CALISTOGA, CALIFORNIA (REUTERS) - Firefighters in northern California on Tuesday (Sept 29) struggled to make headway against two wildfires, one of them threatening towns and wineries in Napa Valley, and another blaze that killed three people in the Cascade foothills closer to the Oregon border.

The three fatalities in the so-called Zogg Fire that erupted on Sunday in Shasta County, about 322km north of San Francisco, were reported Monday evening by the local sheriff. As of Tuesday there were still no details on how or when they perished.

All three were civilians, and their deaths brought to 29 the number of people killed since mid-August in what now stands as the worst year on record for California wildfires in terms of acreage burned.

Farther south, the Glass Fire also raged for a third day in wine country. Already it had destroyed a highly rated restaurant and buildings at a winery whose architecture was inspired by a 13th-century Tuscan castle.

Another popular winery, the mansion-like Chateau Boswell, went up in flames on Sunday night.

Wine industry officials said the longer-term ramifications to their industry of the Glass Fire and a spate of other blazes that came before it is likely to be a 2020 vintage of diminished volume because of grapes spoiled by heavy exposure to smoke.

Some 80,000 people have been placed under evacuation orders, including all 5,300 residents of Calistoga, a resort town known for its hot springs and mud baths and the site of the landmark castle complex of Castello di Amorosa.

"We still have high heat and low humidity that can spread large fires," said Ms Lynette Round, a spokesman for the California Department of Forestry and Fire Protection, or Cal Fire.

Both fires were zero-per cent contained, though calmer winds could give firefighters an edge on Tuesday, Cal Fire officials said.

The Zogg fire, burning near the town of Redding, has destroyed 146 structures and charred more than 161 sq km of grassy hillsides and oak woodlands thick with dense, dry scrub.

About 15,000 structures were listed as threatened, and 2,200 residents were under evacuation orders or advisories.

After merging with three other blazes, the Glass Fire had spread across more than 170sq km in Napa and Sonoma counties, incinerating at least 80 homes and 32 other structures, according to Cal Fire.

Napa Valley residents Matthew Rivard and Amanda Crean parked their car by a sign reading "Welcome to the World Famous Wine Growing Region" on Monday night and watched flames surround the Schramsberg Vineyards, known for its sparkling wines.

WINE COUNTRY HAVOC

A short distance to the northwest, flames destroyed a farmhouse containing a trove of bottled wine and offices at the Castello di Amorosa winery, but its distinctive castle remained intact, the San Francisco Chronicle reported.

To the east, the three-star Michelin restaurant, the Grill at Meadowood, and a golf shop were destroyed at the Meadowood Napa Valley resort, the newspaper reported.

In Calistoga, the evacuation left its main street, known for boutiques and tasting rooms, looking like a ghost town, according to a Reuters photographer.

As of Wednesday, no wineries were reported to have burned in neighbouring Sonoma County, though a "couple of outbuildings and accessory buildings" were damaged, said Mr Michael Haney, executive director of the Sonoma County Vintners trade group.

The Glass Fire struck midway through the traditional grape-harvesting season in Napa and Sonoma counties, both world renowned among California's wine-producing regions and still reeling from a cluster of large wildfires earlier this summer.

The full affect on the region's wine business remained to be seen. But Mr Haney said vintners would likely scale back production of certain wines due to smoke exposure to grapes still on the vines when the fires struck.

"I do know there are wineries saying we have been impacted and we won't be making as much wine," he said. Several Napa Valley growers said recently they would forgo a 2020 vintage altogether due to smoke contamination of their crop.

The blazes in Shasta County and wine country marked the latest flashpoints in a destructive spate of wildfires this summer across the Western United States.

California fires have scorched over 15,378 sq km since January - far exceeding any single year in state history. They have been stoked by increasingly frequent and prolonged bouts of extreme heat, high winds and dry-lightning sieges that scientists attribute to climate change.

More than 7,000 homes and other structures have burned statewide this year.

Red-flag warnings for extreme wildfire risk remained posted for much of northern California.