Truck owners swear by their favorite brands — Ford, Chevrolet, GMC and Dodge. In California table grape production, growers prefer either white- or green-colored plastic row covers to protect late-season berries from inclement fall weather.

Some California grape growers farm late-season table grapes — including Crimson Seedless, Redglobe, and Autumn King varieties — to gain higher grape prices at the end of the crop year. A key to successful late-season production includes the use of polyethylene row covers over the grapes and vines to shed rain water away from the berries to reduce moisture-based grape diseases.

Many growers have a personal plastic preference — an opaque white film or a more transparent green film. Yet there is an absence of published scientific data to show how the plastics compare to each other and impact vine physiology, grape quality at harvest, and fruit in cold storage.

Until now.

UC Davis’ Extension specialist Matthew Fidelibus and UCCE farm advisor Stephen Vasquez, Fresno and Madera counties, launched a trial last fall to study the performance of different plastic row covers used in late-season table grape production.

Fidelibus is based at the Kearney Agricultural Research and Extension Center in Parlier while Vasquez is based at the UCCE office in Fresno.

“The purpose of the study was to compare cover performance and generate a database which could be used to help assess the performance of new cover products when available,” Fidelibus said.

The study will also make note of any potential downsides of the covers.

“Undesirable cover characteristics can include excessive heat buildup in the canopy caused by the plastic,” Fidelibus said. “Another potential issue is the amount of condensation associated with the different plastic covers and its impact on grape quality.”

The row cover research is grower funded.

Vasquez and Fidelibus placed covers on one acre of Redglobe grapes in a commercial vineyard near Easton in southwestern Fresno County.

Redglobe is a large-seeded grape developed by Harold Olmo of UC Davis in 1980. The variety is grown on about 12,000 acres in California. Redglobe is the most popular seeded grape and ranks fourth overall behind Thompson, Flame, and Crimson Seedless.

Fidelibus says the initial results suggest green-plastic covers allowed more light to pass through creating higher temperatures and condensation in the canopy compared to the more opaque white plastic covers.

“My thought on why it occurred is white plastic may have reflected more light than the transparent green plastic,” Fidelibus explained. “Light transmitted through the plastic is absorbed by the vines which heats them, and then increases air temperatures under the covers similar to a greenhouse effect.”

He added, “A more transparent plastic may have a greater effect on canopy temperatures, vine transpiration, and humidity. Nightly cooling results in condensation which may be greatest under the covers with the highest absolute humidity during the day.”

Fidelibus and Vasquez emphasize that the one-season findings are just that — one-year results.

More data needed

Additional data are needed to confirm that similar results will occur each year. Additional trials will be conducted this fall in commercial vineyards in the Easton area and near Madera in Madera County.

The additional trials will also allow Fidelibus and Vasquez to measure the long-term impact of row covers on grape vine physiology.

“When plastic row covers are installed, the growing environment changes instantaneously,” Vasquez said. “We also want to determine if row covers over time can have long-term positive or negative impacts on the vine.”

Growers keep late-season grapes on the vine to prolong fruit quality. Table grape quality is maintained when fruit remains on the vine versus harvested grapes placed in cold storage. Once harvested, the berries and rachis (cluster stem) lose moisture faster which in turn decreases storage and shipping durability.

Vasquez said, “Holding the fruit longer on the vine allows growers to pick and pack fruit as needed to meet current sale orders and provide consumers a fresher product.”

In the San Joaquin Valley, plastic row covers are installed over late-season table grapes in late August to early September. Without the covers, rain on the fruit can lead to bunch rot which can render the crop unmarketable. This can be disastrous since most late-season grapes have higher sugar content.

In addition to using plastic covers, growers apply fungicides against rot fungi. Late season grapes often receive additional applications which help minimize storage rot issues.

“Grapes harvested at peak maturity have the potential for the most problems,” Vasquez said.

Fidelibus added, “Exposing ripe table grapes to rain can result in serious rot problems.

Table grape growers want more information about plastic covers to help them make the best, bottom-line decisions.

Vasquez estimates row cover costs from $600 to $1,000 an acre including the cover itself, installation, and removal for single-year use. Costs vary due to the specific plastic and trellis system which determines the size of the work crew needed to install the plastic covers.

Industry research is underway to develop new cover technology. Desirable characteristics of new covers would include lower cost, reusability, and lesser impact on the canopy temperature, humidity, and condensation.

Plastic row covers are only used in late-season grapes in California; not in early-season grapes.

California’s estimated 550 table grape growers produce about 98 percent of the nation’s crop or about 100 million boxes of grapes, according to the California Table Grape Commission.

Per capita consumption of fresh grapes in the U.S. is 8.4 pounds per person. About one third of the California crop is exported to more than 50 countries. California table grapes are available May through January.

To help share information about the production of all California grapes — table, wine, and raisin, Fidelibus and Vasquez are tapped into social media. Kearney-based UC Davis staff research associate Kimberly Cathline suggested the social media avenues. Related photos are also available at the photo-sharing service Flickr, www.flickr.com.

About 800 people follow the Twitter grape tweets at www.twitter.com/grapetweets. About 400 gather information through Facebook at www.facebook.com/viticulture. Also available is a general viticulture blog at http://ucanr.org/sjv-viticulture. 

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