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66 WINES&VINES February 2018 GRAPEGROWING E very year, winemaker Greg Allen waits as Napa Valley Sémillon takes its sweet time to mature. His staff and a team of researchers from the Univer- sity of California, Davis, have demonstrated that grapes from John's Creek vineyard are as hard at work in early November as the Far Niente field crew gearing up for late harvest. Part of the 11,000-acre Coombsville sub- AVA, the 17-acre vineyard straddles some of what's left of a volcanic vent that collapsed under its own weight 5 million years ago. The crescent-shaped caldera borders the Napa River to the west, the Vaca Range to the east and Mount George to the north. Warm air rising from the valley floor draws coastal fog off the San Pablo Bay, moderating daytime temperatures and fueling a seemingly endless summer of misty mornings, warm days and moist nights. A temperate autumn extends the vintage, lengthening hang time and delay- ing harvest. The valley's maritime climate, hillsides that block the wind and trap humidity, and well- drained, nutrient-poor soil make the grapes ripe for Botrytis cinerea. Unlike bunch rot, which saps life from vines under a swath of moist mid-summer air, noble rot enhances the aromas, color, flavors and texture of wine made from late-harvest grapes, such as Far Niente Dolce. By late fall, the grapes have ripened. Botrytis, a fungus that overwinters in the soil and on the vine, has drained moisture and nutrients from the infected berries and is re- writing the sensory profile of Sémillon grapes. "Noble rot reprograms berry development and metabolism," says Dario Cantu, plant ge- neticist and professor from the Department of Viticulture and Enology at UC Davis. "The in- teraction of the fruit and fungus stimulates metabolic pathways otherwise inactive in white- skinned berries, leading to the accumulation of additional compounds and creating the unique flavor and aroma of botrytized wines." When conditions are right—still air, high humidity, light rain and air temperatures that range from 58° to 82° F—the velvety gray fungus penetrates cracks in the cuticle of the grape, softening and dehydrating the berry. Threadlike hyphae trigger the nuclei of host cells to express genetic traits that redefine the sensory attributes for late-harvest wine. The sugar, acid and trace minerals that the mold leaves behind help define its character. Sweet idleness Twenty years of experience cultivating, har- vesting and fermenting late-harvest grapes has turned Greg Allen's world inside out. "The key to making late-harvest wine is to reverse con- ventional wisdom," he says. Most vignerons look for ways to reduce mold in the vineyard. Allen searches for ways to cultivate it. The vineyard team has developed a "T-top" trellis for ripening late-harvest grapes. "Nor- mally, a vineyard manager would create an open canopy allowing sunlight into the fruiting zone," he says. "We've developed a trellis that shades grapes from sunlight, protects them from the wind and traps humidity." Each step—from growing Sémillon grapes to aging Dolce late-harvest wine—lengthens the journey and intensifies the vintage. While the pink hue of Botrytis can show up in individual grapes anytime from Septem- ber through December, crew members do not harvest these thin-skinned, tightly clustered grapes until the berries turn purple. Once the KEY POINTS Botrytis cinerea triggers the nuclei of host cells to express genetic traits that redefine the sensory attributes for late-harvest grapes. Unlike bunch rot, noble rot enhances the aromas, color, flavors and texture of late- harvest grapes. Noble rot accelerates the development of grapes beyond what's merely ripe; red blotch turns back the clock shortly after véraison. How Semillon Grapes Adapt to Botrytis Researchers describe the effects of noble rot on Far Niente Dolce By Thomas Ulrich Noble rot prompts Sémillon grapes to express a unique combination of metabolic activities, according to Dario Cantu of UC Davis.