The affliction is a grapevine-killer, delivered by insects from the sharpshooter family. They feed on a plant’s vascular tissue, known as xylem, and introduce the bacterium Xylella fastidiosa. As the flow of xylem through the plant is constricted, infected vines are clogged and starved of water and nutrients. Leaves turn yellow, the grapes dehydrate and the vine eventually dies.
The affliction can deliver a devastating economic blow. In California alone, Pierce’s disease costs the wine industry more than $100 million annually. It’s also hit vineyards spanning from Florida to South Carolina and all along the Gulf Coast.
Unsurprisingly, Pierce’s disease-resistant grape varietals have major appeal. Such is the impetus of Dr. Andrew Walker and Dr. Alan Tenscher of the University of California at Davis’s Department of Viticulture and Enology, the driving forces behind a slew of new pest and mildew-resistant cultivars. Five hybrid varietals, known as Walker varieties, were released from commercial grape nurseries in limited quantities in 2020 and made more widely available the following year.
“I’m confident that these materials will make a significant impact on the wine industry, as they exhibit strong resistance to powdery mildew and Pierce’s Disease,” says Luis Diaz-Garcia, who currently leads the grapevine breeding program at U.C. Davis. “[These varietals] also boast excellent wine quality.”
But are these new varieties all they’re cracked up to be? Early plantings in the Southeast and Texas suggest that the hype is real.
Decades in the making
The debut of these new varietals took more than 20 years of development in the lab of the now-retired Dr. Walker. Each variety a cross between Vitis vinifera, which encompasses popular European grape varieties, and Vitis arizonica, a grape indigenous to the U.S. Southwest that carries a gene resistant to Pierce’s disease.
Over time, Walker’s lab gradually increased the percentage of vinifera by backcrossing the most recent iteration of the hybrid with the vinifera base. This was done to help boost the appeal of the resulting grapes, as vinifera varieties are most familiar to mainstream wine drinkers.
Early studies in Texas and Georgia—led by Jim Kamas, an associate professor at Texas A&M; professor Elina Coneva at Auburn University; and Dr. Violeta Tsolova of Florida A&M University’s Center for Viticulture and Small Fruit Research—suggest the varieties are even more successful than researchers had first anticipated. Scientists in these regions were particularly interested in these varietals due to the intense pressure of Pierce’s disease in the area that suffocates most vinifera vines.
“We’ve confirmed that they are indeed disease resistant and are much higher quality than previously evident, when vinifera levels were 88%,” says Kamas. “So, we are at the dawn of a new age.”
All of the varietals—three reds and two whites—are named for derivations of the word “walk” in Spanish or Italian. Among the reds, there’s Camminare Noir, which contains 94% vinifera. It includes 50% Petite Sirah and 25% Cabernet Sauvignon, exhibiting characteristics of both. The grape is the most widely planted of all the Walker varietals, due to its concentration of color and tannins. Another red is Paseante Noir—similar to Zinfandel, it’s 97% vinifera, including 50% Zinfandel, 25% Petite Sirah and 12.5% Cabernet Sauvignon. Finally, Errante Noir, which most closely resembles Cabernet Sauvignon, is 97% vinifera with 50% Sylvaner and 12.5% each of Cabernet Sauvignon, Carignan and Chardonnay.
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