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.