Mechanical harvested dried-on-the-vine raisin production is helping growers reduce their reliance on hand crews in the traditionally labor-intensive crop.

However, there remains an intensive hand labor job — cutting canes prior to harvest — that a University of California viticulture specialist hopes to eliminate using abscission agents.

Without cutting canes, individual grapes hold tightly to the cluster and rupture during machine harvest, leaking juice that can create a sticky, dirty mess on drying raisins. These broken berries also attract insects and missed canes can reduce the overall quality of the raisin crop.

UC Cooperative Extension Viticulture Specialist Matthew Fidelibus from the Parlier, Calif., UC Kearney Ag Center is exploring the use of abscission agents in raisin grapes to get individual berries to fall easily from clusters with their cap stems intact. He told growers at the recent San Joaquin Valley Grape Symposium in Easton, Calif., that spraying abscission agents on raisin grapes could eliminate the laborious chore of cane cutting mechanically harvested raisins.

“In continuous tray raisins, we see great potential implications here. Right now we are severing canes on continuous trays to facilitate shatter and minimize juicing, but while severing canes will facilitate shatter, it's not creating a biological abscission process,” Fidelibus said. “Basically it just makes the cluster framework more fragile and it tears apart more easily.”

Abscission agents, by comparison, trigger a physiological process that encourages berries to drop off on their own, with little damage to the individual grapes. While grapes do very little abscission naturally, two years of study by Fidelibus at Kearney show that the fruit is susceptible to applications of abscission agents to trigger the process.

Abscission agents, typically plant hormones, are commonly used in other fruit crops such as oranges to get individual fruit to drop cleanly from the stem. They work by triggering tissue to weaken at the point where fruit connects to the stem allowing it to detach cleanly with the cap stem intact. Abscission agents have not been researched in grapes.

“There is really hardly anything known about the biology of abscission in grapes so we are starting from ground zero,” Fidelibus said.

The most promising, a natural plant hormone called methyl jasmonate (MeJA), has been evaluated by Fidelibus for two seasons with promising results. A biological mimic of MeJA called coronate also appears promising although Fidelibus said quantities of the biological were in short supply and he was not able to replicate his 2006 trials in 2007.

Fidelibus compared four different rates of MeJA and found that abscission agents at higher rates performed as well as cane cutting to increase fruit detachment.

Grapes harvested with MeJA and coronate fell easily from the grape cluster at rates above 10 millimeters and had a smooth protective scar at the stem end revealing no mechanical damage to the berry. Lower rates provided no real abscission effect, but at higher rates of 10 millimeters, Fidelibus said about 25 percent of the berries fell from clusters on their own. Rates at 20 millimeters actually caused in excess of 50 percent natural fruit drop, a rate Fidelibus said appears too high. Fidelibus said there was no canopy damage with MeJA applications.

“In 2007, we verified that 10 to 20 mm rates do cause a lot of abscission on their own and it does happen very quickly,”

But he noted that commercial air blast applications in 2007 performed differently than hand-held sprayer applications in 2006. He suspects sprays must thoroughly coat the clusters, which could be difficult with the protective canopy in conventional raisin vineyards.

So while treatments appear promising, Fidelibus said research challenges remain before this alternative method for loosening raisin grapes can be commercialized.

“After two years we know we have two effective compounds in MeJA and coronate, we just don't know how to manage it yet,” he said.

Still, Fidelibus said there is strong potential for this new labor saving technique for raisin growers and he plans to continue researching abscission agents to identify spray technologies, rates and other specifics for their future use in mechanically harvested raisin vineyards.