Materials hold promise to reduce stone fruit crop loads economically

By Marni Katz

For decades, stone fruit growers have been searching for chemical fruit-thinning methods to replace< costly hand thinning.

Finding an affordable material that’s safe to the crop and works under West Coast conditions has been elusive until now.

But University of California researchers say they are on the cusp of a solution as three chemicals look promising enough to move from controlled plot tests into commercial field trials this season.

“Particularly with thinning costs being the second highest expense that fresh shipping fruit growers put out, anything that they can do to eliminate hand labor is helpful,” says Harry Andris, a UC Cooperative Extension farm adviser in Fresno County.

Mike Harvey, who growers peaches, plums, pluots and other stone fruit at Harvey Farms in Delano, Calif., estimates he spends $1,000 to $2,000 per acre thinning his fruit, depending on bloom, set and variety. He says the rising cost and shrinking supply of quality labor makes the current hand-thinning approach impractical.

“I think we are going to have to face the music and do something eventually,” Harvey says.

Borrowing from the South

California researchers began to look for an alternate chemical thinner in the 1970s, when a commonly used denitro herbicide was banned as a commercial fruit thinner.

Since then, researchers have looked at other herbicides, plant-growth regulators, carbamate insecticides, desiccating agents, fertilizers and a number of other compounds to find a safe, affordable replacement.

“We’ve been working on this for over 70 years in California, and it is still eluding us to have a good product,” Andris says.

For the past three years, he and other scientists at UC’s Kearney Agriculture Center near Parlier have based their research on trials by Clemson University in Clemson, S.C., that used soybean oil. Although the soybean oil doesn’t work under California climate conditions, Andris says experiments with cottonseed oil and wetting agents showed enough promise in controlled experiments to expand the project to commercial trials this year.

In 2006, seven treatments were applied to Rich Lady peaches and Summer Fire nectarines at full bloom using a 500-gallon air-blast sprayer. Of the treatments, Andris says combinations of three are worth evaluating in large-scale commercial trials this season.

One promising candidate is a 20 percent emulsified cottonseed oil, which works similarly to soybean oil in the South and suffocates developing fruit buds.

The second is 1.5 percent ammonium thiosulfate liquid nitrogen fertilizer. Both of those treatments performed well combined with 5 percent Activator 90, a spreader-sticker from Loveland Products Inc. of Greeley, Colo., that appears to give the best results of all wetting agents in the trials.

The third material is a high-end wetting agent called Tergitol TMN-6, a Union Carbide Chemicals and Plastics Co. product commonly used in the textile and film industries. Tergitol is federally registered and labeled for use in the East, but it does not have a California registration.

Both Activator 90 and ammonium thiosulfate are registered, affordable and readily available in California, Andris says.

Significant fruit-load reduction

Peach trees were sprayed at bloom with 10 percent open flower. An application of cottonseed oil and Activator 90 at that stage provided a 32 percent reduction in fruit set. Ammonium thiosulfate and Activator 90 provided a 38 percent reduction.

Tergitol performed the best with a 56 percent reduction, but cost and availability are likely to limit its potential, Andris says.

Applications to nectarines were made when the bloom was less advanced—buds were starting to swell, but no pink was showing. At that stage, a tankmix of cottonseed oil and Activator 90 reduced fruit set by 42 percent. Ammonium sulfate and Activator 90 provided 35 percent  reduction, and Tergitol showed a 39 percent reduction.

“It looks like it’s actually better to apply cottonseed oil a little earlier when we have buds starting to swell but no pink showing,” Andris says.

Of all the treatments, Andris says ammonium thiosulfate with Activator 90 is perhaps the most promising because it works consistently, does not damage the trees and is inexpensive.

“Ammonium thiosulfate sold by the ton last year for $185, so the material was $1.02 per gallon,” Andris says. “Activator 90 is about $14 a gallon, so when you add those costs of materials combined, last year it was $71.50 an acre for a single spray at full bloom.”

Cottonseed oil in a 20 percent solution with Activator 90 costs about $350 per acre.

The downside for all chemical thinning treatments, he says, is they do not provide a uniform thinning throughout each branch, and growers may need to come through later in the year and break up clusters to reduce the crop load to desired levels.

Start small

Andris recommends that growers try the treatments on a few rows of heavy-setting varieties the first year to see whether orchard location or varieties affect the results.

“We just want to do it and see what it does to the trees and what type of result we get, how it spaces the fruit set out and if we can live with the results,” Harvey says. “If it is something we can live with, then maybe we’ll do half a block next year. It’s tempting to go in big, especially if we can cut our labor down by 50 percent, but I think we need to find out how it works first.”