Publication 360, Fruit Production Recommendations: Factors affecting Response to Thinning Agents of Apples

1. Factors Influencing Thinning Response
   • Length of wetting period and temperature for material absorbtion
   • Cultivar sensitivity
   • Weather conditions
   • Pollination and bee activity
   • Tree age and vigour
   • Density of foliage
   • Spray coverage
2. Other Information on Thinning and Growth Regulators

   • Chemical Thinning of Apples
   • Fruit Size Distribution in the Tree Canopy
   • Chemicals Available for Apple Thinning
   • Timing of Application of Apple Thinners
   • Steps in Thinning
   • Thinning Under Special Conditions
   • Suggested Rates for Chemical Thinning of Apples
   • Key Concepts and Benefits of Hand Thinning
   • Promalin to improve fruit shape
   • Ethephon to promote fruit colouring
   • Chemical control of preharvest drop
   • Recommendations for the use of ReTain
   • Preventing heat stress and fruit sunburn with Surround
   • Apogee for vegetative growth control
3. Other Information on Apples
   • Apple Growth Stages
   • Apple Nutrition
   • Apple Calendar
   • Guidelines for Protecting Non-Bearing Apple Plantings
   • Notes on Apple Diseases
   • Table 4-7. Products Used on Apples
4. Chapter 4: Apple Calendar - PDF 357 kb
5. Related Links

Cultivar Sensitivity (Mature Trees)

• Easy-to-thin or easy-to-over-thin cultivars include non-spur Red Delicious, Idared, Crispin (Mutsu), Jonagold strains, Jerseymac, Honeycrisp, Gingergold, Silken, Creston, Cameo and Golden Supreme.
• Moderately difficult cultivars include Empire, McIntosh, Northern Spy, Cortland and Ambrosia.
• Harder-to-thin cultivars include Golden Delicious, Paulared, Gala strains, Fuji, Goldrush, Spartan, Wealthy, spur-type McIntosh and spur-type Red Delicious.

Weather Conditions

These weather conditions encourage absorption of chemical thinners which may increase the thinning response:

• cool, wet weather and low light conditions before or after application
• prolonged period of high humidity without rain either before or after application
• high temperatures after cool periods
• frost or near-freezing temperatures before or shortly after application
• prolonged period of dry weather before or after application
• excessively warm conditions following fruit set

A decrease in thinning response can result from sunny and warm daytime temperatures accompanied by relatively cool nighttime temperatures before and after thinning treatments have been applied. This results in a high net accumulation of carbohydrate reserves in the tree.

Pollination and Bee Activity

Pollination and bee activities are closely tied with weather conditions. Pollination conditions and bee populations can make fruit either more or less difficult to thin. Fruit set under good pollination conditions, which results in increased seed count, is harder to thin. Fruit set on king blossoms is more difficult to thin than fruit set on side blossoms. Single fruit are more difficult to thin than clustered fruit. Even a light or spotty set with clustered fruit should be thinned, in order to break up the clusters.

Tree Age and Vigour

• Young trees are more easily thinned than mature trees that have cropped well for 2-3 years, so hand thinning young trees of high-value cultivars is recommended. Even low rates of thinners that work on mature trees can easily over-thin young trees.
• Mature trees with winter-injured buds, or trees that are low in vigour, are more easily thinned.
• Trees bearing a heavy crop one year with good bloom the next year may be more easily thinned.
• The heavier the bloom, the more stress on the tree and the easier to thin. Producing flowers and pollen requires nitrogen and adds stress to the tree.
• Stress from the previous season makes a tree easier to thin (e.g. excessively dry, excessively wet, nutritional deficiencies, insect or disease injuries).

• Poorly pruned dense trees are thinned more easily than well-pruned open trees with stronger fruit buds.
• Partially shaded spurs on low branches of well-pruned trees thin more easily than upper spurs..
• Late thinning sprays require increased water volumes for adequate coverage of the rapidly developing tree canopy.

Spray Coverage

As with all plant growth regulators, complete coverage is essential for good results. Chemical thinners will act only where deposited.