A week after unwanted rains began to hit vineyards in Sonoma County, some grape growers are reporting significant signs of damage to the county's $400 million crop.

For many growers, it's too early to quantify just how much of the crop has fallen victim to the bunch rot called botrytis. Most are too busy attempting to control the damage Mother Nature wrought — harvesting if the sugar levels are right, spraying fungicide on grapes that need more ripening time, and pulling leaves out of canopies to help eventual sunshine and winds dry out the clusters.

"It's taken a toll on everyone, on the grapes as well as everybody's outlook," said Saralee McClelland Kunde, owner of Saralee's Vineyard in the Russian River Valley. "There's a lot of botrytis out there. We're all seeing it."

Rainy weather returned to Sonoma County on Monday after a three-day dry interlude. Monday's storm dropped less than a quarter-inch of rain on most parts of Santa Rosa, according to the National Weather Service. Forecasters expect a few showers early Tuesday morning, and then warm, dry weather until the weekend. But Saturday may bring another bout of rain.

"It's hard to say what it's going to be like then," said Chris Stumpf, meteorologist. "I imagine there will probably be some rain there Saturday afternoon."

The botrytis problem is becoming serious for some growers in the Russian River Valley, where the cooler climate delayed ripening of chardonnay grapes. Glenn Alexander, owner of Bacchus Vineyard Management, said he lost 50 percent of one zinfandel vineyard to botrytis, and lost 15 to 30 percent of his chardonnay grapes to the fungus.

"It's going to be worse in a couple of days," Alexander said. "Its growth rate is exponential."

The rot problems caused by the rain are hitting growers twice, Alexander said. First, growers have to pay for crews to cut leaves out of the canopies, to expose the fruit to the drying sunlight and wind. And then, the grapes that are infected with botrytis are unusable, so there is a smaller crop to harvest and sell.

"It's been here with a vengeance," said John Clendenen, owner of Clendenen Vineyard Management in Dry Creek Valley, about the spread of botrytis. "Wine grapes are not designed to take this kind of rain at this time in the vineyards."

Clendenen was checking weather forecasts every 15 minutes on Monday, trying to plan the best course of action.

For the grapes that haven't yet reached the desired sugar levels, the decision of whether or not to pick is a gamble: Growers must balance their desire to leave them on the vine and let those sugars develop, against the risk of allowing rot to spread through the vines.

As a result, some wineries are dropping the minimum sugar levels they require, said Duff Bevill, founder of Bevill Vineyards Management. In other cases, growers may end up renegotiating prices with the wineries that contracted to buy their grapes, he said.

"Typically in the grape contracts, there's some minimum sugars that are expected," Bevill said. "So if the sugar levels are below what the contractual specifications are, then you typically engage in some kind of discussion."

The extent of rot problems throughout Sonoma County varies depending on the location of the vineyards, the type of grape, and the techniques used by the grower, said Rhonda Smith, viticulture farm advisor at the UC Cooperative Extension in Sonoma County.

"It's very site-specific," Smith said. "It would be really difficult to paint a broad brush stroke of the percent of damage across the county."

Weather conditions were favorable enough for some growers to harvest all of their varietals that are susceptible to rot before the rains began.

"The damage was not as bad as we had feared," said Christopher Silva, president and CEO of St. Francis Winery & Vineyards. "The sugar levels are still in reasonable range."

Most thinner-skinned grapes like chardonnay were already harvested at St. Francis. But the intermittent rains are making Silva wonder whether late-ripening Bordeaux varietals, like cabernet sauvignon or merlot, will achieve their sugar levels before winter temperatures settle in.

"There will come a point when we will have to call in all of our fruit. This is not that point," Silva said. "Anyone who thinks that harvest should be calm and predictable is in the wrong business."