Grape growers in Sonoma County have harvested nearly half of their $400 million crop, fighting rain and rot for a week as they hustled to get fruit off the vines.

But despite their extraordinary efforts, the overall crop yield could be the smallest in seven years, according to estimates Friday by Nick Frey, president of the Sonoma County Winegrape Commission.

Frey said chances are "pretty good" that harvest volumes will fall below 2008 levels, when growers brought in 168,992 tons. The last time yields fell below that level was in 2004.

"It's not going to be good for growers on the income side," Frey said.

A week ago, only about 20 percent of the grapes had been harvested. More than half of the county's largest grape crop, chardonnay, is now in winery tanks, Frey said. But of all the varietals, the chardonnay that remains on the vines is the most susceptible to rain damage, due to its thin skin and the amount still left in the vineyard.

"The good news is the sun's out, and there was a breeze yesterday so it dried things out," Frey said. "Growers are out removing leaves, getting some air through the clusters, so hopefully with that the grapes will maintain good health until they get ripe, and probably until they're harvested."

Growers said it was still too early to tell the extent of damage caused by botrytis, or "bunch rot," which is already present in small amounts in the vineyards, and may have been encouraged by the rains. It takes up to a week after rain for real botrytis problems to appear. A small grayish dot, the size of a dime or quarter, appears on a grape as one of the early signs, said Duff Bevill, founder of Bevill Vineyard Management.

"In a very short period of time that thing, almost in hours, can engulf a major part of that cluster," Bevill said. "The right kind of organisms are out there, they're just waiting for the right conditions."

About 90 percent of the chardonnay grapes grown in the Russian River Valley, which produces some of the most valuable chardonnay in the county, is still on the vine, Bevill said.

"We've got the potential for picking on some of the biggest blocks of chardonnay, hopefully this weekend and into Monday and Tuesday," Bevill said. "We're trying to get the go-ahead from wineries, but they're still impacted by the big rush over the weekend."

The influx of grapes into wineries in the past week — as much as 50,000 tons — amounts to a huge traffic jam in fermentation tanks.

"The ground, when it's wet, it makes a bunch of humidity, which allow the spores to multiply," said Steve Sangiacomo, partner at Sangiacomo Family Vineyards. "But you never know. It's a &‘wait and see' game."