Thinning Peaches with ACC (Accede™)

Thinning in Stone Fruit Production

Peach trees set thousands of flowers each year, but only about 15-20% of blossoms must be pollinated and maintained to reach a target crop load. Target crop load varies by variety but generally ranges from ~45,000 fruit per acre in large-fruited cultivars to ~100,000 in small-fruited cultivars, with a typical goal of 6-8 fruit per meter of fruiting lateral length (Kumar et al., 2026).

Hand thinning during the fruitlet stage remains the dominant thinning method in stone fruit production. However, it is extremely time and labor intensive, exacerbated by high labor prices and difficulties finding work during the limited thinning period.

Blossom thinning provides an opportunity to reduce crop load earlier in the season and decrease the amount of hand thinning needed later.

Chemical Blossom Thinning

Chemical blossom thinning is a method of early crop load management using a chemical product to remove flowers or prevent fruit development. The goal of blossom thinning in stone fruit is to reduce the amount of hand thinning needed later in the season – not eliminate it. While blossom thinning has long been common in apple production, limited product availability has restricted its use in stone fruit. This has changed with the recent registration of Accede.

Blossom thinning can also improve fruit size compared to fruitlet thinning (Myers et al., 2002; Osborne & Robinson, 2008). Final fruit size is determined early in the season, influenced by the number of fruits competing for the tree's available carbohydrate resources. When resources are limiting during this early developmental period, fruit growth potential may be stunted and cannot be fully recovered later in the season (Day & DeJong, 1998; DeJong et al., 1991). Blossom thinning removes unwanted flowers earlier in the season, preserving resources that would have typically been wasted if they were distributed to fruitlets that were later hand-thinned.

Economic analysis has estimated the benefit of blossom thinning in peach orchards ranging from $250 to $850 per acre (Schupp et al., 2018), emphasizing its potential to reduce production costs while improving fruit quality.

Accede – The First Chemical Blossom Thinner for Stone Fruit

Accede, developed by Valent Biosciences, was registered in 2021 and is currently the only commercially available chemical thinner for use in stone fruit in Pennsylvania.

Accede is a hormonal thinner with the active ingredient ACC, a natural ethylene precursor. It is registered for use across most of the United States, including all major stone fruit- producing states and all Mid-Atlantic states except Rhode Island. Accede was initially released as a liquid concentrate but is now only available as a soluble granule formulation.

ACC and How it Works

The active ingredient in Accede is 1-amino-cyclopropane-1-carboxylic acid (ACC), a naturally occurring amino acid and an ethylene precursor. Ethylene is a gaseous plant hormone involved in ripening, aging and development, stress responses, and more. When ACC is applied during bloom, it is believed to be absorbed through the flower pedicel (the stalk connecting the flower to the shoot) (Racsko, 2025) and quickly converted to ethylene within the plant. Ethylene then stimulates the abscission zone, the tissue where flowers and fruits detach, leading to flower drop and the desired thinning response (Ruperti et al., 1998; Tonutti et al., 1997).

Ongoing research is investigating the potential of ACC to have independent effects outside of its role as an ethylene precursor, but this is not well understood at the moment.

In most higher plants, ethylene is produced through a pathway (Figure 2) that begins with the amino acid methionine. Methionine is converted to S-adenosyl-L-methionine (SAM), then to ACC, and finally to ethylene (Pattyn et al., 2021). The detection of ethylene can further stimulate the plant to synthesize ethylene (Dai et al., 2023; Guo et al., 2021), resulting in a rapid increase in ethylene production.

When ACC is applied as a thinning treatment, it enters partway through this pathway and is quickly converted to ethylene.

Using Accede According to the Label

The information below summarizes usage instructions based on the Accede SG product label (available at valent.com) and commercial practice. Always consult the current product label for complete instructions and remember – the label is the law.

Accede SG is a soluble granule formulation that is 40% active ingredient (ACC) by weight.

Application Method

Accede is applied as a foliar spray, most commonly using airblast sprayers, tower sprayers, or backpack/handgun sprayers.

Commercial applications are typically done with an airblast sprayer with a spray volume of 100 gallons of water per acre. It is recommended to use a non-ionic surfactant at 0.05% v/v with Accede applications.

Application Time

Accede is labeled for application between the pink and petal fall stages of bloom. Application is most often done with a single spray, but additional applications may be made 3-10 days later if necessary, given that the tree remains within the labeled bloom window.

