If you’ve thinned and thinned and there are still too many apples on the trees, there is one more growth regulator treatment you can use before resorting to hand thinning.

That is a rescue thinning treatment using Ethrel (ethephon)—a treatment that is considered risky but has three benefits: It reduces the cost of hand thinning; it can be done earlier than hand thinning, in time to enhance return bloom the following year; and it increases fruit size.

The treatment was discussed during an International Tree Fruit Association workshop in Boston in February. Thinning was a special topic this year because of the short crop across much of the eastern United States last year and the expectation of snowball return bloom. The penalty for failure to thin could be small apples this year and fewer apples next year.

The rescue thinning advice was developed by Dr. Wes Autio at the University of Massachusetts and Dr. Win Cowgill at Rutgers University in New Jersey.

“It is important to understand that ethephon can be tricky,” they reported. “If conditions or concentration are wrong, complete crop removal can occur, so be careful!”

Autio and Cowgill did five years of research to define appropriate rates and conditions, which vary by variety. Their recommendations are:

•     Treat when temperatures are in the 70s the day of treatment and for two days after. Do not treat when temperatures are below 70 or above 80.
•     Treat when fruit are 15 to 25 millimeters in diameter.
•     Use the following concentrations, in 100 gallons of dilute spray, by variety:
—McIntosh and Macoun, 200 to 300 parts per ­million (0.7 to 1 pint)
—Spur-type Delicious and Fuji, 300 to 375 ppm (1.0 to 1.5 pint)
—Golden Delicious, Rome Beauty, and August ­varieties, 120 ppm (0.4 pint)
—Gala, Cameo, and GoldRush, 225 ppm (0.75 pint)
—Enterprise, 150 ppm (0.5 pint)
—Jonagold, 150 to 225 ppm (0.5 to 0.75 pint)
•     Use the right concentration along with a half-pound of Sevin (carbaryl) and a surfactant.

“Hand thinning is usually performed in early July,” they said. “Because of this timing (late), it has very little effect on return bloom the following year, since most flower-bud formation occurs in June. So, it may be possible to gain some fruit size with hand thinning, but if the set is heavy before hand thinning, bloom may be light the next year and trees may even become biennial.”

The benefit of the rescue treatment is that it would be done by June.