By Laura Dobbyn (OMAFA-summer student), John Cline (UofG), Sofia Franzluebbers (UofG M.Sc.), Kathryn Carter (OMAFA), Catherine Bakker (UofG), Younes Mostofi (UofG)
Background
Fruit trees commonly set more fruit than they can support. Fruit thinning is an essential practice used to reduce crop load, maximize fruit size, improve fruit quality and promote return bloom. Thinning also reduces the risk of limb breakage caused by excessive crop loads, preventing structural damage and future yield loss. Chemical thinners are not currently registered for peaches in Canada, and hand-thinning is required.
All peach cultivars are hand-thinned annually. Generally early season peach cultivars are thinned earlier than mid- and late-season cultivars, because of the shorter growing season to gain adequate fruit size. Hand-thinning is labour intensive, time sensitive and costly, with labour costs estimated at $600 to $1800 per ha (OMAFA, 2023). Hand-thinning typically occurs within 20 to 40 days of full bloom. Early flower thinning in peaches is generally more effective at increasing fruit size than late fruit thinning, especially for early-ripening cultivars which have a shorter development time. Blossom thinning allows the trees to redirect resources to the remaining fruit, producing larger fruit. It also increases the risk of frost related yield losses.
1-aminocyclopropane-1-carboxylic acid (1-ACC), is a new chemical thinner recently registered in the United States for use in peaches and plums and currently under review in Canada. Studies in the United States indicate that 1- ACC can effectively manage crop load and reduce labour costs in peach orchards (Schupp, 2023).
Research Trials
In 2023, Dr. John Cline and Sofia Franzluebbers (M.Sc. Candidate) (University of Guelph) initiated a multi-year study with the aim of reducing the labour required for hand-thinning and improving marketable fruit size and quality in peaches.
Trials were conducted in commercial orchards in southern Ontario in 2023 and 2024 on peach cultivars ‘Harrow Dawn’ and ‘Vivid.’ Trees were treated with 300, 400, and 500 mg·L-1 ACC applied at full bloom and when fruitlets were 18 mm fruitlet diameter and compared with an untreated, hand-thinned control treatment.
Evaluations were conducted to evaluate the impact of the treatments on fruit set (pre and post June drop), leaf phytotoxicity (leaf yellowing or drop), labour requirements (time) for hand-thinning, yields, fruit size, and fruit marketability.
Trial Results
Fruit set and crop load was very light in 2024 due to a severe frost event occurring on 26 April (–5.7 °C) which reduced the initial bloom by roughly 50% before chemical thinners were even applied. Cool temperatures post-bloom (<7 °C) further reduced natural fruit set. As a result, peach yields in both cultivars were significantly reduced in all treatments. 1-ACC applications were applied for research purposes, even though additional thinning was not necessary. The manufacturer (Valent BioSciences) cautions against the use of 1-ACC in years with late spring frost events. Cold temperatures in the spring are known to increase the thinning effect of 1-ACC and potentially increase the risk of phytotoxicity.
Yields
Across both cultivars, all 1-ACC treatments over-thinned the trees, resulting in inadequate yields. Relative to the hand-thinned control, all 1-ACC-treated trees had lower fruit set after ‘June’ drop, and lower yields, especially when applied at full bloom (FB).
(FB) applications of 1-ACC had a greater thinning response (reduced fruit set and yields) compared to fruitlet (15-20mm diameter) applications for both cultivars. The thinning response increased in a linear fashion with increased concentrations of 1-ACC applied at FB, with a 52% to 65% reduction in yield from the control for ‘Vivid’ (Figure 1).
Fruitlet stage applications of 1-ACC on ‘Harrow Dawn’ trees significantly reduced fruit set and yield compared to the hand-thinned control, though fruit set remained higher compared to applications made at FB. In contrast, fruitlet stage applications on ‘Vivid’ trees did not produce a consistent effect on total yield per tree as compared with the control.
With ‘Harrow Dawn,’ 31% of peaches in the control treatment were unmarketable as a result of surface blemishes, soft fruit, or split pits. FB applications of 1‑ACC increased the proportion of unmarketable fruit by an average of 49%, whereas fruitlet‑stage applications resulted in unmarketable fruit levels similar to the control. For ‘Vivid,’ 1‑ACC applications did not affect the proportion of unmarketable fruit.
Fruit Size
1-ACC treatments influenced fruit size distribution during the first and second harvests with ‘Harrow Dawn.’ When data from both ‘Harrow Dawn’ harvests were combined, fruit size peaked at 63.5-67 mm for the control, >73 mm for 1-ACC applied at FB, and 63.5-67 mm when 1-ACC was applied to fruitlets (15-20 mm diameter). However, it should be noted that the increased fruit size in 1-ACC did not offset the reduction in marketable yields as a result of overthinning. In contrast, ‘Vivid’ trees treated with 1-ACC sprays applied at FB had fewer fruit in the larger size categories resulting in a significant reduction in marketable fruit weight compared to the control and 1-ACC sprays applied at the fruitlet stage.
Fruit Maturity and Quality
1-ACC treatments had little to no effect on fruit maturity as assessed by the amount of marketable fruit picked on the first harvest date compared to the second harvest date. With ‘Vivid,’ there was a tendency for bloom applications of 1-ACC to result in more fruit harvested on the first harvest date, but there not any consistent trends with respect to different rates of 1-ACC applied. 1-ACC treatments also had no effect on fruit quality as assessed by color, firmness, acidity, and sugar content.
Phytotoxicity
There was slight leaf yellowing (less than 10% of leaves affected) and leaf drop observed after FB applications of 1-ACC. Additionally, greater phytotoxicity to flowers and shoots was observed in 2024 compared to previous years. This could be a result of the cold temperatures that occurred during application. 1-ACC treatments at FB had greater shoot dieback; however, overall injury to the tree was less than 1% for all treatments, except for the 500 mg L-1 1-ACC treatment applied at bloom and 400 mg L-1 1-ACC applied at fruitlet stage, where injury affected an estimated 1-5% of the tree.
Take home message
- 1-ACC is effective at thinning fruit on peach trees, and reduces fruit set and crop load, but monitoring weather conditions is critical to avoiding overthinning.
- Cold weather before and after application can increase the effects of 1-ACC, resulting in overthinning, and increased leaf phytotoxicity.
- Bloom applications of 1-ACC were more effective in thinning fruit than fruitlet applications.
- Cultivars vary in their response to 1-ACC.
- This trial will be continuing until 2026. Stay tuned to the ON Fruit Blog for more updates on this project. Visit the OFVC to see Dr. Kumar share the results of his research on the use of chemical thinning peach orchards in Pennsylvania.
- 1-ACC (Accede) shows promise as a chemical thinning agent for peaches and is currently in the registration process for peaches.
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