Dr. Duane Greene at the University of Massachusetts is exploring new thinning strategies that can be used with existing thinning products.

NAA (naphthalene acetic acid), one of the oldest thinning materials around, is usually used postbloom, when fruitlets are about ten millimeters in diameter, but Greene said it can be used at petal fall at higher rates—perhaps twice the dose. Greene defines petal fall as immediately after the bees are out of the orchard and you can get a sprayer in.

Amid-Thin, or NAD (naphthaleneacetamide), is also an old compound. It is widely used in the West but not the eastern United States. This also can be used at petal fall, though it is more often used when the fruitlets are ten millimeters in diameter. The product can cause pygmy fruit, but this is less of a problem at the earlier timing.


Greene said combinations of thinners allow growers to use lower rates and yet get better thinning responses. He has been testing MaxCel (benzyladenine) with NAA. The combination can cause small fruit, but the size effect seems to vary by cultivar, and is more likely with Fuji and Red Delicious, he said.

He has also been testing a product called Sysstem-CAL, a calcium and copper foliar fertilizer, with NAA. Sysstem-CAL, a product sold around the world, is a phosphite-based product that aids penetration of some compounds. Greene had heard anecdotal information that it enhanced the activity of thinners so he did experiments with McIntosh last year at both petal fall and ten-millimeter timings. It was not effective at petal fall, but the later timing did provide some thinning.

A combination of MaxCel (100 parts per million) and Sysstem-CAL (two quarts per acre) reduced fruit set to 30 percent of the control. A combination of Sysstem and NAA (7.5 ppm) was less effective, reducing fruit set to 70 percent of control.

“This compound appears to work primarily with MaxCel but not NAA to enhance thinning,” Greene concluded. “Any time we had Sysstem-CAL and MaxCel together, we got rather good thinning.”

The MaxCel and Sysstem-CAL combination also resulted in larger fruit (235 grams in weight versus 156 grams for the control), which Greene said was probably due to enhanced penetration of the MaxCel because the gain was beyond what would be expected just from thinning the crop. Fruit in the NAA and Sysstem-CAL treatment was the same size as the control treatment fruit. The MaxCel and Sysstem-CAL treatment had no other extraordinary effects on the fruit.