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Vineyard Vistas

Vineyard scene along Oakville Cross Road this month .

As the historic Mendocino Complex fire rages a safe distance to the north, hillsides in the upper Napa Valley were recently wreathed in its signature haze for days on end.

Clear skies have returned, but many have wondered if the temporary stratum of smoke had any ill effect on the ripening grapes in vineyards dotting those hills?

Hardly, growers say.

Caleb Mosley, senior viticulturist with Michael Wolf Vineyard Services, recalled a string of days when the smoke enveloped the hillsides. “There were maybe a handful of mornings that it was really bad; maybe three or four days,” he said.

To Mosley’s knowledge, the valley floor escaped the haze untouched.

“I’ve heard of a few people that have done smoke taint tests and so far nothing has come back with any positives,” he said.

One of the key providers of smoke taint testing and wine analysis in the valley is ETS Laboratories in St. Helena, whose co-founder, Gordon Burns, was blunt: “I think there’s not a story here.”

“There’s no evidence at this point that it’s in any way analogous to what happened last year,” Burns said. “In short, we don’t have any evidence at this point that there’s something to be concerned about for the Napa Valley.”

Last year, some late harvesting vineyards in the Napa Valley did have smoke taint from the October wildfires that burned on three sides of the valley for days.

The Napa Valley Grapegrowers also dispelled the possibility of harm from the haze, stating this week, “There is no indication of any fire-related issues to Napa Valley’s 2018 grapes.”

Mosley said the risk factor for smoke taint lies along what he called a “gradient of susceptibility.”

The gradient begins around veraison, when cell division in the berries stops and the vines begin to load sugar into the fruit. As veraison progresses, the skin of the grapes softens, raising the risk that smoke compounds may combine with the sugar molecules in the fruit, leading to the dreaded smoke taint.

“It’s not like it starts off at 100 percent susceptibility,” he explained. “It’ll start off at a very low level and ramp up as ripening continues.”

Today, more questions than answers exist on the exact causes of smoke taint. How much smoke does it take at a given point in the grapes’ development to make a significant impact? Would light smoke for a few weeks be worse than a fire burning near a vineyard for a few hours?

“I just don’t know,” Mosley said. “And I don’t think anyone knows really how to quantify the amount of smoke and the duration that it’s out there and the results in smoke taint you might end up with. It’s still so darn new.”

Ironically, hillside grapes that may have been in contact with the smoke have also been offered some protection by their elevation. Unlike those growing on the valley floor or in the Carneros region, which are now beginning to be harvested, Napa Valley’s hillside grapes largely begin to ripen later in the growing season.

“The nice part about those higher elevation vineyards is that they’re behind in terms of their development,” Mosley said. “The window for susceptibility hasn’t even opened yet.”

At this point in their phenology, he pointed out, grapes higher up would have been less at risk than those on the valley floor, which went untouched while the smoke was present.

On the opposite end of the gradient, grapes that are at their ripest naturally would tend to be the most at risk. Such was the case when the October wildfires began last year. Though the majority of grapes had been harvested when the fires began, those that were still on the vine likely would have been at or near their peak of susceptibility.

“But we’re certainly not anywhere near that point yet this year,” Mosley said.

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Wine Reporter / Copy Editor

Henry Lutz covers the local wine industry. He has been a reporter and copy editor for the Register since 2016.