Choosing Better Grapes for Warm Climates

Some lesser-known varieties have significant potential; mechanical pruning may reduce rot

by Paul Franson
wine grape crush california 2016
Some of the most-crushed wine grapes in California rarely appear on labels. Source: California Grape Crush Report, Preliminary 2016

Davis, Calif.—The grape varieties grown in California’s warm Central Valley were chosen largely based on their overall popularity, growing qualities and tradition, not rigorous research. As a result, some varieties may not be optimum for the area’s warm climate, particularly as the earth grows ever warmer.

To address this issue, Lindsay Jordan, the UC Cooperative Extension’s viticulture advisor for Madera, Merced and Mariposa counties in the San Joaquin Valley, has been conducting research on wine grape varieties for warm-climate wine production.

Jordan presented a brief summary of her findings to date at the Current Wine and Winegrape Research seminar held Feb. 13 at the University of California, Davis.

Jordan noted that nine varieties accounted for about three-fourths of the total 2015 crush: Chardonnay, Cabernet Sauvignon, Zinfandel, French Colombard, Merlot, Pinot Noir, Rubired, Pinot Gris and Muscat of Alexandria (though Colombard and Rubired are almost never identified on wine labels).

But opportunities may exist for other varieties. Though most consumers reach for familiar Chardonnay and Cabernet, others are interested in trying new things.

Jordan also noted that the popularity of red wine blends might make it easier for growers to sell superior but lesser-known grape varieties. Some of these varieties may have desirable blending characteristics including flavors, color, tannin and acidity. While red blends predominate, white blends are popular too.

In addition, growers and wineries may find markets for wines that are unique, distinctive or limited in supply.

Rising temperatures intensify the need
The rising temperatures worldwide also may encourage growers to plant new varieties. Many of the hottest years on record have occurred recently, with each of the past three years setting a new record.

Jordan said, “We can grow quality wine grape varieties in the San Joaquin Valley. We just need to find those naturally suited to the characteristics we want.”

Desirable characteristics include:
• Good yield potential;
• Adaptable to mechanization/minimal management;
• Appealing flavor and wine-quality attributes like aromatic or high-acid whites and aromatic/flavorful or colorful reds.

Jordan is conducting trials at the Kearney Agricultural Center in Parlier, Calif., in a vineyard with an altitude of 350 feet above sea level. The area recorded 2,716 growing-degree days in 2015.

The 1103P rootstocks were planted in 2008, and scions were grafted in 2009. Some varieties were grafted over in 2013 and 2016, depending on the results. As of 2016, the test varieties include 25 red varieties and 26 white varieties. Some of the varieties were configured for both spur pruning and mechanical pruning for additional data.

wine grape warm climate hot varieties
Source: Lindsay Jordan/UC Cooperative Extension

Researchers measured yield, yield components, simple fruit chemistry and rot incidence at harvest (~22° Brix for whites, ~25° Brix for reds) from four vines per plot.

Fruit from different varieties was vinified by Constellation Brands and the UC Davis teaching winery.

Pruning weights were collected around January.

What we know so far

Jordan said, “The San Joaquin Valley is capable of producing fruit with high-quality wine attributes, and some varieties present characteristics that we want for blending and/or varietal winemaking.”

She added, however, “There is a huge amount of diversity.” For example, the date of bud break varies widely as do vigor, growth habit and water use as well as grape characteristics.

Researchers calculate the “ideal” range of crop load for grapes by the Ravaz Index as from 4-10 kg yield/kg pruning weight for “quality” winemaking.

The crop load or balance of fruit mass and canopy size of vines is described by the Ravaz Index. It is calculated by using yield at harvest and comparing it to the dormant pruning weight measured in the winter following harvest (crop load = vine yield/dormant pruning weight).

Values in this experiment ranged for spur-pruned varieties from 2.0 (Carmenere) to 25.8 (Parellada). Most were within the normal range, but there were a range of possibilities in the San Joaquin Valley.

However, the index isn’t meaningful for mechanically pruned varieties as it produces values above 15.

Some interesting varieties
Jordan notes a few varieties that have desirable or undesirable characteristics.

Fiano, a white wine variety originating from southern Italy, exhibits high acid and good flavor, for example. It was consistently a high-quality wine producer and had 7.0-9.2 titratable acidity over four years.

It ripened early with an average calculated yield over four years of approximately 9.4 tons per acre, with a calculated average of 5.2 tons per acre in 2016.

It also exhibited no rot.

On the other hand, Marsanne was prone to rot, with 25% rot in 2013, 55% in 2014, 11% in 2015 and 37% rot in 2016.

Mechanical vs. spur pruning
Fourteen of the grape varieties were trained to both spur and mechanical pruning. This was obviously looking to the future, when labor issues may make mechanical pruning more important, but mechanical pruning may also improve yields compared to under-cropped spur-pruned varieties.

Mechanical pruning can also reduce cluster size and thereby decrease rot.

Morrastel, a European red variety that is a crossing of the vinifera grape Graciano, ripens fairly early (Aug. 22- Sept. 6). Its average calculated yield over four years was approximately 11.1 tons per acre (8.4 tons per acre in 2016) with low incidence of rot, including none in two years.

The researchers applied mechanical pruning to part of the vines for the past few years to see if yields could be increased, and they were: The yield increased to 13.7 tons per acre compared to 8.4 for spur pruning.

The incidence of rot was significantly lower for most mechanically pruned vines and the yields were higher. In Arneis, for example, rot was 3% for mechanically pruned vines compared to 60% for spur pruned, even though yield was 18.3 tons per acre compared to 11.7 tons per acre.

Jordan noted that in 2016, water stress from heat waves starting in June severely hindered some varieties and reduced yields overall. Some varieties were prone to sunburn or raisining.

In 2016, a number of other varieties were added to the tests by grafting Petit Bouschet, Grand Noir, Nero d’Avola and Assyrtiko onto existing vines.

Jordan summarized the results of these trails so far, saying, “The varieties tested exhibit a wide range of vigor, productivity and fruit quality, but many have desirable characteristics for the San Joaquin Valley—notably, high yields, low rot and good quality.” She added that some varieties may be suited for quality-focused production, while other would be better others for bulk blending.

She also highlighted the opportunity to use mechanical pruning to improve fruit quality for some varieties.

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