UC research will help table grape growers face the rainy season

An early October rainstorm in the San Joaquin Valley provided UC viticulture specialist Matthew Fidelibus an ideal opportunity to evaluate the effectiveness of plastic vine covers for protecting late-season table grapes from inclement weather.

Fidelibus, who is based at the Kearney Agricultural Research and Extension Center, installed the covers in September on a Redglobe vineyard near Easton. Some farmers choose to grow late season table grapes – such as Autumn King, Crimson Seedless and Redglobe – to market in the fall when prices are typically highest. However, they run a greater risk of being rained on. Exposure to moisture within six weeks of harvest can cause rots and molds to render the grapes worthless.

Many growers with late-season table grapes cover their vines with sheets of plastic film to protect them from rain. Growers may choose between a relatively transparent green film, or a more opaque white film, but data distinguishing the differences the two films might have on vine physiology or fruit quality at picking, or after storage, are not available. Buying, installing and removing the plastic is very expensive, so Fidelibus is working to provide growers with objective information about the effects of the different films. Growers can track the progress of the trial in real time by following Fidelibus’ Twitter feed, http://twitter.com/grapetweets.

Five days after the trial was installed, a winter storm rolled into the San Joaquin Valley, dropping nearly an inch of rain on the table grape trial. As expected, the fruit on the covered vines stayed dry while the uncovered grapes were soaked, however, the covers posed some problems of their own.

“In some places we found pools of water on the plastic covers,” Fidelibus said. “In fact, the weight of the water displaced the covers, exposing the vines in some places. A few pools apparently grew until reaching a vent hole, releasing a water stream powerful enough to wash soil from roots.”

After the storm, the soil under the covered vines remained dry, but wind and sun quickly dried the grape clusters and soil around uncovered vines.

Data loggers in the grapevine canopies are collecting temperature and humidity readings – measures that Fidelibus will use to help describe the effect of the different covers on the environment within the grapevine canopies. He also installed atmometers, special instruments that help determine the canopy’s “evaporative potential.” Research has already shown that the greater the evaporative potential, the lower the incidence of bunch rot.

“The covers protect the grapes from rain, but when it’s not raining, they create a hotter and more humid environment under the plastic,” Fidelibus said. “The high temperatures can damage some of the vines’ leaves, and condensation on the inside of the plastic can precipitate onto the grapes. We want to know if either film provides a superior environment for table grapes. These data will also serve as a baseline against which new covering systems may be compared.”

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