Applying Lessons From Pierce's Disease

UC Davis researcher uses grape breeding techniques to fight powdery mildew

by Paul Franson
Andy Walker from the University of California, Davis, is taking grapevine breeding techniques that foster resistance to Pierce’s disease (left) and using them to combat powdery mildew (right). Photo source: University of California
Napa, Calif.—The Napa Valley Grapegrowers, Napa Valley Technical Viticulture Group and University of California Cooperative Extension joined forces March 4 to present a summary of new findings about pests and diseases as part of the Grapegrowers’ Sustainable Vineyard Prices series.

Andy Walker of the University of California, Davis, Department of Viticulture & Enology discussed using the lessons learned from breeding grapes for resistance to Pierce’s disease (PD) to produce grapes resistant to powdery mildew. While creating a vine with that resistance is challenging, he reported significant progress breeding PD-resistant vines and says they will be ready for planting in a few years.

While maintaining the resistance to PD, Walker is also trying to breed grapes resistant to powdery mildew and to combine powdery mildew resistance with PD resistance.  Powdery mildew is a North American fungal disease.

V. vinifera is susceptible, and vineyards are protected by application of fungicides at seven- to 21-day intervals; failure to apply fungicides can result in complete crop loss.

All North American native Vitis species resist powdery mildew to some extent, but resistance also exists in V. romanetii, V. piasezkii from China, and in some Central Asian V. vinifera varieties including Karazdhandl, Kishmish Vatkana and a few others Walker’s lab has discovered.

Many resistant hybrids with North American grapes exist, but they do not have vinifera fruit quality, and most have complex resistance difficult for breeding.

Australian and French researchers found single-gene resistance to powdery mildew in the muscadine grape, M. rotundifolia. Walker’s group is using these sources and those from China and Central Asia to stack resistance genes and produce wine grapes with durable resistance to powdery mildew, and to incorporate it with PD resistance.

They know that grapes will need multiple resistance genes and have mapped a wide range of resistances to different chromosomes, making it possible to combine and follow multiple unique resistances. Dario Cantu, a new genomics specialist in the Department of Viticulture & Enology at UC Davis, is aiding these efforts.

PD-resistant vines nearer

Walker summarized efforts to breed PD-resistant grapes, noting that Lenoir (Jacquez, Black Spanish), a Vitis aestivalis x V. vinifera hybrid, is resistant to PD but doesn’t make very good wines. “It has a wild character, is astringent and blue-purple in color.”

Hundreds of years of breeding have been hindered by multigenic resistance in grapes, which greatly reduces the number of progeny with resistance and high fruit quality. One of the best examples is Blanc du Bois, a resistant variety which is used to make wine in the southern United States. “It’s also a sub-par wine,” noted Walker, who added that they have 150 vines growing in Napa for experimental purposes.

Typically multiple genes are involved in disease resistance, but Walker discovered that a Southwestern and Mexican grape variety, V. arizonica (b43-17), has single dominant gene resistance to PD.

Using modern genetic mapping tools, he created multiple generations of crosses with that grape to produce vines that are 97% vinifera.

His objectives were to develop large seedling populations at the 97% vinifera level in diverse, high quality vinifera wine grape backgrounds and then intercross advanced selections with resistance to Xylella fastidiosa (the bacteria responsible for Pierce’s disease) from other sources.

He is now characterizing additional unique resistances so that he can stack different forms of resistance to make broadly and durably resistant varieties

Scientists are also genetically mapping these resistances so that marker-assisted selection of genes can be used to identify progeny seedlings with resistance to PD. This selection, combined with aggressive growing techniques, was used to get seed from crosses to resistant parents in two years.

Resistant progeny were then selected for the lowest possible bacterial levels, and the best were crossed back to high quality vinifera parents so that with subsequent breeding to percent high quality vinifera went from 50%, 75%, 88%, 94% and 97%.

The marker-assisted selection optimizes classical breeding and is not genetic engineering.

This process produced vines that were 97% vinifera in about 12 years, far less than would otherwise be possible. “We went from grapes that made peppery, herbaceous wines with blue-purple pigments to high quality vinifera characters,” he said.

Walker and his colleagues have conducted field-testing of PD-resistant selections at Davis and at a Beringer Vineyard in Yountville, Calif., along the Napa River—a hot spot for Pierce’s disease.

They also made wines from the trials of 75%, 88% and 94% vinifera and compared them with wine from classic vinifera cultivars made at the same small scale and place.

They’re also growing 88% and 94% vinifera vines in Fredericksburg, Texas, and 88% grapes in Auburn, Ala., all areas with high PD pressure.

In 2014, they planted new plots in Temecula, Texas and Alabama, all challenging locations, as well as two in Napa.

The final step in the breeding process is to send advanced high-quality selections to Foundation Plant Services for certification. They have sent many PD-resistant 94% and 97% vinifera selections to FPS and will continue to select and send additional resistant selections while they finalize decisions about which have the best wine quality based on field and wine trials.

The first release to nurseries will be the 94% vinifera red wine selection 7355-75. They’ve made wine three years from multi-vine trials in Davis and Napa, and it performs well in repeated severe greenhouse testing.

The next steps are to combine resistances from other backgrounds to ensure durable, broad-based resistance, which will be done with other grape species Walker has collected across the southwest.

Walker ended his talk with a note that viticulture has too long relied on “traditional” varieties, and the lack of new variety development makes wine grape growing one of the most backward forms of horticulture.

Part of the problem is the marketing of new varieties given the emphasis on varietal labeling. For instance, you can’t call a 97% vinifera hybrid “Cabernet Sauvignon.” He hopes that resistance breeding may alter people’s perception and willingness to accept new varieties—and climate change may have an even more compelling impact on our need to create and use new varieties.

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