Chemical Thinning of Pears

By  Spencer Leidl (Summer Student),

Kathryn Carter and Amanda Green (OMAFRA)

Thinning is a process where a grower removes fruit from a tree to allow the remaining fruit to grow larger, be of a higher quality at harvest, and get a better price. Traditionally, pears have been hand-thinned, but this takes time and is very labour intensive. In recent years, increased labour costs and difficulty obtaining available labour has increased the use of chemical thinners in pear orchards. In this article, we will discuss the benefits of chemical pear fruitlet thinning, the timing of chemical thinning, and options for chemical thinning that are available in Ontario for growers.

What are the Benefits of Chemically Thinning Pears?

Fruit trees often set more fruit than they can support. Excess fruit set results in increased competition for resources (carbohydrates), resulting in smaller fruit, and weaker trees which are more susceptible to disease (Ingels, Geisel, Unruh, & Lawson, 2001). Excess fruit set in one year often results in a reduction in fruit set the following year (biennial bearing), having a negative impact on yield and productivity. Chemical thinning pears can result in many benefits, including:

• Increase in the quality of the crop through enhanced size, colour and flavor

• Prevention of over cropping
• Reduction in limb breakage
• Evenly spaced fruit and increased air flow, decreasing spread and likelihood of disease
• Encourages return bloom
• An overall more uniform crop

Figure 1. Pear fruitlets in Niagara on June 11^th,2020

What are the Challenges of Chemically Thinning Pears?

Chemically thinning can be challenging even in crops like apples where considerable research has been done. Moreover, in pears, there are fewer proven products registered as chemical thinners, which limits the options available and the time window for application.

The efficacy of the chemical thinners is affected by weather conditions before and after the application of the chemical and include cloudiness and temperature. Other factors such as humidity, frost, and pollination also affect chemical efficacy (Atlantic Committee on Tree Fruits, 2010). The condition of the tree can also have an impact on the thinning process. Young trees and trees with a heavy crop the previous year are easier to thin. Additionally, any tree which is undergoing a stressor, such as disease or injury, will be easier to thin (Green & Carter, 2016). However, it is important to recognize that chemical thinning in pears is inconsistent, and results may vary from year to year (Slingerland & McFadden-Smith, 2012). When using chemical thinners it is important that a grower accurately records the concentration of chemical, spray technique, date, and weather conditions (before and after prays), as these may impact the efficacy of the chemical thinners.

When is the Best Time to Chemically Thin Pears?

Chemical thinners for pears should be applied ideally when the fruit is between 5-10 mm in diameter and can be sprayed until the fruitlet is up to 14 mm in diameter (Green & Carter, 2016).

Environmental conditions can have a significant impact on the efficacy of chemical thinners. Higher temperatures allow for faster absorption of the chemical, while cool and wet temperatures allow for slower absorption, and therefore more thinning than usual will occur (Atlantic Committee on Tree Fruits, 2010). The optimal weather conditions for applying chemical thinners are calm days with temperatures in between 20-25℃, with the following 2-3 days above 18℃ (Green & Carter, 2016). If temperatures are above these conditions, growers may consider decreasing the application rate of the chemical to avoid over thinning. Avoid thinning when temperatures are above 30℃ (Valent BioSciences, 2019).

What Options are Available for the Chemical Thinning of Pears in Ontario?

In Canada, 6-BA (6-benzylamminopurine) (Cilis Plus and Maxcel) have been registered for chemical thinning on pears. Both of these chemicals are classified as plant growth regulators and influence the growth and differentiation of plant cells. For more information on the timing and application of chemical thinners on pears, refer to table 1.

Name	Registration Number	Preharvest Interval	Restricted Entry Interval	Application Rate	Total Application
Cilis Plus*	29210	28 days	12 hours	50-200 ppm	21.3 L/ha
MaxCel*	28851	86 days	12 hours	50-200 ppm	22.5 L/ha

*Ontario Ministry of Agriculture, Food, and Rural Affairs. (2020).

