Study Investigates Savings From Naturally Drying Raisin Grapes

Harvesting raisin grapes can be a labor-intensive process. With many grape varieties, about two weeks before the harvest, crews must cut the canes, which are the long branches that actually produce the grapes. Once the canes are cut, the grapes begin to wilt on the vines so that they can later be shaken onto trays.

That may change soon. A new raisin grape developed by USDA/Agricultural Research Service (ARS) scientists in California dries naturally on the vine, eliminating the costly step of cane cutting.

‘Sunpreme’ is a green raisin grape similar in size and quality to ‘Thomson Seedless,’ an industry standard. It was bred and patented by retired ARS horticulturalist David Ramming, and its release is being directed by ARS plant geneticist Craig Ledbetter and his colleagues at the San Joaquin Valley Agricultural Sciences Center. So far, five nurseries have a license for ‘Sunpreme,’ and some are already taking orders from grape producers. “We expect these grapes to become extremely popular,” Ledbetter says.

His optimism is based on economics. Cane cutting costs about $130 per acre, which is roughly 36% of total harvest costs, Ledbetter explains. ‘Sunpreme’ was bred for raisin production in the San Joaquin Valley, where almost all of the nation’s raisins are produced. There are roughly 200,000 acres of vineyards producing raisin grapes in the San Joaquin Valley, so ‘Sunpreme’ could annually save millions of dollars in production costs.

To give growers added guidance, Ledbetter and his colleagues compared the overall quality of ‘Sunpreme’ grapes grown under three irrigation levels and two pruning techniques. The three irrigation levels were: replacing 100% of the water used by the vines and soil; reducing to 50% water at berry softening; and a “shock treatment,” where the vine was initially irrigated, then denied water for two weeks, and finally given a 50% water treatment.

Results from six years of irrigation showed that the “shock treatment” saved the most water, but it could be tricky for growers to implement. A better alternative is to irrigate at a 50% replacement level, Ledbetter says.

After two years of study, Ledbetter confirmed that ‘Sunpreme’ vines only need to be “spur pruned,” which generally requires less skill than “cane pruning.” In spur pruning, thick, permanent branches known as “cordons” run along a support trellis. Canes growing off the cordons are pruned back to short “spurs,” which produce the fruit. In cane pruning, the canes run directly along the trellis, and some are pruned back so that others can produce more robust fruit. Knowing which canes to prune back can take training. Grapes from spur-pruned vines were comparable in size, quality, and number to grapes from cane-prune vines, Ledbetter says.