feasibility regarding crop load, and they define precision crop load management.

Table 1. Effect of fruit size in relation to crop load (Number of apples required per tree for given tree density (trees/acre), desired yield (bu/ac) and fruit size)

Nibble Thinning

The nibble thinning approach is to thin a little at every opportunity, including the application of chemical thinners at various growth stages (blossom, petal fall/early fruitlet and early fruitlet stages). Nibble thinning is a great way to thin apples, as it allows growers time between applications to assess their crop load and get closer to the target amount of fruit. Another benefit of utilizing nibble thinning is that you don’t have to rely on a small number of applications or wait for the perfect thinning weather within the staging window. Another potential benefit is the reduction of biennial bearing crop loads in varieties like ‘Honeycrisp’, as the crop load will be adjusted with earlier bloom applications.

Although there’s some reluctance regarding the application of chemical thinners before determining the fruit set after a spring frost event or other trauma, apple trees are resilient and only require 5 to 10% of blooms to achieve a full crop load. Growers should know that more confidence in utilizing chemical thinners will be achieved as more information regarding apple frost injury, bee activity, pollination and fertilization becomes available. Starting early with chemical thinners allows you the opportunity to achieve your ideal crop load over time and lowers the chance of over-thinning at later growth stages.

One of the most important aspects before managing your crop load is to assess the health of your trees and observe your fruit set. Assess fruit buds for signs of damage following freezing temperatures or onset of frost. The ONfruit article “Evaluating Frost Damage on Buds in Tree Fruit” and “Frost Damage Evaluation On Apple Buds” on the OMAFRA website go into more detail regarding how to assess your apple crop from frost damage.

Regarding thinning with frost damage, please refer to the following articles:

For information regarding suggested rates and various chemicals used for chemical thinning, refer to the Thinning of Tree Fruit, apple section on the OMAFRA website or the Ontario Crop Protection Hub. Information regarding ATS and Lime Sulphur specifically can be found in the February 2022 and April 2021 issues of the Orchard Outlook Newsletter (ONNL) titled “Bloom Thinning with Lime Sulphur and ATS” and “Preliminary Results of Blossom Thinning with Lime Sulphur and ATS”, respectively, by Dr. John Cline.

Thinning Models

Below are brief descriptions of each modelling system, how to utilize each, and the benefits and challenges of each. Remember that every modelling system should be utilized in conjunction with previous thinning decisions, grower experience and weather forecasts and that these systems become better through incorporation and use within the orchard. I also want to point out that you do not have to utilize these strategies on every variety – maybe it’s only worth the time to perform these data collection methods for high-value or biennial cropping varieties, but that decision is up to you.

Carbohydrate (Carbon Balance/MaluSim) Model

This model was developed by Dr. Alan Lakso and Dr. Terence Robinson from Cornell University and has progressed into estimating the timing and amount of chemical thinning applications for improved efficacy. The model is based on the theory that carbohydrates (energy) are needed to keep fruitlets growing and to remain on the tree. When there is low energy or carbohydrates, fruitlets should thin more easily and drop, where the opposite would occur if generous amounts of carbohydrates are available for fruitlets to resist thinning and set fruit well. The Carbohydrate model predicts daily energy supply and demand to determine fruitlet sensitivity to chemical thinning. This is why the model can be applied throughout the season, utilizing knowledge of growth habits and staging within apples. This model assists in predicting how sensitive fruitlets will be to thinning strategies.

Benefits
Challenges
More precise then estimating the effectiveness of thinners
Requires a weather station to utilize
Once started it is easy to continue
Web application only available in the U.S.
Can be used in collaboration with other thinning tools currently available
Takes time to set up and become comfortable with
Can be used at multiple time points during the thinning period
Growers have to count, enter and interpret data
Doesn’t factor in tree architecture, specific varieties, or training systems

Pollen Tube Growth Model

The Pollen Tube Growth Model (PTGM) is one of the newer modelling systems, currently being utilized across the U.S. This modelling system was developed by Virginia Tech and is only available for certain varieties. A “universal” PTGM is currently being developed by Dr. Greg Peck. The PTGM model uses the ideal number of king blooms for a target crop load, weather data and king style length to determine the best timing for bloom thinner application. This modelling system uses the theory that based on style length, a certain amount of time needs to pass for proper fertilization of the flower.

How To Use:

  1. Assess orchard to determine target crop load
  2. Move to step 3 when king blooms equal target crop load
  3. Measure 25 to 50 king styles
  4. Input style length into modelling system
  5. Model provides best time and date for effective bloom thinning application using weather data
Benefits
Challenges
Early bloom thinning tool
Requires weather data
One-time data collection
Precise timing for thinning applications
Only available for certain varieties
Not available and currently being validated in Ontario
Unknown cost associated with use

Fruitlet Growth (Fruitset) Model

This tool is utilized to help predict thinning efficacy 7 to 9 days after thinning application. The Fruitlet Growth Model was developed by Dr. Duane Greene from the University of Massachusetts and utilizes the growth rates of fruitlets to determine chemical efficacy. This is based on the assumption that the fruitlets that will not abscise will be growing at a faster rate than the abscising fruitlets. In the past, growers have found that two measurements separated by 3 to 5 days suffices. This model can be used at multiple points during the season and can be continuously used with other tools and modelling systems.

