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Scientists are making headway in the search for effective chemical thinning agents for cherries, though specific recommendations aren’t available yet.

Chemical thinning research is complex and time consuming because trials are subject to the vagaries of weather, both before and after application, said Dr. Matthew Whiting, horticulturist at Washington State University’s research and extension center at Prosser.

“But after four years into it, we can look back and start to select winners and losers,” he said. “Losers are easier to identify.”

A fish oil-lime sulfur combination has been the most consistent material tested in the four years, Whiting noted. The second most consistent agent has been ATS (ammonium thiosulfate) at 2 percent concentration, followed by tergitol, a new chemical added to the trial in 2005.

Other researchers have found tergitol to be an effective thinner in apples, pears, and peaches. Whiting observed positive results in cherries and will study it further.

“The one material that has not been consistent in the trial is the vegetable oil emulsion,” he said. “We will be tossing that one out of the trial and focusing on the other three.”

In 2005, thinning materials were applied at 20 percent and 80 percent bloom in different cherry blocks at different locations—Rainier on Mahaleb rootstock, Rainier on Gisela 5, and Bing on G.5.

In addition to the “spray-and-pray” trials, where Whiting involves many blocks at different locations to gather data, researchers are also investigating the mode of action of the different thinners.

Understanding the mode of action should help scientists develop more predictable results when applying thinning agents.

Chemical thinners work by causing a physiological or physical response, he explained. An example of those that work by a physiological mode is one that is absorbed by the leaf tissue and interferes with photosynthesis. Burning the flowers and stigmas through a caustic is physical and interferes with pollination.

Whiting said that ATS and the fish oil-lime sulfur combination act predominantly on the leaves, while tergitol is effective on the leaves and flowers.

Missing piece

“But what we’re still missing for growers is an early pregnancy test—a rapid, portable, analytical tool to use in the field to assess how many flowers are going to set,” he said. “We know we can consistently make big mistakes in overthinning. Last year, we did improve fruit quality, but not without reducing yields.”

Growers have no way of knowing how many of the flowers will actually set fruit, he added. Is it 50 percent or 20 percent? Some years, when only a small percentage of flowers will turn into fruit, thinning agents are not needed.

While scientists may someday develop technology to assess fruit set, in the meantime, Whiting is working on a postbloom component to the thinning trials.

“You have a better idea of fruit set after bloom,” he explained, adding that they applied chemical thinners as a postbloom application last year, waiting 14 days after full bloom before making the application.

The postbloom application of fish oil-lime sulfur resulted in a physical response and dropped fruit.

“The good news is that we proved last year that you can wait two weeks for a better assessment of fruit set before applying a thinner—and get results,” he said.

Whiting’s cherry thinning research is supported by the Washington Tree Fruit Research Commission.