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

cblake@farmpress.com