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:
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:
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:
Trees are difficult to thin when:
For more information:
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