California varietal wine grape growing has become all about clones.

And for good reason. However, according to Mathew Fidelibus, assistant Cooperative Extension viticulture specialist at the University of California Kearney Agricultural Center in Parlier, growers may not be digging deep enough to discover differences.

Fidelibus’ research into various clones of traditional cultivated wine grape varieties has yielded some surprising nuances in terms of yield, disease tolerance, fruit composition, and other production and quality characteristics.

“Most of California’s wine industry is based on a single species of grape – Vitis vinifera,” he says. “Over the years, old cultivars have accumulated many clones due to mistaken identity or mutation or both.”

Variation is not necessarily a bad thing. In fact variation within cultivars can help a grower select a clone to better fit a specific growing situation.

“Clone selection is almost untapped in California,” Fidelibus surprisingly says. “There are huge differences that occur even within clones that can work to a grower’s advantage.”

Technically, a clone is defined as a population of vines that are propagated asexually from a single mother vine.

“Nurseries offer many different clones of a given cultivar,” Fidelibus says. “They may differ dramatically with respect to yield, yield components, fruit composition, and susceptibility to pests and diseases. Additionally, the performance of a given clone can depend on the climatic region where it is grown.”

Clone differentiation

Fidelibus conducted several trials looking at clone differentiation in the San Joaquin Valley. Cultivars included Chardonnay, Merlot and Zinfandel/Primitivo.

In the Chardonnay clone trial Fidelibus evaluated six clones including Clone 4 which is the industry standard. “Chardonnay is the most widely cultivated wine grape in California,” he says. “There are more than 70 different registered Chardonnay clones in California. Clone 4 is characterized by high yields and good fruit composition. It has large heavy fruit clusters, but it has a drawback. It is very susceptible to sour rot.”

“Clone 4 had good yields and good fruit composition as expected, but it was more susceptible to sour rot at 16 percent than any of the other clones in the trial,” he says. “Clone 15 had 10 percent lower yields than Clone 4, but it only had 4 percent incidence of sour rot. That’s a big difference if you’re growing Chardonnay in an area where sour rot is a problem.”

In the second cultivar trial, Fidelibus looked at six Merlot clones. “Merlot is among the most important red wine grapes in California,” he says. “Forty-five percent of the state’s Merlot yield is grown in the Central Valley. Clone 3 is the standard with consistent set, yield and good fruit composition. However, its performance as compared to other clones had not been tested.”

The results of the trial showed that growers might not be optimizing their yields or quality if they go with the industry standard instead of considering their options. “Clone 10 performed best with consistently higher yields of fruit with low pH and high tritratable acids,” Fidelibus says. “Clone 11 was undesirable. It had the largest berries, was the most susceptible to rot, and had high pH. Clone 3 performed similar to Clones 1 and 9 but was not superior to Clone 10.”

Zinfandel trial

In the Zinfandel trial, Fidelibus again looked at six different clones including three Primitivo clones. Primitivo is clone of Zinfandel origin and is so closely related Primitivo berries can legally be marketed as Zinfandel.

Zinfandel is characterized, in general, by big berries, large clusters, high fruitfulness and high yields. It is also very susceptible to “sour rot” in warm climates. It often ripens unevenly, and yields typically decline as the season progresses. Additionally, color can be poor, especially in warm climates.

“There were few differences between the Zinfandel clones tested,” Fidelibus says. “The Primitivo selections generally were better. They were earlier maturing. They had similar or higher yield, and similar or less incidence of sour rot.”