Machines could dramatically reduce expense of thinning
By DAN WHEAT
ROCK ISLAND, Wash. -- Brandon Mulvaney drives a tractor with a 10-foot-tall spindle of plastic strings mounted on its front. He steers it along a row of Bartlett pear trees, powers up the spindle and sends a flurry of white blossoms flying from the trees like an isolated spring snowstorm.
By destroying thousands of the beautiful blossoms, Mulvaney is thinning the crop. He's doing in a few minutes what a few weeks later would take hours of hand labor, picking tiny green pears to leave one about every six inches for maximum growth and quality.
Mulvaney, 24, is a research assistant for the Washington Tree Fruit Research Commission in Wenatchee. The commission and Washington State University Extension is involved in a four-year, four-state, USDA Specialty Crop Research Initiative project, funded through the National Institute of Food in Agriculture.
The project is testing commercial thinning machines mainly in soft fruit -- apricots, nectarines and peaches.
"We've had great success in apricots and nectarines and are very excited about what we've been able to do in removal of bloom and reduction in cost of green fruit thinning," said Karen Lewis, WSU Extension, Ephrata, and Washington project leader.
Mechanical thinners have removed 30 to 75 percent of blossoms in nectarines, she said, reducing hand thinning that can cost up to $900 an acre in organic blocks.
An April 13 test in Josh Koempel's Rock Island pear orchard, 11 miles southeast of Wenatchee, resulted in good blossom removal on the outer edges of Bartletts, but the 24-inch-long plastic strings didn't reach blossoms on short limbs closer to tree trunks.
For better results, Koempel, 34, said he would have to remove larger limbs that stick out and prevent the machine from getting closer. He has to weigh loss of yield from such cutting against the gain of reduced hand thinning and gain in fruit size and quality, he said.
Lewis said pears are not in the project. There's risk in pears, she said, of mechanical thinning spreading disease.
Koempel sprayed a protectorate on the row right after it was thinned.
The Darwin machine used in his orchard is a non-selective, two-dimensional thinner that knocks off blossoms where the strings strike. It can remove from 20 to 80 percent of blossoms, depending on the speed of the tractor, speed of the spindle, number of strings and their arrangement.
Other machines being tested have arms to reach inside the canopy of limbs, and some have sensors to selectively thin blossoms.
"In cherries, we like fruit on the sides and bottom, not the top of limbs," Lewis said. "On any species, whether nectarine, cherries, apple or pears, there is a specific bloom you want to keep more than others."
The Washington Tree Fruit Research Commission and the Oregon Sweet Cherry Commission are helping fund testing of a hand-held string thinner, which Lewis said could be very useful in the hands of experienced thinners.
Traditionally, cherries have not been thinned, but larger crops have stoked interest. "Most say an engineering solution is the most promising," Lewis said.
Timing is another variable. Thinning too early in apple bloom knocks out whole clusters, and thinning too late damages fruit, she said.
"We are trying to develop a matrix of when to use it, just like in chemical thinning," she said.
A drum shaker, which originated in citrus, will be tested in stone fruit for green fruit thinning in Washington next year, she said.
Other states involved in the project are California, South Carolina and Pennsylvania.
"The California Canning Peach Association is a big player in the project," Lewis said. "They want the hand-held device."