Sonoma County olive growers are bracing for a disastrous harvest, one that could wipe out the supply of fresh local olive oil.

The late-season rains of 2011 drenched the North Coast about the same time olive trees were blooming, killing the pollination process and leaving trees with little to no fruit. Some growers are estimating that the small, niche crop will be a tenth of normal size.

"We knew as soon as the weather report said it was going to rain that we were going to have trouble," said Ridgley Evers, co-owner of DaVero, a high-quality olive oil producer in Healdsburg.

"But we didn't know how much trouble we were in until we saw how much rain we got," he said.

That was three months ago. Now, the proof of what Evers has described as a slow-moving train wreck is illustrated by the bare branches of his Tuscany-bred olive trees.

"Economically, this is a full-on disaster," he said.

Even before the late rains, growers knew it wouldn't be the best harvest for Sonoma County. Olives are an alternate bearing crop, and this was supposed to be a down year.

But the moisture was a nail in the coffin for this year's harvest.

Paul Vossen, a farm advisor with the University of California Cooperative Extension, said that in an average low-yield year olive orchards produce 50 to 60 percent less.

"We have some places where they have almost nothing," said Vossen.

Unlike some plants and flowers, olives are pollinated by the wind, not insects. Sonoma County olive trees bloomed in late May and early June, when the North Coast was being hit by one storm after another.

High humidity alone will cause the fine pollen produced by olive trees to bind together, hindering its ability to travel through the air for cross pollination. With rain, pollen simply clumps together and doesn't go anywhere.

Olive production in Sonoma County is tiny compared to wine grapes and apples, according to the 2010 county crop report. That year there were 567 olive olive orchards in the county, 465 of which bore fruit for a total raw crop value of $176,100.

In contrast, the raw value of Sonoma County wine grapes in 2010 was more than $390 million.

But the true value of local olives is derived from their conversion to olive oil.

"This is the lightest yield of olives I believe I have seen here since I began producing our Estate Picholine Olive Oil in 1990," said Bruce Cohn, president B.R. Cohn Winery & Olive Oil Company in Sonoma Valley. "Everyone should stock up on last year's olive oil!"

Cohn said Vossen's assessment is on target. "I do not see much on our trees at all."

Vossen said a fruitless harvest would likely affect speciality buyers of fresh, locally produced olive oil, which in Sonoma County is mostly Tuscan-style varieties such as Frantoio and Leccino.

He said Sonoma County, which consumes more olive oil than it produces, likely will have to get its fresh oil elsewhere.

The Central Valley did not get as much rain during its bloom season, Vossen said. Also, olives grown in the valley are mostly Arbequina, which are less alternate-year bearing.

Meanwhile, DaVero's Tuscany-style olive trees, which were grown from cuttings from an 800-year-old grove outside of Lucca, could come back next year with a vengeance.

Evers said he has contacted his roster of 1,000 buyers to let them know there will be no fresh olive oil this year. And he has asked them to come back in 2012.

"That's assuming we have fruit next year," he said. "If we don't, we're going to be in the olive tree lumber business."