Applications should be made during slow drying conditions, such as early morning or evening, with low wind speed and moderate temperatures.

Do Not Apply Accede…

• Through irrigation or fertigation systems.
• When temperatures are predicted to be below 32°F or above 90°F on the application date.
• To injured or stressed plants.
• If rainfall is expected within 8 hours of application.

Late frost events may result in flower damage, bud damage, and a reduction in crop load. If frost is possible during bloom, wait until damage can be assessed before determining thinning needs or adjusting rates.

Application Rate

The labeled application rates are as follows:

Nectarine, Peach: 300-600ppm (10-20oz Accede SG per acre in 100gal water)

Apricot, Sweet and Tart Cherry, Plum (Chickasaw, Damson, Japanese), Plumcot, Fresh Prune: 100-600ppm (3.35-20oz Accede SG per acre in 100gal water)

Rate selection should consider:

• Crop load
• Desired thinning level
• Varietal thinning response
• Frost or injury risk

Stressed or damaged trees may be more sensitive and at-risk of overthinning or phytotoxicity.

Tank Mixing

Currently, no information is available concerning Accede's compatibility with other products.

Results from Recent Trials

Ongoing research evaluating the local use of Accede and factors that influence its efficacy is being conducted by Valent researchers, Dr. Jim Schupp at the Penn State Fruit Research and Extension Center, Dr. Shan Kumar and graduate student Michael Evans, and Pennsylvania growers.

Key findings are summarized below:

Application Timing

Applications made at three different times within the labeled pink to petal fall window (Figure 3) did not significantly change thinning response, fruit size, or fruit quality. However, applications made at full bloom numerically produced the strongest thinning response.

In mixed-variety blocks, selecting a reference variety that blooms mid-season compared to other varieties and applying when that variety reaches full bloom may be a practical timing approach.

Rewetting

Rewetting after ACC application does not appear to influence overall thinning efficacy. With some thinning products, it has been suggested that rewetting may allow dried product residues to be taken up by the plant and therefore increase efficacy. Applying a 100 gallons of water per acre one day after ACC application may have accelerated fruit abscission but did not change final fruit set or fruit quality. Later rewetting treatments had no significant effect. This suggests that rewetting may influence early thinning activity but does not influence overall thinning efficacy.

Dose Response

Trials evaluating Accede across a range of application rates demonstrated that the product is rate responsive, with greater thinning observed at higher doses. Increasing the rate applied per acre consistently produced stronger thinning effects and improved fruit size response, confirming that Accede performs in a predictable, dose-dependent manner.

Higher rates increase the amount of active ingredient available for uptake across the plant surface, driving a more pronounced physiological response and greater thinning efficacy.

For large peach trees, hard-to-thin varieties, or areas with limited prior response to ACC, consider increasing the application rate to achieve the desired level of thinning.

Varietal Response

Peach and nectarine varieties differ in their response to ACC.

Three years of thinning trials in Adams County conducted by Dr. Jim Schupp have resulted in the table below (Figure 4) showing 31 varieties' thinning ranking each year. All trees received the same rate of ACC and were treated on the same day. Varieties with a lower ranking (red) were thinned more strongly, while varieties with a higher ranking (blue) experienced less thinning.

In general, varieties tended to rank similarly year-to-year. Varieties like John Boy, Vinegold, and Sentry were consistently strongly thinned each year, while varieties like SilverGlo, SilverGem, and Victoria were minimally thinned. Varieties such as July Rose, Blushingstar, and Avalon had variable responses year-to-year.

While the relationship between variety and thinning response is not completely understood, it currently does not appear that flesh texture classification (melting, non-melting), stone type (clingstone, freestone), flesh color, shape, pedigree, flower size, or seasonality predict a variety's response.

Ongoing research is investigating possible differences among varieties in ethylene production and sensitivity following ACC application with the hope of developing variety-specific recommendations.

Application Method

Recent work from Valent has explored the impact of many application method factors on ACC efficacy including tractor speed, fan speed, droplet size, and spray pressure.

Improved thinning was associated with:

• Slower fan speed
• Smaller droplet size
• Standard tractor speed (~3mph)

These settings likely improve canopy coverage by reducing spray blow-through and improving solution deposition on flowers.

