Is there a Rescue Thinner for 15 to 25 mm Diameter Apple Fruitlets?

It was June 2015, and there was uncertainty over fruit set because of the severe post-bloom frost damage. During the development stage when chemical thinners were effective (8-15 mm fruitlet diameter), it was unclear which fruits might remain, so the risk of over-thinning using chemicals was too high. By the time it was evident how many fruitlets had survived, the crop load was heavy, and the ideal thinning window had closed. A rescue thinner was needed.

In a two-year study on 'Gala,' 400 mg/L-1 ACC (VBC-30160) and 300 mg/L-1 ethephon (Ethrel®) were applied at three different fruitlet diameters (~15, 20, and 25 mm) to determine their late-thinning efficacy. Treatments were compared with carbaryl (Seven® XLR Plus) alone, and hand thinning. During 2014, 14-yr old 'Royal Gala'/M.9 trees were used at 'Site A' the University of Guelph Simcoe Research Station. In 2015, ten-yr-old 'Imperial Gala'/B.9 trees were used at 'Site B,' a commercial orchard in Windham Centre, ON.

When the fruitlet diameter reached 10 mm, all treatments were sprayed with 960 mg/L-1 carbaryl for a first attempt at thinning. The eight treatments (Figure 1) were replicated and randomized. All the chemical products were tank mixed with 500 mL/L-1 Regulaid® surfactant and 0.5-1.0 mL of an antifoaming agent. Foliar sprays were applied to whole trees using a commercial airblast sprayer calibrated at 1000 L/ha-1 .

Final fruit set was determined on two flagged limbs on each tree by counting the initial number of fruit prior to treatment application and then the remaining fruit after 'June' drop. Trees were visually inspected for phytotoxicity including leaf drop and yellowing at 24 hours and seven days after application.

Prior to harvest, ten representative fruit from each tree were assessed for fruit ethylene evolution and maturity using the starch-iodine index, blush and ground colour, firmness, soluble solids and titratable acidity.

Fruit were harvested to determine the total and marketable yield, fruit weight and crop load. A sample of 70 fruit/tree was graded using commercial sorting equipment at the Norfolk Fruit Growers' Association, and they were sorted into ten size categories according to their weight. Return bloom was assessed the following spring based on the percent of flowering spurs.

In 2014, trees from the carbaryl treatment had a relatively low crop load at 5.9 fruit per cm2 of trunk cross-sectional area (TCSA), indicating natural fruit drop was adequate without rescue thinning. Crop load was significantly lower for the ACC treatments applied at 15 and 25 mm compared with the carbaryl treatment, demonstrating a crop load reduction from thinning. ACC applied at 15 mm thinned by reducing fruit set by ~8.3% compared with the carbaryl treatment (Figure 1A). All other treatments had similar fruit set compared with the carbaryl treatment.

In 2015, crop load for the carbaryl and the hand-thinned treatment was high at 18.6 fruit cm-2 TCSA and 14.4 fruit cm-2 TCSA, respectively. Applying ACC at 16 mm reduced the crop load to 7.5 fruit cm-2 TCSA, which was significantly lower than carbaryl and hand thinning. Fruit set on trees treated with ACC at 16 mm was reduced by 36% compared with the carbaryl treatment (Figure 1B).

Figure 1. Fruit set of 'Gala' apple trees when thinners were applied in A) 2014 and B) 2015. Means with the same letter are not significantly different.

Figure 1. Fruit set of 'Gala' apple trees when thinners were applied in A) 2014 and B) 2015. Means with the same letter are not significantly different.

No treatments reduced the marketable yield, however, when thinners were effective and resulted in less fruit per tree, fruit weight was increased. In 2015, the ACC treatment at 16 mm had significantly less fruit per tree by ~292 fruit and higher fruit weight by 54 g compared with carbaryl. In both years, the size distribution for ACC treatments had a higher proportion of large fruit compared with the carbaryl treatment (Figure 2A-B).

Figure 2. Size distribution of fruit on 'Gala' apple trees when thinners were applied in A) 2014 and B) 2015.

Figure 2. Size distribution of fruit on 'Gala' apple trees when thinners were applied in A) 2014 and B) 2015.

The use of ACC and ethephon did not negatively affect fruit quality at harvest, and the ACC treatment at 16 mm that promoted thinning improved fruit quality by increasing soluble solids and weight. However, ACC did cause some yellowing and drop of leaves. In the spring of 2016 return bloom was enhanced, as there were ~23% more spurs with flowers when ACC was applied at 16 mm compared with the hand-thinned treatment.

Ethephon was ineffective for late 'rescue' thinning. This lack of a response may have been due to too low a concentration of 300 mg/L-1 . While there were indications by other research for the use of ethephon for thinning during late development, this was not supported by the results of the current study on 'Gala'.

The thinning effect of ACC is consistent with previous research. However, leaf yellowing may deter commercial use of ACC. Future work is needed to determine the maximum concentration of ACC application without causing leaf phytotoxicity on 'Gala,' to understand the environmental conditions associated with leaf phytotoxicity, and whether 6-BA can be combined with ACC to reduce phytotoxicity. Subject to registration, ACC is a prospective thinning compound for 'Gala' when used at the ~15 mm fruitlet diameter stage, providing an additional thinning option to what is currently available.

Acknowledgements

The authors appreciate the generous financial support for this project: University of Guelph - Ontario Ministry of Agriculture and Food Plant Production research program, Queen Elizabeth II Graduate Scholarship in Science & Technology, Ontario Apple Growers, and Walter and Laura Scott Tree-Fruit Scholarship. The technical support of Ms. A. Gunter and research station staff is gratefully acknowledged. The authors are thankful for the generosity of Chris Hedges who welcomed the research onto his orchard during 2015. The authors also appreciate the help of Hayden Dooney with commercial sorting equipment at the Norfolk Fruit Growers' Association.

Selected References

  • Greene, D.W. 2002. Chemicals, timing, and environmental factors involved in thinner efficacy on apple. HortScience 37, 477-481.
  • Marini, R.P. 1996. Chemically thinning spur 'Delicious' apples with carbaryl, NAA, and ethephon at various stages of fruit development. Horttechnology 6, 241-246.
  • McArtney, S.J. Obermiller, J.D., 2012. Use of 1-aminocyclopropane carboxylic acid and metamitron for delayed thinning of apple fruit. HortScience 47, 1612-1616.
  • Schupp, J.R. Kon, T.M., Winzeler, H.E., 2012. 1-Aminocyclopropane carboxylic acid shows promise as a chemical thinner for apple. HortScience 47, 1308-1311.

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