The Biology of Letting Go: How Chemical Thinners Work

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Plant growth regulators are used for many purposes within the apple orchard, but its most pronounced use is for chemical thinning. The nuances of chemical thinning are involved and vary substantially based on environmental factors, but why? In this article I will go over how each current product works to take off fruitlets, giving you a better understanding and hopefully more confidence in utilizing various products to reach your goal.

Competition in the Canopy

Before delving into the nitty gritty of plant growth regulators (PGRs) there are a few physiological aspects that should be understood. Apple trees have a dynamic that is essential for growth of vegetative and reproductive tissue, termed sources and sinks.

  • Source Organs: produce and export carbohydrates, Source organs convert light, water and CO2 into sugars (photosynthesis).  
    • Example: Fully expanded, mature, sunlit leaves are a primary example.
    • Strength: depends on how much sugar is produced.
  • Sink Organs: consume and store carbohydrates. Sink organs unload sugars from the phloem to drive cell division and growth.
    • Examples: Developing fruitlets, roots, buds and young expanding leaves are examples.
    • Strength: depends on how strongly an organ demands sugar.

The relationship between sources and sinks is a driver in chemical thinner efficacy for many PGRs. These localized sink ratios aid chemical thinners as the ratios generate differences in growth that chemical thinners can exploit.

These include differences in:

Seed Development

Seeds produce auxin, where the more viable the seed is, the stronger auxin signal it sends. This means that fruitlets with higher seed viability will be more difficult to thin. If seeds are damaged (less viable) auxin production is reduced, and these fruitlets would be easier to thin.

Fruit Position

King blooms flower first, being the first to access carbohydrates and become the dominant sink in comparison to laterals. Chemical thinners take advantage of this difference by decreasing resources to weaker fruitlets.

Laterals always have a disadvantage because they are typically behind in staging compared to the king, but if the king has been damaged – perhaps due to frost – then one (or more) of the laterals would become the dominant sink(s). This is why waiting until size differences are obvious between laterals after a significant frost event is suggested before applying chemical thinners.

Leaf-to-Fruit Ratio

The number of functional leaves supplying a single fruitlet. This ratio focuses on the spur leaves, as they typically provide almost all the energy for fruitlet growth and development.

Generally, 20 to 40 leaves per fruit allow most cultivars to reach premium size targets. This number changes as cultivars have variable leaf sizes, thickness, and photosynthetic capacity.

Spur leaves are small and not fully photosynthetically active early in the season. During this time the tree will rely on stored carbohydrates to sustain fruitlet needs until the leaves mature. This is the period in which chemical thinners are typically applied and why a carbohydrate deficit is needed to increase chemical thinner efficacy.  

Remember that: Not all fruitlets are treated equally on the tree, which is why chemical thinners work.

Label is law. Be sure to follow the label.

Products & Their Performance

AccedeTM (40%), 1-aminocyclopropane-1-carboxylic acid (ACC)

What

ACC is a new plant growth regulator for apples in Ontario. It has been researched by Dr. John Cline at the University of Guelph and continues to be studied.

How

ACC works by enhancing ethylene production, as ACC is a direct precursor to ethylene. ACC drives the fruit drop process by initiating the ethylene pathway. It is suggested that ACC has a more localized effect compared to NAA and 6-BA.

When

The product can be applied anytime between full bloom to 25mm, although it is intended to be used as a ‘rescue’ thinner and is more efficacious between 15 – 20 mm.

Mixing

Research is ongoing about tank mixing and use is season with other products. Initial studies in Ontario have shown that carbaryl use with ACC has been successful.

For more information on ACC, please refer to the following articles:

Brevis 150 SC (150 g/L), metamitron

What

Another recent addition to the Ontario apple thinning toolbox and originally registered as a herbicide in sugar beets, metamitron shows promise as a chemical thinner. Dr. John Cline continues to study this product.

How

Generates carbon stress, as it inhibits photosynthesis. It directly reduces carbohydrate supply to fruitlets due to the inhibition of photosystem II, this stress causes fruitlets to abort.  

When

Product can be applied from petal fall to 20 mm fruitlet size. This product comes with a model to aid in application timing of the product.

Mixing

It is recommended to apply alone. Research is ongoing into tank mixing and use in season with other products. Initial trials by Dr. John Cline show that efficacy mixing with carbaryl or 6-BA increases efficacy, while multiple applications caused over-thinning.

For more information on metamitron, please refer to the following articles:

Fruitone® L (3.1%) or MaintainTM 3.5 L (3.1%), naphthalene acetic acid (NAA)

What

NAA is an auxin-type thinner that interferes with auxin signaling from the seeds. NAA has also shown to affect the ethylene pathway, and research has also shown that NAA impairs photosynthesis.

How

A reduction in supply and auxin flow results in the loss of fruit, while impairing photosynthesis contributes to carbon stress as it is limiting source carbohydrate generation and mobility. NAA has multiple pathways of affecting fruitlet abscission, which is why can be variable in its response.

When

NAA products can be applied from full bloom to 30 days after full bloom. Application between 5 to 10 mm fruitlet size has shown to be more effective.

Mixing

Efficacy has been improved with use of a non-ionic surfactant (NIS), rates should be adjusted when using NISs.

Cilis Plus® (2.0%) or Maxcel (1.9%), 6-benzyladenine (6-BA)

What

6-BA contains a cytokinin that promotes cell division. This alters carbohydrate distribution, meaning the sink strength of the larger fruitlets increases.

How

The change in sink competition drives the large fruitlet to gain more carbohydrates while reducing the auxin flow in weaker fruitlets. The reduction in auxin causes fruit to abscise.

When

6-BA products are most effective between 5 to 15 mm diameter and can be applied from 7 to 21 days after full bloom.

Mixing

Research has shown that mixing with carbaryl surfactant improves thinning efficacy. Do not apply NAA product in same tank or separate sprays in same season, as it can result in pygmy fruit (Delicious and Fuji).

Sevin® XLR (99.5%), carbaryl

What

A previously registered insecticide in apples that also causes fruitlet thinning is absorbed through the fruit, not the leaves.

How

Fruitlet biochemical processes are interfered when using carbaryl, as the product ceases growth, causing fruitlets to drop.

When

Carbaryl can be applied between late petal fall (bees need to be removed or are not active in orchard) to 25 days after full bloom.

Mixing

Carbaryl has been studied with success for use with NAA and 6-BA products.

PGRs influence hormones locally, meaning spray coverage plays a significant role in efficacy.

For more information on apple thinning PGRs, check out the links below:

Erika DeBrouwer

OMAFA Tree Fruit Specialist

erika.debrouwer@ontario.ca