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The Darwin string thinner is attached to the front of the tractor. The Bonner is in the background.

The Darwin string thinner is attached to the front of the tractor. The Bonner is in the background.

Mechanical thinning equipment has been tested for several years during bloom on apples and soft fruits, such as peaches and nectarines, but is now being tested prebloom for cherry thinning.

Two mechanical string thinners are being studied: the Darwin, which is commercially available in Washington State through Blueline Manufacturing of Moxee, and a prototype of the Bonner, which was developed at the University of Bonn in Germany. The Darwin, which has a single spinning spindle with cords, is most suited to a narrow canopy. The ­Bonner, which has three adjustable arms that can be placed into the canopy, can be used with any type of tree architecture including those that can’t be accessed with the Darwin.

Karen Lewis, Washington State University Extension educator for Grant and Adams counties, said that the Darwin thinner was tested in 2009 during bloom in the cherry orchard of Mark Hanrahan at Zillah, Washington, which is trained to the UFO (Upright Fruiting Offshoots) ­system.

“We did break up some of those big roped-on clusters last year, and we were pleased with the results,” Lewis reported during WSU’s annual cherry field day at Prosser in June. “It’s all about the cord hitting the target, so when we get a good placement into a block, like the UFO, we can do very well.”

This spring, she did further tests with the Darwin on a number of varieties in Hanrahan’s orchard at three timings: dormant (February), bud swell (March), and 50 to 60 percent bloom. Lewis said there were interesting results from the dormant treatment that will change how they approach field tests next year.

Rolf Lühs, an intern with the Washington Tree Fruit Research Commission who is assisting with the project, said the bud-swell timing—when the buds are 5 millimeter (about a quarter of an inch) in diameter—had the best effect. During the dormant season, it’s too difficult to hit the buds precisely, he said. When the thinning is done during bloom, it appears to knock off too many leaves, creating areas of blank wood, though more research needs to be done, he said.

Lewis said the dormant-season treatment removed about 5 to 10 percent of the full cluster buds and 20 to 35 percent of flower buds but did not damage the wood. “I was worried about blank wood then, but it turns out there was much less blank wood there than when we went in at bloom.

Lewis said results of chemical thinning on soft fruit do not translate directly into apples or cherries; apples have a lot of leaves at bloom and different flower ­morphology. “This is a key finding for us.”

The revolutions per minute and the ground speed can be adjusted on the mechanical thinners, depending on the tree arrangement and the desired effect. The rpm on the Darwin can be set between 200 and 400, and the ground speed is usually between 2.5 and 3.0 miles per hour. Changing the cord arrangement also has an impact, Lewis said.

More trials on cherries during the ­dormant and bud-swell stages will be ­conducted in the Southern Hemisphere this year with funding from a Specialty Crop Research Initiative grant and a grant from the Washington Tree Fruit Research ­Commission.

Kevin Wang and Karen Lewis discuss a new hand-held string thinner, which has several different heads. Hand-held thinner

Kevin Wang, a graduate student in WSU’s department of biological systems engineering, demonstrated a prototype hand-held thinner that was adapted from a commercial string trimmer. It has several different heads (known as end effectors) with a variety of types, lengths, and widths of cord. Lewis is working with WSU automation engineer Dr. Qin Zhang on the project, which is funded by the Washington Tree Fruit Research Commission and the Oregon Sweet Cherry Commission. WSU is applying for a patent for the concept, Lewis said.

Preliminary tests in UFO, steep leader, and Spanish bush systems indicate that the spindle speed can impact the amount of bloom removed. At a high speed, 55 to 70 percent of the flowers were removed; at medium speed 19 to 54 percent; and at low speed, 6 to 41 percent.

However, Lewis said that the adapted string trimmer, with its gas engine, is too heavy and noisy for a person to work with for long periods of time. The end effectors will be developed with universal mounts so that they can be used with other power sources. She envisions that a tool could be developed with rechargeable batteries or an electric version could be used on a platform where it could be plugged in and set on a mount.

Dr. Matt Whiting, research horticulturist at the Irrigated Agriculture Research and Extension Center, said the challenge with bloom thinning of cherries—whether mechanically or chemically—is the unpredictable fruit set. Graduate ­student Allison Stewart is testing benzyladenine (BA), gibberellic acid (GA), naphthaleneacetic acid (NAA), ethephon (Ethrel), and six aromatherapy oils as potential postbloom thinners for cherries.