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CORVALLIS, Ore. - Researchers have determined that the pathogen that causes sudden oak death, a major concern for both forests and nurseries in the United States, has three distinct lineages whose origins are still unknown but have been around for hundreds of thousands of years.

The separate lineages and the development of genetic markers is now allowing experts to track this deadly plant pathogen more accurately, and hopefully help control its spread, researchers say. But the existence of three lineages also opens the door to sexual, rather than clonal reproduction - which could allow the pathogen to "shuffle" its genes fairly quickly and become even more virulent.

Even now, sudden oak death is killing forests on the West Coast, causing 80 percent tanoak mortality in some places, and has been a costly problem for the export business of Pacific Northwest and California nurseries since first identified in the U.S. in the 1990s.

"We still don't know where sudden oak death evolved, but it's clear there are at least three separate lineages that have been separated for 100,000 to 500,000 years," said Niklaus Grunwald, an associate professor of plant pathology at Oregon State University and researcher with the U.S. Department of Agriculture's Agricultural Research Service.

"Two of these lineages are of one mating type and the other of the opposite, raising concerns about sexual reproduction," Grunwald said. "This could allow what's already a very destructive pathogen to change and more easily adapt to new hosts or become more virulent.

"At one point I thought this was inevitable," Grunwald said, "but it may be the lineages have been separate so long they can't sexually reproduce. We just don't know for sure yet."

What is clear, researchers say, is that the threat posed by this pathogen, called Phytophthora ramorum, is serious and growing. The presence of three different lineages in the U.S. suggests that it was introduced on at least three separate occasions. Since those introductions, plants infected with the pathogen have been shipped to many states, but aggressive inspections have helped to control its spread.

Researchers from OSU and the USDA are continuing to work on control measures, and have traveled to Asia looking for the native home of the pathogen, so far without success. Sudden oak death can also cause non-lethal damage to many types of plants and trees common to the Pacific Northwest, including azaleas, big leaf maples, huckleberries, California bay laurels, camellias, myrtles, honeysuckles, Pacific madrones, Douglas firs, rhododendrons and others.

The newest findings on the genetic makeup and movement of the different lineages of this pathogen were recently published in PLoS Pathogens, a professional journal. The work has been supported by the USDA.