Good Fruit Grower / August 2010 / Insights into ­cherry thinning

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Insights into ­cherry thinning

British Columbia research project compares different treatments and their effect on fruit.

Cherry trees don’t seem to respond to fruit thinning in the same way that apple trees do, observes Dr. John Cline, tree fruit physiologist with the ­University of Guelph at Simcoe, Ontario, Canada.

“With apples, you remove the crop and the fruit get bigger. We do lose yield as well, so that’s a compromise,” he said. “With cherries, the increase in size seems to be less than you would achieve with apples. That’s why we’re experimenting to see what real benefit there is.”
This season, Cline began a cherry thinning experiment at the Pacific Agri-Food Research Centre in Summerland, British Columbia, where he was on sabbatical. He compared a number of different thinning treatments applied during bloom to sixth-leaf Skeena trees planted on Gisela 6 rootstocks. Overcropping and small fruit can be problems on self-fertile cherry varieties on dwarfing rootstocks. Preliminary results suggest that many treatments did not work, Cline reported to an international group of fruit growers who ­visited the plot this summer.
The treatments included:

—Ammonium thiosulfate (ATS) applied at 2 percent. There was some reduction in the crop load and very little phytotoxic effect. There seemed to be some mild benefit.

—Sylgard 309, an organosilicone surfactant at 0.25 percent. Cline said research in Israel suggested that a surfactant might have some effect, but so far it does not appear to have had an effect.

—Copper sulfate applied with 0.7 percent mineral oil to soften the phytotoxic effect. There was some damage to spur leaves and very little thinning.

—A highly refined, organic mineral oil at 2 percent. This treatment achieved very little thinning.

—Maleic hydrozide, an antisprouting compound used on potatoes and onions in storage. It is registered on ornamental crops, but not fruit. Cline said it shut down the terminal growth, discolored leaves, and thinned off all the fruit. However, it stimulated shoot growth in areas of blind wood, so there might be positive side effects. In the future, he will test lower rates.

—Mechanical thinning using an adapted weed trimmer to simulate the Darwin thinner. The head was composed of four eight-inch-long nylon strings spinning on a drill bit. The mechanical thinning, which took about three minutes per tree, caused damage to spur leaves and removed about 30 percent of the flowers, not the 50 percent he had been hoping for. He would do it more heavily in the future.

—Hand thinning of 50 to 60 percent of the flowers. The thinning was done at the popcorn stage, slightly before full bloom. By summer, the crop still looked fairly heavy, but the fruit were spaced out more nicely than on other treatments. Cline said it is not a practical treatment, as it took him and his colleagues 10 to 15 minutes to thin each tree. It was done more for comparison purposes as a way to assess the effect of thinning without causing any physical or chemical damage to the tree. However, hand thinning might offer some advantages if there is a ­significant fruit size response, he said.

This was the first year of the study. It will be repeated next year. Cline will also do the same experiment in Ontario next season.
The tour of Northwest cherry growing districts was organized by Susan Pheasant of Wenatchee, Washington, and Mauricio Frias of Curicó, Chile.