Slowing tractor speed to increase spray volume showed reduced thinning efficacy compared with maintaining standard speed. When increasing spray volume, use nozzles with a higher volume flow rather than reducing the speed of the tractor to deliver a larger volume.

Like the aforementioned water volume trial, this work once again highlighted the importance of adequate coverage when using ACC.

Recap of Practical Recommendations

• Apply Accede at 300-600ppm (nectarine, peach) or 100-600ppm (apricot, cherry, plum, plumcot, prune) between pink bud and full bloom stage.
  • Adjust ppm based on desired thinning level, tree health, frost risk, and varietal response.
• In mixed blocks, consider targeting full bloom of a mid-season reference variety.
• Use a non-ionic surfactant at 0.05% v/v.
• Use high spray volumes to ensure thorough coverage. Consider 150 gallons per acre for large trees, hard-to-thin varieties, or nonresponsive blocks.
• Improve coverage by using a slower fan speed, smaller droplet size, and standard tractor speed (~3mph).
• Be aware of varietal differenced in thinning response. Strong responses have been observed in John Boy, Vinegold, and Sentry, while SilverGlo, SilverGem, and Victoria have shown minimal response in Pennsylvania trials.

Resources

Accede Product Label

2026-2027 Penn State Tree Fruit Production Guide

References

Dai, J., Xu, Z., Xu, Y., Fang, Z., Shah, K., Kang, T., Wu, H., Zhang, D., Xing, L., Ma, J., Liu, H., Hu, Y., & Zhao, C. (2023). A novel NAC transcription factor, PpNAP6, is involved in peach ripening by activating ethylene synthesis. Postharvest Biology and Technology, 201, 112363. doi.org/10.1016/j.postharvbio.2023.112363

Day, K. R., & DeJong, T. M. (1998, June 16). Improving Fruit Size: Thinning and Girdling Nectarines, Peaches, and Plums. Curso Internacional de Fruticultura de Clima Templado-Frio.

DeJong, T. M., Johnson, R. S., Day, K. R., & Beede, R. (1991). Feasibility of Increasing Cling Peach Yields by Early Thinning: 1991 Cling Peach Report. University of California.

Guo, Z.-H., Zhang, Y.-J., Yao, J.-L., Xie, Z.-H., Zhang, Y.-Y., Zhang, S.-L., & Gu, C. (2021). The NAM/ATAF1/2/CUC2 transcription factor PpNAC.A59 enhances PpERF.A16 expression to promote ethylene biosynthesis during peach fruit ripening. Horticulture Research, 8(1), 209. doi.org/10.1038/s41438-021-00644-6

Kumar, S. K., Brunharo, C., & Weber, D. (2026). Part I. Cultural Information. In 2026-2027 Tree Fruit Production Guide. The Pennsylvania State University.

Myers, S. C., Savelle, A. T., Tustin, D. S., & Byers, R. E. (2002). Partial Flower Thinning Increases Shoot Growth, Fruit Size, and Subsequent Flower Formation of Peach. HortScience, 37(4), 647–650. doi.org/10.21273/HORTSCI.37.4.647

Osborne, J. L., & Robinson, T. L. (2008). Chemical Peach Thinning: Understanding the relationship between crop load and crop value. New York Fruit Quarterly, 16(4).

Chemical Peach Thinning: Understanding the relationship between crop load and crop value.Racsko, J. (2025, June 23). Uptake of 1-Aminocyclopropane-1-Carboxylic Acid (ACC) When Used as a Peach and Apple Chemical Thinner. International Society of Horticultural Science Plant Growth Regulator Symposium.

Ruperti, B., Bonghi, C., Tonutti, P., & Ramina, A. (1998). Ethylene biosynthesis in peach fruitlet abscission. Plant, Cell & Environment, 21(7), 731–737. doi.org/10.1046/j.1365-3040.1998.00305.x

Schupp, J. R., Baugher, T., & Heineman, P. (2018, June 4). Blossom thinning peaches. Fruit Growers News.

Tonutti, P., Bonghi, C., Ruperti, B., & Ramina, A. (1997). The Modulation of Ethylene Biosynthesis and ACC Oxidase Gene Expression During Peach Fruit Development and Fruitlet Abscission. In A. K. Kanellis, C. Chang, H. Kende, & D. Grierson (Eds.), Biology and Biotechnology of the Plant Hormone Ethylene (pp. 149–153). Springer Netherlands. doi.org/10.1007/978-94-011-5546-5_20