New Chemical Thinning Compounds for Apples

It's a few months before bloom, but not too soon to be thinking about your chemical thinning strategies for 2019. There are some new products being researched that will hopefully become available to Canadian apples producers in the next few years. The purpose of this article is to briefly highlight two new compounds being researched in Canada and provide a brief review of currently registered products. In addition, a summary of environmental and tree factors that can influence the ease of thinning is provided.

What is the purpose of chemical thinning:

The intention of chemical thinning of apples is to:

  • Increase fruit size to optimize financial returns
  • Ensure adequate return bloom and prevent biennial bearing
  • Improve fruit colour, disease and pest control through reduced fruit clustering
  • Reduce limb breakage
  • Improve harvesting efficiencies

New Compounds on the Near Horizon

Metamitron: Metamitron is a herbicide used on sugar beets that was first used and is now registered in Europe for thinning apples. Research has been conducted for two years at the University of Guelph, Simcoe to evaluate its efficacy under our growing conditions and cultivars. Marketed under the name Brevis, metamitron is a photosynthetic inhibitor that temporarily (7-10 days) reduces carbohydrate supply to developing fruitlets, triggering earlier and enhanced fruit abscission. Thinning levels are concentration-dependent and can be enhanced under specific conditions. As with all thinning agents, the thinning response can vary based on cultivar and weather conditions at and following application. We are actively working with Adama Canada to provide data to support the registration of metamitron in Canada and are encouraged by the thinning response we've seen.

ACC: 1-aminocyclopropane carboxylic acid (ACC) is a molecule that enhances ethylene production in developing fruitlets and triggers fruit abscission. ACC is being developed by Valent Biosciences and our research in Simcoe shows promise when used alone or in combination with 6-BA, NAA, Carbaryl or s-ABA (another new compound). What is unique about this compound is its ability to be effective beyond the traditional thinning 'window' of 15 mm fruitlet diameter. Research in the USA has demonstrated ACC thins fruitlets as large as 25 mm diameter, which some have referred to as 'rescue' thinning. The federal pesticide regulator (PMRA) has indicated that it will conduct minor use trials beginning in 2019 on 1-ACC (Project AAFC18-006), giving growers optimism that it will be available for use soon.

Canadian Registered Thinning Products

Carbaryl (Sevin XLR): Carbaryl is the most widely used thinning agent. This mild thinner is used at rates of 0.5 to 3.2 L of Sevin XLR per 1000 L water, assuming dilute application. Sevin XLR contains a surfactant incorporated in the formulation to aid in leaf and fruit uptake. Sevin XLR can be applied from late petal fall (after bees have been removed or are not active in the orchard) to 25 days after full bloom. The use rate will depend on the apple variety, tree size, row spacing, and the weather conditions at the time of and following application

NAA (Fruitone L): Naphthalene acetic acid (NAA), used at rates between 2.5 and 20 ppm, is the most potent thinner registered in Canada. Because NAA is a more active compound than Sevin, it can cause over-thinning, especially when temperatures exceed 25°C after it is applied. Also, fruit size may be reduced or 'pygmy' formation may increase with late or high concentration applications. The absorption of NAA may be increased by the addition of a surfactant (ie, Agral 90 or Li-700), therefore rates should be adjusted for increased effectiveness under these circumstances.

NAD (Amid-Thin): Naphthalene acetamide (NAD) is less active (hence milder) than NAA. It frequently is used in situations where wilting/flagging of foliage is a problem when NAA is used. NAD is generally applied at 25 to 100 ppm. NAD should not be used on Delicious because there is a greater chance of small seedless fruit forming. It often is used at petal fall on cultivars that ripen before McIntosh and works best when applied under slow drying conditions.

6-BA (MaxCel or Cilis Plus): 6-BA (benzyladenine), a naturally occurring plant cytokinin, is an effective chemical thinner, particularly when applied at higher concentrations or tank-mixed with carbaryl. Generally, to effectively thin easy-to-thin cultivars, such as Empire, Idared, and McIntosh, 50 to 75 ppm is required. Harder-to-thin cultivars require between 75 and 150 ppm and often it is combined with carbaryl which is very effective. Combining 6-BA and NAA in the same year, may result in small ('pygmy') fruit especially on Delicious, Fuji and sometimes Gala. 6-BA thins best and increases size most when the application coincides with active cell division. Therefore, apply 6-BA when 3 days of warm temperatures (20-25°C) are forecast and fruit are between 5 and 12 mm in diameter.

When it is time to chemically thin:

  • Check sprayer calibration to ensure that there is adequate spray coverage
  • Spray the lower limbs less by shutting off one or more nozzles since fruit on lower limbs are easier to thin than those on upper ones
  • Do not to concentrate sprays above 2X tree row volume
  • Do not spray after frost or freezing temperatures.
  • If you must, reduce the concentration 25 to 30 percent.
  • Do not mix chemical thinners with other fungicides or insecticides
  • Recognize that thinning is greater for low vigor trees, light pruning, heavy bloom, poor pollination, high humidity before spraying, and cloudy, cool weather preceding or following the bloom period.
  • review the newly revised section on thinning in the Apple Chapter (Chapter 4) of the 2018-19 OMAFRA Publication 360, Fruit Crop Protection Guide, p. 86-88)
  • Also visit and click on the section "Thinning of Tree Fruit"

A summary of the key features of fruit trees that are either easy or difficult to thin (after Williams 1979; Williams and Edgerton 1981).

Trees are easy to thin when:

  1. Fruit spurs on the lower, shaded, inside branches are low in vigor.
  2. Moisture and nitrogen supply are inadequate.
  3. Root systems are weakened by disease or physical damage.
  4. Bloom is heavy, especially after previous heavy crops.
  5. Young trees have many vigorous upright branches.
  6. Thinners are applied to self-pollinated or poorly pollinated fruits.
  7. Fruit set is heavy on easily thinned cultivars such as 'Delicious'.
  8. Cultivars tend to have a naturally heavy June drop.
  9. Fruit sets in clusters rather than as singles.
  10. Bloom period is short, and blossom-thinning sprays are used.
  11. High temperature is accompanied by high humidity before or after spraying.
  12. Blossoms and young leaves are injured by frost before or soon after spray application.
  13. Foliage is conditioned for increased chemical absorption by prolonged cool periods.
  14. Rain occurs before or after spray application.
  15. Prolonged cloudy periods reduce photosynthesis before or after application of chemicals.

Trees are difficult to thin when:

  1. Fruit set on spurs in well-lit areas of tree (tops and outer periphery).
  2. Trees are in good vigor with no mineral deficiencies.
  3. Older trees in good vigor have a mature bearing habit.
  4. Light bloom or light fruit set occurs with the exception of young trees.
  5. Trees have horizontal fruiting branches.
  6. Insects are active on cross-pollinated cultivars.
  7. Limbs and spurs have been slightly girdled following moderate winter injury.
  8. Biennial bearing trees are in the off year.
  9. Fruit sets in singles rather than in clusters.
  10. Cultivars such as 'Golden Delicious' and heavy-setting spur types are to be thinned.
  11. When ideal fruit growth occurs before and after time of thinning.
  12. Low humidity causes rapid drying, and decreased absorption occurs before and after spraying.
  13. Cool periods follow bloom without any tree stress.
  14. Endogenous ethylene production is low.
  15. Bloom is light, and a high leaf-to-fruit ratio exists.

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