The full impact of this season’s unusual spring freeze in the Napa Valley remains a question mark.

Greg Clark, assistant agricultural commissioner for Napa County, has been collecting reports from grapegrowers around the county. While some crop losses may be evident now, he said, the total impact may not be known for several months or even until harvest.

“The extent of the damage is unknown at this time,” he said.

This year’s frost hit the Napa Valley and beyond, from Carneros, Pope and Chiles valleys to north of Calistoga. Sonoma, Lake and Mendocino counties also saw frost damage from weeks of below-freezing nights after bud break.

So far, about 50 Napa County growers have reported crop loss adding up to about 640 acres, said Clark, who does not have dollar estimates. Napa Valley cabernet sauvignon fetches more than $4,000 per ton while even sauvignon blanc, the least expensive of the dominant grapes here, goes for nearly $2,000 a ton.

“It’s quite possible that we will not be able to make white wine this year,” said Volker Eisele at Volker Eisele Vineyard on Lower Chiles Valley Road, one of the coldest areas in the Napa Valley, where temperatures dropped to the 20s.

Eisele said his family’s 60-acre property experienced 19 frost nights over three weeks this season.

While his sauvignon blanc showed widespread damage, Eisele couldn’t put hard numbers on the damage to this year’s crop.

Assessing the damage on the 5,500 vines on the property at this point is very difficult, said Eisele, as he showed canes carrying both burned and apparently healthy buds. Secondary shoots may push but these may or may not carry grapes. Healthy buds may not necessarily carry much crop.

“How do you assess that? Impossible,” he said. “We won’t have zero. But how much is anybody’s guess.”

The information from the reports the county is gathering from growers could be used to apply for state or federal disaster aid — usually in the form of low-interest loans, according to the Napa County Farm Bureau. The farm bureau is encouraging its members to report the damage to the Agricultural Commissioner’s office.

Al Wagner, vineyard manager at Clos Du Val, reported damage at his vineyards. So did Patty Saldivar, director of vineyard operations at Hall’s estate vineyards. The Halls own more than 300 acres in Napa County, including in Carneros and the Mayacamas Mountains.

The cold temperatures caused damage in vineyards lacking frost protection, such as sprinklers or wind machines.

Wind machines mix warm air with the cold air mass settled on the ground, according to Saldivar, while sprinklers are used to spray water because heat energy produced from the freezing water protects the shoots.

Frost may be the least of the wine industry’s worries this year. Eisele said other factors outside his control may hurt him, like the weak economy.

“How many people will want to pay $20 for a bottle of wine, let alone $40 or $60?” he asked.