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28 WINES&VINES December 2017 Viewpoint I f you are growing one of the 20 or so most popular interna- tional wine grape varieties, you are dealing with a very nar- row slice of the genetic heritage of the genus Vitis. Chances are that the variety in your vineyard originated somewhere in the 15th or 16th century in the middle of Europe. Most certainly, the genetic makeup of your vines predates the introduc- tion of a suite of pests from North America, such as grape phyl- loxera, powdery mildew, downy mildew and black rot. Classic wine grape cultivars and their names have spread throughout the globe, but their genetics are frozen in the Middle Ages. This is in stark contrast to other horticultural and agronomic crops. What if fruit breeding had stopped in the 1600s? Peaches would be the size of cherries, watermelons would have six small pockets of red flesh divided by fleshy white tissue, and bananas would have large seeds. Are traditional wine grape varieties— wonderful as they are—impervious to improvement? Many of the name-brand cultivars are closely related to each other. A decade ago, a USDA unit based at Cornell University produced a custom-made genetic array (dubbed the "SNP chip") that provided a way to detect more than 5,000 single nucleotide polymorphisms (SNPs) and allowed a 100-fold more detailed map of the grape genome than was previously possible. They tested 583 unique grape cultivars from germplasm collections and found that roughly 75% of them were closely related (siblings or parent-offspring) to at least one other cul- tivar (Myles et al. 2011). More than half (58%) were part of a single genetic network through sibling/parent relationships. It's now common knowledge that Cabernet Sauvignon was the offspring of Cabernet Franc and Sauvignon Blanc (Bowers & Meredith, 1997). But the map from Myles et al. (2011) shows that many of the most well-known cultivars are just one or two steps distant from four cultivars: Riesling, Traminer, Pinot Noir, and Sauvi- gnon Blanc (see "Relationships Among Grape Cul- tivars" on page 29). If not siblings or progeny, many are at least first or second cousins. The genetics are frozen, but the rest of the grape- vine ecosystem has not stood still. Phylloxera caused Europeans to graft their classic wine grapes onto re- sistant American rootstocks. Powdery mildew prompted the introduction of sulfur and Bordeaux mixture. And currently, downy mildew, which grows explosively and can defoliate a vineyard in a week under the right conditions, is the key pathogen driving spray programs in Europe and eastern North America. There are newer challenges not encountered by V. vinifera during the species' evolution, such as Pierce's disease and grapevine red blotch disease. These are challenges that the current varieties have not been selected to resist. Consequences The end result of these frozen genetics is that we need to in- tervene—often, and in a timely manner—to bring in a disease- free, high-quality crop. A large part of vineyard management is devoted to applying "Band-Aids" to compensate for notable weaknesses in our current wine varieties. For Chardonnay growers in the humid East, this amounts to the need to apply eight to 15 carefully timed fungicide sprays and to practice careful canopy management in order to combat the five major pathogens that Chardonnay is not genetically equipped to resist. While we have been successful in developing and using these tools, it's not at all clear that continued success is sustainable. Public perception of health risks of sprays (justified or not) grows more contentious and will place new limits on what growers are able to do. Of equal concern, disease organisms adapt quickly to new chemistry (except for sulfur, which is still effective after more than 100 years). But many of the most recent ones have been rendered ineffective by resistance within a few years of their introduction. There's no guarantee that new products will come along at the right time to replace them. To add to this list, climate change will radically reshape our agriculture in the next century. Will our current varieties be able to cope? Will changes in manage- ment offset changes in climate? Or will we need new varieties? Genetic alternatives Many of the weaknesses of current varieties could be addressed by broadening the nar- row genetic base of our current cultivars. V. vinifera germplasm is more diverse than what is represented in the classic cultivars. But even more unexploited genetics exist in North America and China. There are about 30 species of Vitis grapevines in North America, and those vines evolved alongside phylloxera, pow- dery mildew and other diseases that originated in North America. Geneticists and breeders have, to date, identified multi- ple sources of resistance to pow- dery mildew, downy mildew and phomopsis. Several inter- specific hybrids grown widely n TIM MARTINSON The Frozen Genetics of International Wine Cultivars