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A German mechanical blossom thinner has potential to reduce hand thinning in tree fruit orchards, but it also can spread fireblight from infected blocks to clean ones, says a Pennsylvania State University horticulturist.

For the last two years, Penn State researchers have tested two thinning machines in peach, nectarine, and apple orchards to evaluate their usefulness in eliminating the need for hand thinning of blossoms or green fruit. Results from trials involving the Darwin thinning machine, designed by a German grower to thin organic apples, and a drum shaker machine designed by U.S. Department of Agriculture scientists, have been promising, reports Dr. Jim Schupp, Penn State horticulturist. The mechanical thinning has helped increase fruit size and reduce labor costs.

The Darwin thinning machine uses plastic cords to knock off blossoms or small fruit, while the drum shaker uses plastic rods to shake off blossoms or fruit.

After seeing damage to leaves and branch tips from the Darwin machine, Schupp wondered if fireblight would be an issue in apple blocks.

Though an organic apple trial in 2007 that compared three thinning treatments, including the Darwin machine, showed no fireblight incidence, he was still concerned. "The machine does shred both flowers and leaves…and we’re creating fresh wounds," Schupp said.

When he asked European growers who have used the machine for 15 years if fireblight was a problem, he said they responded, "No better, no worse."

Fireblight spread

Schupp initiated a trial in 2008 specifically to see if fireblight could be spread by use of the Darwin mechanical thinner. The study was conducted with York apple trees, which are highly susceptible to fireblight. The unthinned control treatment was compared to the Darwin machine treatment in both a clean section of the trial and an area that was inoculated with a virulent strain of fireblight. Additionally, the Darwin thinner was used in trees in close proximity to the inoculated ones. Rain was simulated during the trial to trigger a fireblight outbreak.

Schupp observed no significant difference in fireblight incidence in the "clean" portion of the block between the Darwin and unthinned control. Moreover, he was not surprised that the neighboring trees near the inoculated ones showed fireblight incidence even when no thinning took place.

"We know that just simply being close to inoculated trees is enough to spread the infection," he said. "But if you go in with the Darwin in the infected trees and then go to the clean, neighboring trees, yes, you can dramatically increase fireblight."

The same story was repeated in a study that involved potted Gala and Golden Delicious trees brought out to the infected site to assess the spread of shoot blight.

"Although they say in Europe ‘no better, no worse,’ with the Darwin, I believe that it applies only to clean apple orchards," Schupp said, advising growers to use the thinner only in clean blocks.

"I would want to have cool weather before using the thinning machine and use it only with dry foliage," he added.

"And, I would make sure that I have strep [streptomycin] in the shed."

Schupp, who spoke at the Great Lakes Expo in Grand Rapids, Michigan, concluded that while the apple industry has several good chemical thinning agents available, the Darwin machine has potential to be an effective thinning tool for organic apples.