Late Thinning

When fruit diameter reaches 18 mm, apples become difficult to thin with NAA and 6BA. Once the fruit reach about 22 mm, they become unresponsive to chemical thinners. Apple fruits grow about 1 mm per day in warm weather, so when fruit are 18 mm in diameter, you have only about four days to apply chemical thinners.

The two chemistries that still have thinning activity at this advanced stage of fruit growth are carbaryl and ethephon. Where mild thinning is all that is required, carbaryl at 0.5 to 1 pound per 100 gallons may suffice. To create a moderately strong late thinning spray, add 1 quart of spray oil per 100 gallons of finished spray mix to the carbaryl. Oil and captan cause phytotoxicity, so if you are using oil in this spray, keep captan out of the orchard for the next two cover sprays.

If a strong thinning combination is called for, then combine ethephon, at 1.5 pints per 100 gallons, with carbaryl and oil. Golden Delicious and Rome are very sensitive to ethephon. Reduce the ethephon rate to 12 fluid ounces per 100 gallons for Rome and to 1 pint per 100 gallons for Golden Delicious.