A discussion of thinning apples can be found on pages 61 through 63 of the “2005 Integrated Orchard Management Guide for Commercial Apples in the Southeast. Just thinking about chemical thinning of the apple crop is enough to keep you awake at night because thinning is an inexact science. Several factors can influence the performance of chemical thinners.Weather conditions before and during application of thinners as well as conditions following application can also impact their effectiveness. The potential to completely defruit the trees always looms in our minds. While this can happen, it is not very likely. The economic impact of not thinning the crop adequately exceeds the loss in income due to overthinning. Thinning not only impacts the current year’s crop, but the following year’s crop as well.
Recently, MaxCel from Valent BioSciences received a label for use as a chemical thinner for apples. Dr. Steve McArtney, North Carolina State University, gave some information regarding this material at a recent Georgia apple meeting. MaxCel can be used by itself or in combination with Sevin where more aggressive thinning is desired. It is with Sevin that Max- Cel seems to offer the most promise. MaxCel is a postbloom thinner. The optimum application time is when the average fruit size is from 5 to 15 mm. To determine the average fruit size, select five to 10 spurs and measure fruit size of all the fruit on the spurs and then calculate an average fruit size. The high temperature on the day of application should reach 65F or higher. With such a wide window in regard to fruit size, it should be possible to wait until temperatures get in the desired range to make the application.
The table below contains suggestions regarding the amount of MaxCel to use and whether Sevin should be added depending on the varietal characteristics and intensity of thinning desired.
The MaxCel plus 1 pound of Sevin 50W combination tends to thin fruit within a cluster, leaving fruit in singles and uniformly spaced within a tree. Until you have more experience with MaxCel, consider using it at 100 parts per million plus 1 pound of Sevin 50W for moderate to difficult to thin varieties. If thinning is not adequate this year, an increase in the Sevin rate might be warranted next year if factors for thinning are favorable.