Penn State's Dr. Jim Schupp says that the inconsistent results with chemical thinners led to researcher interest in mechanical thinners.

Penn State’s Dr. Jim Schupp says that the inconsistent results with chemical thinners led to researcher interest in mechanical thinners.

After a decade of searching for effective chemical blossom thinners for Pacific Northwest stone fruit growers, researchers are still empty-handed.

The Washington Tree Fruit Research Commission has evaluated chemical thinners for stone fruit since 2000, testing a variety of products, including ATS (ammonium thiosulfate), Tergitol (a polyglycol surfactant), NC 99, fish oil and lime sulfur, Armothin (ethoxylated propoxylated tallow alkyl amines), and vegetable oil.

Nearly two dozen trials have been conducted in grower-cooperator plots to assess the effectiveness of chemical thinners.

Results have often showed a reduction in fruit set, but without improved size or yield, said Ines Hanrahan, Research Commission scientist. Hanrahan gave an update on the thinning research during Soft Fruit Day, held in Buena, Washington in mid-February.

“Through the years, we have had materials that do work some of the time,” she said, but added that overall results have been inconsistent. The Research Commission is seeking input from Washington stone fruit growers as the organization considers the future of its in-house chemical thinning research.

Researchers on the East Coast have also had inconsistent results in chemical thinning trials. Dr. Jim Schupp of Pennsylvania State University noted that eastern scientists have looked at a variety of chemistries, including fertilizer salts, oily surfactants, and late dormant applications of vegetable oil.

“It’s a fine line between something that’s effective and something that’s harmful to the tree,” Schupp said. “You’re on the knife’s edge. Some years you get a good thinning response, but you may put the tree in a funk, and then you don’t get increased fruit size.”

Some South Carolina growers have had favorable results from applying 8 to 10 percent concentration of vegetable oil in a late dormant spray about five to six weeks before bud break, he reported. The oil smothers the buds, and the weaker buds don’t survive.

“But most years in Pennsylvania, we have snow on the ground five to six weeks before bud break,” Schupp said. “And, 8 percent vegetable oil is not a cheap ingredient.”