Heat’s not necessarily the friend of grapes, says George Zhuang, University of California viticulture farm advisor for Fresno County, a point he made in spades at an educational field day at Sanger, sponsored by the San Joaquin Valley Winegrowers Association.

For one thing, heat can “take away water and tonnage,” and it can turn grapes into raisins hanging on the vine. “We farm wine grapes — not raisins, he says.”

Heat can place stress on all grapes, he notes, whether for raisin, wine, or table grape production, a point he says was driven home last year by temperatures well into the 100s for three weeks.

The hit was not only high in the Central Valley, but also in Napa and Sonoma, causing sunburn in places. Because in the Central Valley, “California sprawl” trellising has been commonly used, that afforded some protection from the canopy. The sunburn issue was more problematic in Napa and Sonoma, where the VSP (Vertical Shoot Positioning) trellis is more commonly used.

Spraying sun block on the clusters can help prevent sunburn, as well as improving quality, Zhuang says, pointing out that heat doesn’t merely fry the grape — it alters some of its qualities, and can compromise quality and taste.

INCREASINGLY AN ISSUE

Trying to improve quality is increasingly an issue in one of the hottest regions of California, the San Joaquin Valley, which has 40 percent of the state’s wine grape acreage and crushes 70 percent of the state’s wine grapes.

“The priority in the valley has been a focus on yield,” Zhuang says, “but fruit quality is becoming more critical.” While brix levels have been used to decide optimum fruit quality for harvest, other characteristics are being given more weight by wineries, including acidity, phenolics, and flavor compounds.

Desirable qualities in wine include a low pH, high titratable acidity (TA), high anthocyanins, smooth tannins, and flavor.

Low TA is caused by high temperatures, he notes, while high pH is caused by low TA and high potassium. “Higher pH is a problem for wineries,” he says, adding that rootstocks, notably the Freedom rootstock, can be contributors to high pH. “Freedom has a higher uptake of potassium, which leads to higher pH.”

Some growers use potassium fertilizer in foliar sprays, which increases sugar levels in the grapes, but the potassium ends up in the berries, pushing up pH. He recommends that growers check with their winery before using the fertilizer as a foliar application.

OTHER TEMPERATURE EFFECTS

Heat also impacts anthocyanins, Zhuang says. Temperatures over 95 degrees reduce color. Tannins are also influenced by temperature and light. Temperatures above 95 degrees reduce skin tannin, while high light increases skin tannin.

Direct berry exposure, with temperatures above 100 degrees, brings heat damage and sunburn that can cause berries to shrivel, resulting in pre-raisining and a “cooked and raisin flavor.”

Orientation of rows is important, Zhuang says. If they are east to west, that lessens the solar impact. Sprawling trellises also helps, and there should be no removal of leaves on exposed sides of the row. Mechanical leafing should be done on the north or shaded side.

’The earlier you do mechanical leafing, the less sunburn you will have,” he notes. “If you do it late, most likely you are going to get sunburn.” Irrigation management is also important to avoid stress. Canopy nets have been used in Napa and Sonoma, he says, but that is costly and labor intensive.

Sun block sprays on the market include Kaolin (Surround), which uses a clay powder, and CaCO3, calcium carbonate (MicroCal). Both reflect sunlight.

CALLCIUM CARBONATE TRIALS

Last year. Zhuang conducted research into use of calcium carbonate, applying sprays to wine grapes once a month during the growing season at various rates. An air blast sprayer was used, and the first application began when grapes were pea-sized, about two weeks after 50 percent bloom. The material was applied to Syrah grafted onto Freedom rootstock.

It was found that calcium carbonate can lower canopy temperature when vines are water-stressed. It increased leaf photosynthesis rate and stomatal conductance after veraison.

Calcium carbonate increases the catechin/tannin and polymeric anthocyanins/tannin index, Zhuang says. There was no benefit in terms of increased yield.

Wineries vary as to which reflective product they prefer, Surround or MicroCal. Zhuang plans more trials this year for both products.