MSU Extension Apples

Thinning is the most difficult, most important, yet necessary practice a grower must perform each year. Making a mistake will compromise both this year’s crop and next year’s crop. Over-cropping and undercropping will reduce income for a block for multiple years. Today, with a more scientific approach to thinning we can achieve successful consistent annual croploads.

A firm understanding of the thinning materials, thinning stages, the natural background sensitivity to thinning sets the foundation for successful thinning. Thinning begins with pruning in the dormant season to reduce budload. Next, a precise, data-driven approach, including Nibble Thinning and Precision Thinning, can be used to guide thinning decisions. Models including the Fruitset Model and the Carbohydrate Model provide guidance based on block data and environmental conditions to achieve better thinning results. We review each of these concepts, approaches, and tools in this guide.

Table of Contents

Introduction------1

Pruning to Budload: Early Cropload Management------2

Thinning Materials------2

Natural Apple Background Sensitivity to Thinning------2

Nibble Thinning------4

Precision Thinning------5

Theory of Fruitset------5

Carbohydrate Model and MaluSim App------5

Updates to the Carbohydrate Model for 2019------7

Updates to NEWA for 2021------7

Fruitset Model------8

Environmental Factors------9

Effect of Environmental Conditions on Thinners------9

Increased supply: harder to thin.------9

Reduced supply: easier to thin.------9

Low demand: harder to thin.------9

High demand: easy to thin.------9

Low light and warm temps.------9

Thinning Timing------9

Season-specific Recommendations------10

2021 Review------10

2022 Outlook------10

Reference Tables------ 16

Table 1.  Chemical Apple Thinning Materials and Comments.------16

Table 2.  Apple Thinning Windows Considerations.------17

Table 3.  Precision Thinning, Timing, Materials and Predicted Percent Thinning Most Years.------18

References------18