Mechanical tree fruit thinners show progress

Dan Wheat/Capital Press Brandon Mulvaney, research assistant at Washington Tree Fruit Research Commission, thins Bartlett pear blossoms with a Darwin thinner in an orchard on the Columbia River near Rock Island, Wash. Mechanical thinners show more promise in stone fruit and apples than pears, where susceptibility to disease is a concern.

Adjustments allow for better control of crop production


Capital Press

Mechanical blossom thinning is still experimental but holds promise as a labor-saving means of controlling the crop load of Pacific Northwest tree fruit.

Tractor-operated and hand-held thinners are effective. Growers are excited about them but likely will move to them cautiously, said Karen Lewis, Washington State University Extension tree fruit specialist in Ephrata, Wash.

In 2011, 20 to 30 percent of apple blossoms were removed in mechanical thinner trials of Jonagold, Gala, Fuji and Golden Delicious, Lewis said.

A 50 percent goal should be attainable by adjusting cord spacing, spindle and ground speed of thinners, she said. But other issues to resolve are effects on the following year's crop to even out the alternate bearing cycle, not removing too many leaves and avoiding a secondary bloom that increases susceptibility to disease, Lewis said.

"There's concern that open wood and torn leaves increases chances of fire blight," she said.

Just prior to full bloom is the best window for mechanical thinning, which breaks up clusters, leaving one or two flowers per cluster to become apples.

Mechanical thinners may be of most benefit in cherries and stone fruit -- apricots, peaches and nectarines -- where chemical thinning is not effective. Chemical thinning works well in apples. But apples, pears and stone fruit are also hand thinned at bloom or fruitlet stages. Most cherries are not thinned but newer varieties are so the need for cherry thinning is increasing.

Mechanical bloom thinning by tractor costs $55 to $60 an acre while hand bloom thinning of stone fruit runs $700 to $1,000 per acre, Lewis said.

A 30 to 70 percent bloom removal has been achieved in stone fruit but growers are nervous about reducing bloom and then chancing a frost that would reduce it more than they want, Lewis said. Stone fruit blooms earlier than apples and cherries making it more vulnerable to frost.

About 20 percent of cherry buds have been removed in dormant and bud swell stage by mechanical thinners, increasing fruit size. It appears to work better on the Lapin than Sweetheart variety, Lewis said.

Only the tractor-operated Darwin thinner is available commercially and it's probably not selective enough for most growers, which limits the acreage where it will work, she said.

The Darwin has a single, 10-foot spindle of cords that can be operated vertically along a fruiting wall or horizontally over tree tops. It works well in dormant cherry bud removal, Lewis said.

The tractor-operated Bonner has three, 3.5-foot spindles on separate arms that can reach inside the tree canopy. WSU and European-designed hand-held devices are slower than the Darwin and Bonner but are best in reaching all limbs. The hand-held thinners probably will be of most interest to Northwest growers, Lewis said.

Lewis and researchers at the Washington Tree Fruit Research Commission are in the final year of a four-year, four-state, USDA Specialty Crop Research Initiative project studying mechanical thinning for stone fruit and apples and cherries. The other states involved are California, South Carolina and Pennsylvania.

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