Viticulture research examines extended cool growth for wine grapes

Research from California State University, Fresno is exploring a new approach to wine-grape growing in the sizzling San Joaquin Valley that could extend cool-season growth of certain varieties producing higher-quality wine grapes.

The strategy involves a “crop-forcing”practice employed in other places but not in the Valley, – said research scientist Dr. Sanliang Gu, a professor and the Ricchiuti Chair of Viticulture Research at Fresno State’s Viticulture and Enology Research Center (VERC).

“Wine grapes produced in California’s warmer regions amount to 60 percent of tonnage, but only 25 percent of crop value,” Gu said in outlining his research. “If the growing cycle of wine grapes in warmer regions can be delayed or shifted to the cooler period of the growing season, fruit ripening will occur at lower temperatures, and much improved fruit and wine quality should be expected.”

Grapevines have genetic ability to set and bear fruit multiple times a year under certain conditions, Gu explained. It does not occur under natural conditions, but will if buds are forced out of dormancy soon after the berries are set, he noted.

The method involves buds being removed from newly-set berry clusters, as well as pruning back shoots and removing leaves and laterals. In some cases, leaves can be retained to provide protection from sunburn.

When these actions are taken, a second bud break quickly follows and the whole developmental cycle starts anew, Gu said. And since the new cycle starts up to several weeks later, the fruit of the forced crop ripens later in the season, generally under cooler temperatures, with characteristics of cool-region, higher-value fruit.

In the central San Joaquin Valley, harvest time of some wine-grape varieties would be moved from the beginning of September to the middle of November, Gu said.

Crop forcing and even double-cropping of grapes have been employed in Australia and in some tropical areas, respectively, Gu noted. Since hand pruning is labor intensive and costly, however, researchers have experimented with other methods and have found some to be effective. Some of those methods include spraying synthetic and/or natural chemicals and extracts that prompt the vine to break new buds.

Later-season wine grape maturation in the Valley would mean crops with smaller berries, higher acidity, lower pH, deeper color, higher tannins and phenolics, and more intense aroma and flavor – all characteristics that tend to produce higher-quality wines, Gu said.

Since the method is virtually untested in the Valley, many issues must be addressed, Gu said. In other areas, such as Australia, experiments have shown it can negatively affect vine health, capacity and longevity.

However, “with the knowledge we’vegained and the advances we’ve made during the past 30 years in vine physiology, mechanization of cultural practices and chemical defoliation or defruiting, we have the opportunity to address these limitations,” he said.

Gu conducted preliminary experiments in vineyard sites at Fresno State and with industry partners in Madera County in 2009 and 2010. Partial funding was provided by the California State University Agricultural Research Institute.

Gu is encouraged by initial results and seeks new funding to continue the work.

For more information, contact Gu at sanliang@csufresno.edu.

(Text by Steve Olson of the California Agricultural Technology Institute based at Fresno State.)

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