(CNN) -

A 4,300-acre wildfire in California's Napa County was barely under control Thursday and is now threatening at least one vineyard in a nearby area, authorities said.

"There are some small vineyards in the general area but nothing major," said Cal Fire spokesman Daniel Berlant.

The Langtry Estate & Vineyard in Middletown in nearby Lake County was advised to evacuate, Cal Fire spokeswoman Stacie McCambridge told CNN affiliate KGO.

About 500 people have been evacuated in the Butts Fire, Berlant said Thursday.

More than 1,000 firefighters were struggling Thursday to contain the wildfire in Napa and Lake counties. They had only 30% of the blaze under control, Cal Fire said.

Residents described how firefighters defended homes from a spectacular wildfire.

"The firefighters are amazing. We have watched over the past two days as they have successfully defended the borders of the fire as it was approaching residences," said Lori Lynn Todd, who posted photos of the fire on her CNN iReport page.

"Those pilots are beyond brave, swooping down the hillsides and into the billowing smoke to suppress the advance of the fire," said Todd, 50, a finance executive who lives in Pope Valley, California, within a mile of where the fire started. "The fire retardant breaks look like orange stripes across the hill sides."

Todd's home was spared from the fire, she said.

Firefighter Mike Forster said the blaze "is a reminder of just how dry things are here in California," which is facing a drought emergency.

"We are about six weeks ahead of where we normally are in fire season in regards to how dry the vegetation is," said Forster, who also posted photographs of the fire on his CNN iReport page.

He noted how the fire has burned almost seven square miles with less than a third under control.

"So we still have some work to do," said Forster, who's from Walnut Creek, California.

So far, two residences and seven outbuildings have been destroyed, fire officials said.

Firefighters were defending about 180 structures Thursday and were laboring in high heat, humidity and the possibility of erratic winds in steep, rugged terrain in rural areas of the two counties, Cal Fire said.

The cause of the wildfire is under investigation, authorities said.

The blaze has grown 75 percent since it began Tuesday and spread rapidly to 2,500 acres on the same day, prompting the immediate evacuation of about 150 residences more than 20 miles outside the town of Napa, authorities said.

As California faces what officials call probably the worst drought in a century, fire officials say it will exacerbate the state's wildfire season, which appears to have begun months earlier than normal.

Gov. Jerry Brown has asked all Californians to reduce voluntarily their water use by 20 percent, and Cal Fire Chief Ken Pimlott is urging the public to use extreme caution with fireworks during the Fourth of July holiday weekend.

"The drought has increased the threat of wildfire in California," Pimlott said in a statement. "We are urging the public to celebrate our nation's independence by attending a professional fireworks display. ... You don't want to ruin your celebration by becoming financially responsible for a fire caused by your activities."