Table of Contents
Factoring in Frost
Cold temperatures and frost injury have complicated thinning decisions across certain regions of Ontario this season. As outlined by Dr. John Cline the biggest risk after frost is reacting too quickly or waiting too long and missing valuable thinning windows altogether.
Damaged fruitlets do not always drop immediately. Some continue growing for several days before aborting, making orchards appear heavier than they truly are. Before applying additional thinners, reassess viable crop load by cutting flowers and checking for dead floral tissue (more information found in the Evaluating Frost Damage on Buds in Tree Fruit article).
Check out the Thinning After the Cold for more details about adjusting thinning plans based on frost injury and damage.
Precision Over Panic
This year’s strategy should focus on conservative nibble thinning while still taking advantage of every reasonable thinning opportunity. Once fruit size advances beyond optimal windows, options become limited quickly.
In a variable frost year, flexibility and timing matter more than chasing a perfect one-pass program.
Warm, cloudy weather following frost can also increase thinner activity because lightly cropped trees often have greater carbohydrate availability. That means standard programs may behave more aggressively than expected in frost-affected blocks. Refer to Tables 1 & 2 regarding weather implications, along with promoters and suppressors of chemical thinners. For chemical thinning product information refer to Table 3.
Table 1. Weather Implications on Chemical Thinner Effectiveness
Weather Condition(s) | Prediction |
Warm Conditions (greater than 18°C) | All thinners perform more effectively |
High Night Temperatures (greater than 18°C) | Great stress High demand and use of energy for night respiration Greater drop |
Very High Day-Time Temperatures (greater than 29oC) | Great stress High energy demand Greater drop Risk of excessive thinning |
Very Cool Temperatures (less than 18oC) | Reduced stress Reduced energy demand Greater set |
Dark Cloudy Weather (at and/or after application ~3 days) | Greater stress Greater thinning response Greater drop |
High Relative Humidity (greater than 60%) | Slower dry time Greater thinning response |
High Light | Increased supply: harder to thin |
Low Light | Reduced supply: easier to thin |
Low Temperatures | Low demand: harder to thin |
High Temperatures | High demand: easy to thin |
Low Light + Warm Temperatures | Most efficacious thinning |
Table 2. Factors that Promote and Suppress the Response to Chemical Thinners
Factor | Cause | Effect on Chemical Thinner Response |
Lower Light Conditions | Reduced sunlight lowers photosynthesis and decreases carbohydrate supply. Fruitlets with lower carbohydrate status are more susceptible to chemical thinners. | Promotes |
Warm Temperatures | Warm temperatures increase tree metabolic activity and uptake, accelerating chemical thinner response. | Promotes |
Higher Night Temperatures | An increase in respiration causes a reduction in carbohydrate availability, enhancing susceptibility to chemical thinners. | Promotes |
Frost or Near-Freezing Temperatures | Cold conditions can damage flower parts and developing fruitlets, potentially affecting pollination and reducing seed development, increasing chemical thinner sensitivity. | Promotes |
Heavy Bloom | More competition between flowers divides carbohydrate availability, enhancing susceptibility to chemical thinners. | Promotes |
Light Bloom | Fewer fruitlets reduce competition for carbohydrates, strengthening fruit retention and reducing chemical thinner efficacy. | Suppresses |
Poor Pollination | Reduced seed number lowers auxin, weakening fruitlet dominance and increasing susceptibility to chemical thinners. | Promotes |
Fruitlets Set in Clusters (rather than singles) | Clusters compete for carbohydrates. Laterals become weaker compared to the kings and more sensitive to chemical thinners. | Promotes |
Biennial-bearing Trees in an “Off” Year | Lower crop load results in higher carbohydrate levels within fruitlets, increasing retention and reducing chemical thinner efficacy. | Suppresses |
Young Trees with Vigorous Upright Growth | Strong vegetative growth diverts carbohydrates away from fruitlets, increasing susceptibility to chemical thinners. | Promotes |
Leaf Cuticles Formed Under Cooler Periods | Soft cuticles increase chemical absorption, enhancing chemical thinner efficacy. | Promotes |
Comparable Lateral and King Fruitlet Size | Similar strength in fruitlets reduces dominance, decreasing selective thinning of laterals, reducing chemical thinner effectiveness. | Suppresses |
Low Moisture | Water stress reduces photosynthesis and carbohydrate availability, increasing fruitlet susceptibility to chemical thinners. | Promotes |
Weak Root Systems | Water and nutrients have limited uptake, reducing canopy growth and carbohydrate supply, increasing susceptibility to chemical thinners. | Promotes |
Table 3. Chemical Thinner Overview Table
Tradename | Active Ingredient(s) | Rate | Application Timing |
Accede SG | 1-ACC (1-aminocyclopropanecarboxylic acid) | 500–1000 g/ha | Bloom – 25mm |
ATS | Ammonium thiosulphate | Bloom | |
Brevis 150 SC | Metamitron | 1.12–2.24 L/ha | Late petal fall – fruitlet sizing, up to 20mm |
Cilis Plus | 6-benzylaminopurine | 50–200 ppm (2.4–9.5 L/ 1,000 L water) | Petal fall – fruitlet sizing, up to 20mm |
Ethrel1 | Ethephon | Bloom – only non-fruiting trees | |
Fruitone-L | 1-naphthaleneacetic acid | 1.2–9.7 ppm (39–312 mL/1,000 L water) | Bloom – fruitlet sizing, bloom to 30 days after bloom |
Lime Sulphur2 | Lime Sulphur | Bloom | |
Maintain 3.5L | 1-naphthaleneacetic acid | 1.2–10.0 ppm (37–313 mL/1,000 L water) | Bloom – fruitlet sizing, bloom to 30 days after bloom |
Maxcel | 6-benzyladenine | 75–200 ppm (3.8–10.0 L/1,000 L water) | Petal fall – fruitlet sizing, up to 20mm |
Sevin XLR | carbaryl | 0.5–2 L/ 1,000 L water | Petal fall to 25 days after bloom |
Sevin XLR + Cilis Plus | carbaryl + 6-benzylaminopurine | 1–2 L/1,000 L water + 2.5–6.3 L/1,000 L water | Petal fall to 25 days after bloom |
Sevin XLR + Maxcel | carbaryl + 6-benzyladenine | 1–2 L/1,000 L water + 2.65–6.60 L/1,000 L water | Petal fall to 25 days after bloom |
Sevin XLR + Fruitone-L | carbaryl + 1-naphthaleneacetic acid | 1 L/1,000 L water + 5–10 ppm (156–312 mL/1000 L water) | Petal fall to 25 days after bloom |
2. ATS and Lime Sulphur are fertilizers and therefore will not be found in the pesticide label search
For more detailed information about volumes and rates, please refer to the following links:
- Product specific information can be found on the Ontario Crop Protection Hub
- The Thinning & Plant Growth Regulator Section found in the Apple Landing Page on the Crop Protection Hub
- The Apple Section of the Thinning of Tree Fruit webpage at Ontario.ca
- The Pesticide Label Search for specific product searches
I believe in you and good luck thinning this season!
Erika DeBrouwer
Tree Fruit Specialist, OMAFA
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