Publication 360, Fruit Production Recommendations: Timing of Application of Apple Thinners

1. Timing of Application
   • Table 4-20. Optimum Fruitlet Size for Chemical Thinning
2. Sampling method to determine timing
3. Other Information on Thinning and Growth Regulators

   • Chemical Thinning of Apples
   • Fruit Size Distribution in the Tree Canopy
   • Factors affecting Response to Thinning Agents of Apples
   • Chemicals Available for Apple Thinning
   • 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
4. 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
5. Chapter 4: Apple Calendar - PDF 357 kb
6. Related Links

Timing of Application

Application timings vary with the chemical used. See Chemicals available for thinning, and notes on timing for each product. Petal fall is an important stage for making thinning decisions. Petal fall is when 80 % of the petals have fallen but before calyx lobes are closed.

• NAD: Apply at early petal fall. See NAD (naphthaleneacetamide).
• NAA: Apply up to 12 days after petal fall. Avoid bloom to petal fall sprays on Empire due to excessive thinning of Empire.
• NAA, Sevin, Sevin + MaxCel or Cilis Plus, and NAA + Sevin: Apply 7-12 days after petal fall (7 days if warm, 12 days if cool). Early cultivars are an exception.
• Sevin: Apply up to 21 days after petal fall.

For some cultivars, apply chemical thinners based on the average fruitlet diameter. See Table 4-20. Optimum Fruitlet Size for Chemical Thinning, below. Apply thinners when the king bloom fruitlets reach the stated diameter to cause the smaller and weaker fruitlets to drop off. This relationship works best when the king bloom fruitlet is larger than the side bloom fruitlets and when the size and stage of development of all fruit to be thinned is fairly uniform.

Table 4-20. Optimum Fruitlet Size for Chemical Thinning

• Collect 50-60 fruitlets of each cultivar to provide a good estimate of fruit development in a reasonably uniform orchard block.
• Select the two largest developing fruits from each of 25-30 randomly selected clusters.
• Measure the greatest diameter of each using Vernier calipers or a thinning template and determine the average for the sample.