If research currently being led by UC Cooperative Extension Specialist Matthew Fidelibus pays off — and right now that’s a big if — raisin growers may be able to replace the labor-intensive practice of severing canes to loosen fruit in advance of mechanical harvesting with a spray-on hormone to induce abscission.

Biologically, the idea works, he says, but much more work remains to be done to make it a practical harvest aid for growers.

Fidelibus is based at the Kearney Agricultural Center, Parlier, where he has conducted most of the trials. He also has some plots in commercial orchards near Caruthers and Biola.

He is focusing on methyl jasmonate, a hormone extracted from jasmine flowers, that is also used in making food flavoring and perfumes. He selected the hormone the first year he began screening candidates in 2006. When applied to some fruits, including grapes, it causes them to abscise and fall from the stem, like leaves dropping from trees in autumn.

The idea is to cause the grapes to become loose enough that they will fall off easily when the vines are shaken by the harvester. They would then be dried conventionally on continuous trays.

“Originally we thought finding such a hormone would be the hard part,” says Fidelibus. “But, it was just the beginning of a lot of hard work trying to manage this hormone in a way that is useful to growers.”

He’s still testing to see where to target the spray on the vine or clusters for best results. “As long as the clusters are well-sprayed, the treatment works. It doesn’t matter what other part of the vine is sprayed.”

One challenge is developing a formulation of the hormone that is both easy to apply and stays in solution.

Another is determining the best time to spray. For example, when the researchers sprayed some grapes the same day that canes on other vines were severed, the hormone-treated grapes tended to loosen much faster, or even fall off before the harvester came through.

Determining the correct dosage of hormone is another challenge. Because a vineyard doesn’t flower all at once, treating the resulting grapes in such a way that they all loosen at about the same time is proving difficult.

“We’d like to end up with a three- or four-day window when the treated grapes would be loose enough to harvest mechanically, but before they fall off the vine,” Fidelibus says. “We’re still trying to come up with an economical and consistently effective way of using this hormone. It’s a work in progress.”

However, if it does eventually pan out, reduced labor costs might not be the only benefit. When grapes are treated with the hormone, the abscission layer forms right where the berry joins the pedestal or stem. That, says Fidelibus, could eliminate the need to de-stem raisins during processing, which abrades the surface of the raisin, causing sugaring and a gritty mouth feel when eating them.

The hormone might also have an application in the table grape industry, he says. For example, efforts to extend the storage life of grapes by marketing them as individual berries, much like blueberries or strawberries, have been stymied because tearing a grape from a cluster leaves an opening for yeast and bacteria to invade and spoil the berry. The hormone treatment could eliminate this problem, by causing the berry to fall off the cluster without tearing the skin.