Chasing the Silver Bullet

At the Salt Lick’s Pecan Grove building in Driftwood, we sat at no-frills, classroom-like tables set with wine glasses. Our tasting leader, U.C. Davis professor Andrew Walker, was the featured speaker at a full-day workshop and tasting organized by Jim Kamas, Texas A&M professor and Texas AgriLife Extension coordinator.

“These are likely the most expensive wines you’ve ever tasted,” said Walker as he began. The bottles were not adorned with prestigious labels, but still, the room full of Texas grape growers and winemakers attentively swirled and sipped. 

The wines in our tasting were indeed special (and costly) because they were products of more than 20 years of U.C. Davis field studies, genetic testing and research. They were made from grapes that were—based on genetics—produced by Vitis vinifera vines. They had, however, a strategically placed gene from Vitis arizonica—a grapevine native to Texas, the southwestern United States and Mexico that offers a natural resistance to Pierce’s Disease (PD), a disease that’s long plagued many vineyards in Texas. The interest of the tasters was immediately piqued, because Vitis vinifera vines are highly susceptible to PD caused by Xylella fastidiosa (Xf), a bacterium spread by plant-sucking insects like our own native glassy-winged sharpshooter. 

Texas winegrowers have long awaited a “silver bullet” in the battle against PD, where devastation from this scourge has been recorded as far back as the 1800s. Immigrant farmers often brought European grapevines with them, but these vines usually died within a few years, forcing settlers to make lesser-quality wines from native grapes. Since the 1980s, when growing European wine grapes in Texas started as a serious endeavor, PD has remained a major barrier to pursuing Vitis vinifera in East Texas and along the Gulf Coast. In the Hill Country, many vineyards were lost to PD before vineyard practices and pesticides were found to keep PD at bay; however, most consider these measures only a temporary fix. 

But how did Professor Walker’s team get the gene for PD-resistance into the Vitis vinifera grapes? According to Kamas, “Walker went at it the way nature has done it for eons: by producing pollen, seeds and seedlings. No gene guns or trans-species bacteria were used to get the arizonica gene to mix with those of vinifera grapes. These are definitely not GMO grapes.”

Walker’s initial cross-breeding gave him a new grape variety that was 50 percent vinifera and 50 percent arizonica, but it didn’t have the characteristics desired by wine drinkers. Then, he repeated the process several more times with additional vinifera parent plants while using a genetic test to choose only those seedlings that showed positive in the gene for PD-resistance. 

Repeating this technique allowed Walker to increase the amount of vinifera genetic material in the seedlings to 75 percent, 88 percent and 94 percent, while still retaining the anti-PD trait. His premise was that as the vinifera content of his grape varieties increased, his selected PD-resistant seedlings would eventually attain wine characteristics more like commercial wines. 

Walker could readily test for the gene for PD-resistance resulting from a single gene. However, the only way he could confirm something complex like wine quality was through the time-consuming process of growing his seedlings to vines, harvesting grapes, making wine and having knowledgeable tasters evaluate the wines.

Impressions from our group’s tasters were resoundingly favorable—attesting that Walker’s wines had distinct vinifera characteristics. “The reason why I came here is that I want to get hooked up so I can plant these new varieties,” said Messina Hof Winery founder, and tasting participant, Paul Bonarrigo. “My vineyard in Bryan is in high-risk PD country and I’m currently limited to PD-tolerant hybrid grapes like Lenoir and blanc du bois. To get the vinifera we need, I’ve got to work with growers all the way up north around Lubbock. I’m looking for more options of what I can grow on my Bryan estate.”

Walker believes his varieties have reached commercial quality. “I’m planning on getting the 97 percent vinifera…and a few 94 percent…to nurseries this year. They will increase the number of varieties for release to growers in 2019 and 2020. There will be more to follow in successive years.” 

After the long wait, it looks like the silver bullet is finally ready. Now, it’s time for Texas vineyards and winemakers to lock and load, and for consumers to get their taste. 

By Russell Kane

For more on Walker’s PD-resistant grapes, email This email address is being protected from spambots. You need JavaScript enabled to view it.