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By Karen MacNeil
November 30, 2018

This time of year, scores of photographers and wine lovers line up along the roads of every wine region and whip out their cameras and phones to photograph grapevine leaves turning glorious shades of red, crimson, and garnet. It’s like New England in the fall, watching the leaves turn color.

Or is it?

Alas, not. Red leaves on a grapevine are the symptom of a virus. Indeed, where we see beauty, winemakers and viticulturists may see a potential problem. I asked Cathy Corison, proprietor/winemaker of Corison Winery in the Napa Valley, to explain.

“Most red leaves indicate leafroll virus (aka red-leaf virus) or another type of virus,” said Corison. “In the absence of virus, all leaves—from grapevines growing both red and white grape varieties—turn yellow as they deteriorate with age. Eventually, the yellow leaves fall to the ground when the vine goes dormant.”

I first learned about the red leaves/virus connection about 20 years ago. It was startling, and I assumed it was bad news.  Surprisingly, many winemakers back then assured me it wasn’t—necessarily. One winemaker described a vine with virus as being like a person with a slight cold—the cold held you back a little and made you slower and less efficient. This “slowdown,” he said, could be good for a grapevine.

Other winemakers I knew disagreed and were quick to pull out virused plants. And I knew from research that enology and viticulture schools like U.C. Davis disapproved of virused plant material.

Corison makes a famous Napa Valley cabernet from a 40-year-old vineyard called Kronos. It’s not far from where I live and driving by the vineyard, I’ve seen red leaves. I asked Corison how it could be that her most lauded cabernet comes from a vineyard with red leaves.

“In the Kronos Vineyard, I think it just might be my secret weapon,” she said. “All St. George rootstock (which is the rootstock planted in the Kronos Vineyard) has some leafroll virus. I am no expert in plant viruses and they are very complicated, but in my vineyard, the virus seems to slow down sugar accumulation in the grapes, while every other component of ripening moves forward. That allows me to pick fully ripe grapes at lower sugars and hence lower alcohol. And that adds up to a more balanced wine.”

Hmmm. Maybe photographing those red leaves isn’t so bad after all.