How To Use:

  1. Select 5 trees per variety/block
  2. Mark 14-15 clusters on each tree (7 on each side in trellised orchard)
  3. Determine target crop load
  4. Take diameter measurements 3 days after applying thinner
  5. Take second diameter measurement 3 or 4 days after second application
  6. Input data into excel file (output will occur automatically)
Benefits
Challenges
Doesn’t require a weather station to utilize
Takes time to set up and become comfortable with
More precise then estimating the effectiveness of thinners
Growers have to count, enter and interpret data
Once started it is easy to continue
Can be used in collaboration with other thinning tools currently available
Can be used at multiple time points during the thinning period
No cost associated with use

Tools to Assist with Crop Load Management Models

To make the process easier when utilizing various modelling systems for growers, there have been different strategies explored:

Schwallier Spreadsheet

This is the original spreadsheet utilized to interpret the data collected from the orchard after chemical thinning has been applied. The spreadsheet gives growers a better idea of fruitset and requires you to label each individual fruitlet.

Ferri Spreadsheet

This spreadsheet was adapted from the Schwallier Spreadsheet utilizing the same method but doesn’t require labelling each individual fruitlet. The spreadsheet automatically arranges fruitlet size from largest to smallest and will predict how many fruitlets will set and how many will abscise. 

Ferri Model Application

The Ferri Model Application is currently in a beta trial and was developed to give growers a better idea of fruitset after thinning. This application is hoping to give real-time answers during collection measurements and would replace the excel spreadsheet previously listed. Currently only available in apple products.

Perennia: Orchard Tools Application

The Perennia: Orchard Tools Application was developed to capture data for the fruitlet growth rate model. This application assists with collecting data but would have to be exported into an excel sheet or the MaluSim application to fully utilize fruit set prediction. This application can be used in the orchard to collect fruitlet sizes and is currently only available on apple products.

Malusim Application

The MaluSim application can be used for the fruitlet growth model, the carbohydrate model, along with irrigation models and records. Currently, the MaluSim application requires a U.S. address in order to connect with weather stations.

Thinner Effectiveness Table

This table was made by Michigan State University based on research from Cornell University, stating general comments and guidelines on PGR thinner use.

Climate Condition
Prediction
Warm Conditions (>18oC)
All thinner work best
Dark Cloudy Weather
Greater stress, greater thinning response, greater drop
High Night Temperatures (>18oC)
Great stress, high demand and use of energy for night respiration, greater drop
Very High Day Temperatures (>29oC)
Great stress, high energy demand, greater drop
Very Cool Temperatures (<18oC), Greater Set
Reduced stress, reduced energy demand, greater set
High Light
Increased supply: harder to thin
Low Light
Reduced supply: easier to thin
Low Temperatures
Low demand: harder to thin
High Temperatures
High demand: easy to thin
Low Light and Warm Temperatures
Worst
Adapted from Cornell University and Michigan State University

Overall, these are modelling systems and tools that should be utilized in conjunction with previous thinning decisions, grower experience and weather forecasts, along with your own knowledge. These are guidelines for how to manage your crop load effectively and drive your efficiency as a grower. Not all tools are going to be for everyone, but the use of some could make your farm more efficient and help you make decisions throughout the season.

If you have used any of these tools or utilize tools that aren’t currently listed, be sure to share your experience with Erika DeBrouwer through text, email, or social media (226-931-4098, erika.debrouwer@ontario.ca, @FruitWiTheSpurs)

Further Reading and Resources

Carbohydrate Model

Lordan, J., Reginato, G.H., Lakso, A.N., Francescatto, P. and Robinson T.L. 2019. Natural fruitlet abscission as related to apple tree carbon balance estimated with the MaluSim model. Scientia Horticulturae. 247:296-309.

Neumann, L. 2020. Dissertation: Improving the apple carbon balance model MaluSim by integrating and testing water deficit and fruit drop effects. Gottfried Wilhelm Leibniz Univ.

Robinson, T.L. and Lakso, A.N. 2011. Advances in Predicting Chemical Thinner Response of Apple Using a Carbon Balance Model. New York Fruit Quarterly. 19:1.

Schawallier, P. 2013. PGR’s and Thinning Strategies GLEXPO 2013. Microsoft Word – Schwallier Thinning EXPO 2013.docx (msu.edu)

Pollen Tube Growth Model

Kon, T.M., Schupp, J.R., Yoder, K.S., Combs, L.D. and Schupp, M.A. 2018. Comparison of chemical blossom thinners using ‘Golden Delicious’ and ‘Gala’ pollen tube growth models as timing aids. HortScience 53(8):1143-1151.

Peck, G.M., Combs, L.D., DeLong, C. and Yoder, K.S. 2016. Precision apple flower thinning using organically approved chemicals. Acta Hortic. 1137 ISHS.

Yoder, K.S., Peck, G.M., Combs, L.D. and Byers, R.E. 2013. Using a pollen tube growth model to improve apple blossom thinning for organic production. Acta Hortic. 1001, ISHS.

Fruitlet Growth Model

Clements, J. 2021. HRT-RECIPE: Predicting fruit set using the fruitlet growth rate model. CAFE: UMassAmherst.

Green, A. 2018. Using the Fruitlet Growth model in Your Orchard. OMAFRA.

Greene, D.W., Lakso, A.N., Robinson, T.L. and Schwallier, P. 2013. Development of a fruitlet growth model to predict thinner response on apples. HortScience 48(5):584-587.

Schwallier, P. and Irish-Brown, A. 2015. Predicting Apple Fruit Set Model. New York Quarterly 23:1.

Erika DeBrouwer
Erika DeBrouwer

Tree Fruit Specialist, OMAFRA