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California researcher explains why new disease-resistant wine grapevines will be needed in warming climate

M. Andrew Walker, Ph.D., appears to have found what he was looking for - new varieties of grapes which resists a costly and persistent vine disease and produces grapes that turn into pretty good wine.

In December, Walker and the University of California, Davis, announced the creation of five new varieties of wine grapes - three red and two white - that they said helped turn back diseases plus are worthy of making into wine. One variety, ambulo blanc, is similar to sauvignon blanc and has been tested in Sonoma County, Temecula and Napa Valley.

Walker has been on the university faculty since 1989, focusing on breeding grapes, much of the time at creating varieties able to resist common threats to the multimillion-dollar wine grape industry. That includes Pierce's disease that is spread by largely a group of insects called sharpshooters. Turning leaves yellow and dehydrating grapes, the disease eventually kills the plant.

This research clearly took a long time and was painstaking. What inspired you to undertake it?

It is also one of the few diseases that kills grapes quickly and for which there is no treatment.

Pierce's disease is a fascinating disease and combines ecological aspects of the vectors, host plants and the bacteria. It is also one of the few diseases that kills grapes quickly and for which there is no treatment. The project also involves a wide array of wild grape species and their varied resistance mechanisms.

Please detail how many people have been involved in this project.

Three technicians were the most involved in the breeding. Two other technicians were also involved and many (at least 10) graduate students, both M.S. and Ph.D. Most of their projects involved the genetics of resistance and screening techniques, not breeding per se. And many undergraduate student assistants that helped with field training and greenhouse screening.

Have other researchers previously undertaken trying to develop Pierce's disease–resistant varieties?

Yes, several breeders in the Southeast and my predecessor, Harold Olmo.

What has been the history of success with that effort?

Good resistance but subpar fruit and wine.

For those who are nongeneticists or experts in viticulture, what is the process of creating a new variety in simple terms?

Survey the grape species, identify resistance, determine how the resistance works, repeatedly cross the wild species, and then their resistant hybrids with high-quality wine grape (or table or raisin) varieties (multiple times) until high resistance and high quality are attained.

We sped up the seed-to-seed generation time to two years using aggressive horticultural tricks and developed genetic markers to expedite breeding. These markers are used to eliminate 50% of the seedlings in a given population, so that effort can be focused on combining fruit/wine quality with strong resistance.

The information states that developing these five varieties took 20 years, suggesting either trial and error, or the time investment needed until grapevines bear fruit or both. But why did take so long?

It took years to genetically characterize the V. arizonica–based resistance.

Your comments reflect that as it relates to the release of these new varieties, there are implications for the wine industry when it comes to climate change. Specifically as it applies to Northern California, what are the implications as you see them from climate change for the wine industry in Northern California?

A warming climate may facilitate greater vector movement and will expand the range of Pierce's disease given that is now limited by winter temperatures.

A warming climate may facilitate greater vector movement and will expand the range of Pierce's disease given that is now limited by winter temperatures. For instance, Pierce's disease stops in California in southern Mendocino County, yet the vectors and host plants can be found further north.

Have the Pierce's disease qualities of the northern Mexico Vitis arzonica been known for some time, or was it the crossing with the species from the Southwestern U.S. that enabled that single gene to become apparent?

We made the discovery of this resistance in the Southwestern U.S. and northern Mexico V. arizonica. Its single-gene nature was brought to light in the crosses we made.

What are the challenges to be faced with trying to get these new varieties accepted and put into place in the wine industry?

(W)here Pierce's disease is found, it greatly impacts what can be grown. So growers either constantly replant dying and dead vines, or they plant resistant varieties.

Acceptance of new wine varieties is very difficult.

However, where Pierce's disease is found, it greatly impacts what can be grown. So growers either constantly replant dying and dead vines, or they plant resistant varieties. I think of these varieties as blenders in established vinifera vineyards, but they have excellent wine quality and could be used as to produce varietal wines. Using them to produce varietal wines may happen in Texas and the Southeast.

Are these varieties owned by the university and available for sale to the wine industry or are they owned by some other entity?

They are owned by UC and are licensed to certified grape nurseries (on their request) to propagate and sell to growers. As of now, three nurseries have them, although with limited amounts, and they are sold out for this year.

Is Pierce's disease in check enough that there is not likely to be much demand in Northern California for these new varieties?

Most of these (impacted) vineyards are found bordering the Napa River, the Russian River and Dry Creek.

Economic analysis of PD impacts (by Julian Alston and company) found that about 4% of the Northern California vineyards are impacted to some degree. Most of these vineyards are found bordering the Napa River, the Russian River and Dry Creek. Whole vineyards can be impacted, but it is often the first few rows where vines are constantly dying and being replanted. Landscaping around wineries is also a key source of PD infection.

If it is accepted, is it the Southeastern U.S. that is likely to be the best market for these new varieties?

They will be used in the Southern U.S. (Texas to Virginia), but will have to be sprayed to control fungal diseases. They have much higher quality than what is currently being grown.

The descriptions of each variety has very precise percentages on each type of wine - carminate blanc, as an example - 62.5% cabernet sauvignon, 12.5% chardonnay and 12.5% carignane.

What in the process of creating these grapes allows you to determine each so precisely?

M. Andrew Walker, Ph.D. and professor, University of California, Davis (courtesy photo)
M. Andrew Walker, Ph.D. and professor, University of California, Davis (courtesy photo)

It's a reflection of what vinifera were used in the crosses and if they were used multiple times. The last parent in a series of backcrosses will contribute 50% of the vinifera component and it might have been used earlier in the series of crosses.

The first generations also included table grape selections (not cultivars) which can account for about 10% of the overall parentage. In addition, parts of the genome can be linked and disrupt the exact amount of vinifera being transferred. But in the last cross the population of plants do have characteristics of the last vinifera parent.

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