Grapegrowers just can't catch a break this harvest season.

First the damp, cool summer led to destructive mold and mildew in vineyards.

Now the sudden, scorching heat of the last two days has sun blasted their grapes, leaving some vineyards with extensive crop loss.

"We have a few blocks that suffered up to 35 percent sun damage," said Steve Hill, general manager of Durell Vineyard in Sonoma. "That fruit is ruined. It will turn into raisins."

The amount of damage varies from vineyard to vineyard. Grape vines sheltered from direct sun had little or no damage, while areas that were directly exposed to afternoon sun were devastated.

Damage to the county's expected 195,000-ton harvest will become more evident in the coming days as growers survey their vineyards, said Nick Frey, president of the Sonoma County Winegrape Commission.

The sun damage comes just as growers were fighting against the exact opposite problem — not enough sun.

In previous weeks, growers had taken aggressive action to stop bunch rot, a destructive mold that was triggered by this summer's unusually cool, foggy weather.

Growers had pruned back leaves to expose fruit to maximum sunlight in an effort to dry out grape clusters during the day. That strategy prevented bunch rot, but left grapes susceptible to overexposure, said Kyle Cameron, who manages two small vineyards in the Russian River.

"Everyone opened up their grapes to get as much sunlight as they could," Cameron said. "Then the sun came out and fried them."

The recent blast of heat was a setback for growers who had been hoping for a stretch of 90-degree days to kill off the bunch rot and mildew.

"Everyone had been doing a sun dance, but this heat was way over the top," said George Martinelli, vineyard manager at Martinelli Vineyards & Winery west of Windsor. "The skins of the berries never had a chance to acclimate to the sun."

On Tuesday, temperatures began dramatically rising from 48 degrees to a blazing 108 degrees by late afternoon, said Hill, of his Sonoma Valley vineyard.

"The heat was so extreme and so fast, the grapes just weren't ready for it," he said.

The rapid change in weather is likely a record, or close to one, Frey said.

"It was probably about as abrupt a change in weather as you'll see," he said. "And in three days we're going to be right back to where we were with cooler temperatures."

Temperatures this weekend are supposed to drop back into the 70s, and there is even a slight chance of rain on Saturday.

A steady rain at this point could ruin the harvest for many growers because bunch rot would likely reappear, especially in clusters with sun damage.

"We need a nice normal fall with no significant rain until we get into November," Hill said. "That's what we're praying for. I don't know if it will happen."

This season has proven one of the most stressful in recent memory for growers. The economy has pushed grape prices down while leaving some growers wondering if anyone will even buy their crop.

The European grape moth invasion has forced growers to spend extra money fighting against infestation. Add to that the persistent problems with mildew and mold, and now sun burn, and this is a season most growers will want to forget.