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International Grapevine Genetics Conference: Vines without Powdery Mildew and Downy Mildew?

COMMENTARY

By Tim Martinson
Senior Extension Associate, Department of Horticulture, Cornell University

Rootstock
Peter Cousins, USDA Grape Rootstock Breeder, pollinating rootstock in Davis, California, May 2010

I had the opportunity to sit in on a session of the 10th International Conference on Grapevine Breeding and Genetics, held August 1-5 right here in Geneva, New York. This meeting attracted grapevine geneticists and breeders from all over the world—notably Australia, Europe, Asia, South America, and, of course, North America. They came together, as they do every four years, to discuss advances and research results from their respective programs.

These scientists speak a language that is a little alien to a field viticulturist such as myself. Talks are liberally sprinkled with unfamiliar words such as metabolomics, proteomics, SNPs, QTLs, transcription factors, and cryptic names for genes such as REN1 and RPV-4. But, as I listened, I realized that these scientists are closing in on a key goal relevant to growers: vines that are highly resistant to downy mildew and powdery mildew.

Several groups have identified at least three genes for resistance to powdery mildew and an additional two for downy mildew from a variety of wild and cultivated grapevines. Each of these genes provides a distinct mechanism of conferring resistance to these diseases.

But, even more importantly, advances in DNA sequencing have put precise, new tools in the hands of breeders. For the first time, they have a way to rapidly test whether one or more resistance genes are present in seedlings they produce from crosses. With "marker-assisted selection" they use genetic probes to test seedlings for presence of specific DNA sequences associated with resistance genes.

These "markers" sift through the millions of DNA base pairs to hone in on a few hundred that are unique to each resistance gene—much like an internet map that allows you to zoom in from the entire United States to your specific street address. Before these tests were developed, breeders had to rely on much less detailed genetic maps—the equivalent of being able to zoom in on a map from the United States down to New York State, but not much further. While they could use field observations to determine which seedlings showed visible symptoms of powdery mildew and which ones didn't, there was no way for breeders to know for sure which of several possible genes was responsible for the observed resistance. Now they can.

The significance of this is that grape breeders will have the tools to determine which genes are present in each grapevine seedling they produce and rapidly distinguish and select the ones that have several resistance genes instead of just one. This concept, called 'pyramiding genes' offers advantages over 'single-gene' resistance. Having just one resistance gene is akin to using a new fungicide: it works great for a few years, but then the powdery mildew organisms adapt quickly and overcome the resistance (or become resistant to the fungicide). Many genes should provide much more durable resistance than just one—it would be much harder for an individual powdery mildew spore to acquire two different adaptations to two sources of resistance than to develop just one. Durability of resistance is an absolutely essential characteristic for vines that will be growing in one place for many decades.

The final hurdle for grape breeders is the amount of time and number of generations of crosses it takes to get these resistance genes into a genetic background that will produce high quality wines. With each generation taking at least four years from seed to seed, the five generations of crosses often requires 20 to 25 years—the better part of a breeder's career. By using marker-assisted selection, one researcher thought they could produce new, disease resistant cultivars with 97% of their genetic makeup coming from an elite V. vinifera cultivar in roughly half the time.

The need for better resistance to powdery and downy mildew should be obvious to any grape grower. Consider that 35,000 tons of fungicides are used each year in France for these pathogens—and eight to 15 fungicide sprays here in the humid East—because grapevines, and particularly Vitis vinifera varieties such as Chardonnay, Pinot noir, and White Riesling, are so uniformly susceptible to these diseases.

Eliminating the need for these fungicides would be a huge environmental advantage, addressing a serious weakness in traditional wine varieties. This prospect is enough to entice researchers from places like Colmar, France—in the heart of the Alsace region—to consider the possibility that future growers might be persuaded to replant their Riesling and Gewurztraminer vineyards with offspring that share their noble characteristics but are also resistant to these important diseases.

Does that sound improbable? Well, it does to me, too, but continuing to discover and develop new fungicides to stay ahead of these diseases sounds unsustainable in the long run as well. And Europeans are feeling some urgency, because the European Union is considering banning some of the fungicides used for powdery and downy mildew as early as 2013.

So many in the general public associate genetic science with genetic engineering and the controversial "GMOs"—genetically modified organisms—some of which are banned in Europe and parts of the United States. These new varieties will not be GMOs. The plants are produced by traditional crosses. But the way they are evaluated and screened is genetically precise. Instead of relying solely on field observations and guesswork, breeders will now be able to use new genetic tools to select those that contain the right genes and discard those that do not.

The hope of these scientists is that this "marker-assisted selection" will dramatically speed up the breeding process—and get resistant grapevines into growers' hands in 15 to 25 years instead of 50 years. Our Cornell grape breeder, Bruce Reisch, thinks this may be overly optimistic—but is already using 'marker-assisted' selection to screen seedlings from his program and select parents that he knows carry the appropriate resistance genes.

I wish Bruce and his colleagues success in their efforts to come up with more sustainable, disease resistant grapevines that don't require such intensive disease management. And I'm hopeful that the new genetic markers will greatly speed up the process.

Reference:

Program and Abstracts, 10th International Conference on Grapevine Breeding and Genetics, 1-5 August, 2010, Geneva, NY.Cornell University, College of Agriculture and Life Sciences, New York State Agricultural Experiment Station, Geneva, New York.