Stone fruit growers and researchers are experimenting with a variety of chemical thinning and mechanical practices that could reduce the cost of hand thinning, a cost second only to harvest costs, which range from $500 to $1,500 per acre, depending on variety.

For the past 60 years, researchers have—with limited success—tried to find products that would reduce or eliminate the need for hand thinning. There has been little success with mechanical thinners. With chemical thinners, the general consensus, is that while some thinners do a good job in knocking off about half the blossoms on trees, the thinners don’t always work uniformly enough.

"The perfect spray would knock off every other flower and not hurt anything that remains," said Dr. Scott Johnson, a researcher at University of California’s Kearney Agricultural Center in Parlier. "But these materials that we’ve used burn flowers somewhat randomly. They’ll burn flowers off this part of the tree, but not that part of the tree. Or, they’ll burn a lot of flowers off of some trees but not any on other trees."

The lack of uniformity has made growers reluctant to try the chemical thinners on large commercial acreages. However, Harry Andris, University of California Cooperative Extension farm advisor for Fresno County, has come up with promising new research that shows that chemical thinning might be far more effective than growers realize.

Andris became inspired to work with chemical thinners a few years ago when he saw that a colleague of his in South Carolina was having success using soybean and cottonseed oil as thinners on stone fruit. Andris discovered, though, that when he did multiple applications of soybean and cottonseed oils on California stone fruit trees, the trees developed problems with phytotoxicity.

Andris conducted several experiments of his own from 2004 to 2006, comparing chemically thinned stone fruit to hand-thinned stone fruit. The three treatments which stood out as being the most promising were concentrations of 20 percent cottonseed oil plus 5 percent Activator 90 (an adjuvant); a mixture of 1.5 percent ammonium thiosulfate (ATS) plus 5 percent Activator 90; and 2 percent Tergitol.

"We found that these treatments did a considerable amount of thinning at different timings," Andris said.

The first two materials—the cottonseed oil and the ATS —are registered, nontoxic, and relatively inexpensive. Tergitol, which is used as a wetting agent in the film industry, is not registered for agricultural use but has been studied by tree fruit researchers in several states.

Andris tested the cottonseed oil combination in peaches and in nectarines. In nectarines, he applied the mixture when the buds were at 0.5 percent bloom and observed 48 percent reduction in flowers. When he applied the cottonseed oil mixture at 10 percent bloom in peaches, he saw a 32 percent reduction in flowers. The earlier you apply the cottonseed oil, the better a thinning effect it seems to have," Andris said.

Andris applied the ATS treatment on peaches and nectarines when the buds were at full bloom. In nectarines, flowers were reduced by nearly 35 percent; in peaches, flower reduction was 38 percent.

Tergitol also got good results—over 50 percent flower drop—in the peaches and nectarines at full bloom. Tergitol, which would cost $500 per acre, though, would not be economically feasible to use in agriculture. He estimated that cottonseed oil would cost about $350 an acre while the ATS treatment would be around $70 an acre.

In the past, some farmers used a fertilizer compound to chemically thin stone fruit trees. The problem was that the material had to be applied at the end of summer or early fall when the trees were going into dormancy. This meant that the fertilizer would affect the next year’s buds.

In addition to testing the efficacy of these different treatments on flower drop, Andris also compared the overall yield and fruit sizes of the chemically thinned trees to hand-thinned trees. The initial worry was that since none of the chemicals thinned fruit as evenly as hand thinning, there might be a reduction in overall fruit sizes or yield in the harvest.

"We found, though, that there were no statistical differences in fruit sizes between the hand-thinned and the chemically thinned trees," Andris said.

Some growers are so encouraged by the research that a few of them are going to try chemical thinning this year.

Bill Chandler, who grows about 250 acres of stone fruit in Selma, California, plans to try chemical thinning on 10 acres this year. Although the chemical thinning will cut back on hand labor, it won’t completely eliminate it, he said.

"We’ll still have to have workers go by and thin each tree, but instead of spending maybe an hour on a tree, we might be able to cut the time in half to 30 minutes a tree," Chandler said. "The bottom line is that we have to start trying some different things. Labor is getting harder to find, and the labor force is also getting older."