Crop Forcing Improves Winegrape Quality

Fresno State Grape Day highlights research, department news

by Jon Tourney
csu fresno state grape day
Viticulture professor Dr. Sanliang Gu of California State University, Fresno, discusses the crop forcing research trial while standing in row of Cabernet Sauvignon in the campus vineyard.
Fresno, Calif.—Information gleaned from a “crop forcing” vineyard trial in Fresno shows good promise for improving winegrape quality from the San Joaquin Valley and other warm climates by managing vines to shift the grape-ripening period from hot summer months to cooler months later in the growing season.

At the Aug. 14 Fresno State Grape Day presented by the California State University, Fresno, Department of Viticulture and Enology, CSUF viticulture professor Dr. Sanliang Gu provided an update about a crop-forcing trial in a Cabernet Sauvignon block of the campus vineyard. Cabernet clusters on control vines that developed under normal season conditions had completed veraison and were within two weeks of harvest. In contrast, crop forced (CF) vines in the same rows had clusters of small green berries that likely will be harvested in November.

In recent years CSUF viticultural research has focused on methods to improve winegrape quality to enhance the demand for—and reputation and prices of—San Joaquin Valley grapes. More than 60% of California’s winegrape production comes from warmer regions such as the San Joaquin Valley, however this represents just 25% of the state’s crop value. Temperature is one of the most important factors influencing grape development and quality such as development of phenolic compounds, color and acidity. As Gu explained, “In many of the world’s grapegrowing regions, the vines go into dormancy about the time of, or just after, harvest. Here in Fresno, when we typically harvest the vines still go for two to three more months of the growing season before they go into dormancy.” 

Grapes in Fresno typically ripen in July and August, when daily average low and high temperatures range from 68° to 98°F (July) and 66° to 97°F. (August). In contrast, October average temperatures range from 53° to 80°F. Gu said with CF, the flowering to veraison period is shorter, occurring during the warmer summer months. But most importantly, the ripening period is longer, with veraison to harvest shifted into the cooler months from September to November.

CF takes advantage of grapevine physiology by which a vine can bear fruit more than once per year if the compound buds are forced out of dormancy. Gu has been conducting trials since 2009. (See the story “Crop Forcing Can Delay Winegrape Ripening” from August 2010.) This week, Gu reported on the successful trial outcomes regarding methods for vine management and timing of crop forcing; he also provided grape quality and chemistry data from the 2011 harvest comparing CF fruit with control fruit. In 2011, CF shifted veraison from July 25 to Sept. 26, and harvest from Sept. 1 to Nov. 18. 

CF fruit had smaller berries, lower pH and higher titratable acidity (TA) at veraison and at harvest compared with control fruit. CF fruit at harvest had lower Brix and pH, but higher TA and malic acid as well as higher amounts of skin anthocyanins, tannins and total phenolics than the control fruit. Gu noted that overall yields are lower for CF fruit—6-6.5 tons per acre compared to an average of 7.5 tons per acre for control fruit. He believes an average of 5-6 tons per acre would be a good range for CF fruit to achieve good quality parameters.

Fine tuning CF management
During the first two years of trials in 2009 and 2010, a range of crop-forcing periods was studied in which vines were stripped post-bloom at intervals as short as every seven days from May through July. CF in May was still too early for significant quality improvement. Waiting until July was too late, as one year green berries were damaged by frost in early December. Gu has concluded that CF during June is the best time to achieve favorable conditions for ripening and quality. In 2011, shoots were hedged and first growth clusters were removed June 24, about six weeks post-bloom.

Trials in the initial years also examined different degrees of vine hedging and stripping ranging from six to two nodes per cane. The best treatment for the Cabernet trial appears to be hedging growing shoots to six nodes per cane and removing summer laterals, leaves and primary clusters. Another issue is nitrogen (N) management for CF vines to maintain vine vigor and yield. Trials in 2011 indicate that N applications through the drip system timed before and after CF at proper intervals can prevent N deficiency in CF vines and maintain vigor. Gu observed, “Vine transpiration temporarily stops when the vine is stripped, but this hasn’t been a problem, because our growing season is so long.”

Gu and his research crew also are working on one of the main management challenges, finding a mechanical or spray method to hedge and remove clusters for CF, so it can be economically viable in   larger acreage vineyards.

Gu believes Cabernet Sauvignon is one of the easiest and most suitable varieties for CF. He is also conducting trials with Chardonnay, Sauvignon Blanc and Zinfandel, and this year began testing Sangiovese and Pinot Noir. He said, “I’m excited about working with Pinot Noir this year, and so far it’s looking beautiful. With CF, I’m hoping Pinot Noir can be grown more in the San Joaquin Valley.”

He observed that CF should be used with mature vines that have adequate vigor to support the process. Based on his research to date, Gu said, “Our worries about adequate vine fruitfulness are gone, and yield is not a concern, as we are getting consistent yields year-to-year from CF vines.  All we’re doing is just physically shifting the fruit development and ripening to later in the season.”

The CF research at CSUF also has other potential benefits beyond the San Joaquin Valley:
• It could enable expansion of winegrape production, and the planting of more cultivars, in regions considered too warm for quality winegrape production.
• It could be a tool to adapt to global climate change.
• It can be used to produce a crop after primary clusters are destroyed by spring frost, if the remaining growing season is long enough.
• It could aid in grapevine breeding by synchronizing bloom and development periods between cultivars that grow at different seasonal rates.
• It can be used in campus and teaching vineyards to enable students to see all stages of vine and fruit development at the same time, or in a short period, in the same vineyard.

Kennedy provides department update
The biennial Fresno State Grape Day featured updates from faculty and student research projects as well as tours of department facilities including the Fresno State Winery, the wine sensory lab and the V.E. Petrucci Library that houses research and information materials available for viticulture and enology students and the wine and grape industry.

Department chair Dr. Jim Kennedy updated attendees on the V&E department and program that has grown in enrollment to 200 undergraduate students. The department now has approval to begin a formal internship program with class credit to enable all students to gain industry experience by the time they graduate. “For those of you in the industry who would like to mentor students, our internship program now offers more opportunities for you to partner with our program, “Kennedy said.

In spite of university-wide budget constraints, the department has received approval and is conducting searches for two new faculty positions—a winegrape viticulturist and a wine chemist. In addition, Fresno State Winery has an opening for a business and marketing specialist. Kennedy also said that the campus vineyards would begin a replanting process next year, starting with a 20-acre block. Vineyard redevelopment will provide new teaching opportunities with more winegrape varieties and a new raisin vineyard.

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