It is important to note that thinners used in apples may not be suitable for use in pears. For example, carbaryl (Sevien XLR) can be used in apples, but in pears it will cause misshapen fruit (Green & Carter,2016). Research with chemical thinners conducted by John Cline in Ontario has found that cultivars of Bosc and Cold Snap pears respond to chemical thinning in a similar fashion. In a 2017 study where various chemical thinners were applied in different concentrations to both cultivars, all chemical thinning treatments, except for those applied at a low rate, had a thinning effect on the trees. Fruit set was reduced with 6-BA in Cold Snap varieties throughout all years of the project, though not significantly. There was a rate response between the use of 75 ppm and 150 ppm of 6-BA, with the high rate of 150 mg/L causing a significant reduction in fruit yield (Cline, Carter, & Green, 2017). In the final two years of the project, 6-BA had a positive effect on fruit size in Bosc pears, especially at 150 ppm. In Cold Snap, there was a positive effect on fruit size in the second year, but it was not significant. From an economic perspective, the use of chemical thinners did not increase crop value when compared to the control fruit (Cline et al.,2017). However, the use of chemical thinners may help to reduce labour costs, especially in years where access to labour is an issue.

Conclusion

Environmental conditions (temperature and sunlight), pollination, tree health, grower experience, and type of cultivar can impact the efficacy of chemical thinners on pears. Careful records should be kept and the weather, dates of sprays, concentrations used, and results should be recorded (Atlantic Committee on Tree Fruits, 2010). Chemical thinners can be unpredictable in pears, so it is also good practice to keep chemical thinning on a small scale until it is properly evaluated (Slingerland & McFadden-Smith, 2012). However, in years like this when access to labour is challenging, the use of chemical thinners in pears may help to improve labour efficiency in orchards. For additional information on thinning pears see Thinning of Tree Fruit .  

Reference

Atlantic Committee on Tree Fruits. (2010, March) Thinners and growth regulators for fruit trees. Atlantic Committee on Tree Fruits. https://www.perennia.ca/wp-content/uploads/2018/04/thinners-growth-regulations.pdf

Cline, J.A., Carter, K. & Green, A.C. (2017, December). Expanding fresh market potential of Ontario’s “Bosc” and “Cold Snap” pears- An agricultural adaptation council project implementation program. Canadian journal of plant science 98(4). Doi: 10.1139/CJPS-2017-0258

Einhorn, T. Sandlin, M. Pasa, M. Arrington, M. Blaine, G. (2014). ABA chemical fruit thinning in Bartlett pears. (Final Project Report).

Green, A. & Carter, K. (2016). Thinning of tree fruit. Ontario Ministry of Agriculture, Food, and Rural Affairs. http://www.omafra.gov.on.ca/english/crops/hort/thinning.htm#pears

Ontario Ministry of Agriculture, Food, and Rural Affairs. (2020). Crop production guide for tender fruit. Ontario Ministry of Agriculture, Food and Rural Affairs (Publication 360D).

Ingels, C., Geisel, P.M., Unruh, C.L.,& Lawson, P.M. (2001). Fruit trees: Thinning young fruit. University of California (Publication 8047). http://homeorchard.ucdavis.edu/8047.pdf.

Roper. T.R. (2005). Plant growth regulator use in apples. University of Wisconsin Extension (Publication A3524). https://polk.extension.wisc.edu/files/2014/02/Plant-Growth-Regulators-in-Apples-A3524.pdf

Slingerland, K. & McFadden-Smith, W. (2012). Pear production in Ontario. Ontario Ministry of Agriculture, Food, and Rural Affairs.

Utah State University Extension (n.d.). Petal fall (post bloom). Photograph. Retrieved from https://intermountainfruit.org/cold-effects/pear-stages.

Valent Biosciences. LLC (2019). Maxcel. Libertyville, Illinois.