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New Clues in Oak-Scourge Puzzle

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The pathogen responsible for Sudden Oak Death first got its grip on California’s forests outside a nursery in Santa Cruz and on Mt. Tamalpais in Marin County, before spreading and eventually killing millions of oaks and tan oaks along the Pacific Coast. A new study led by Matteo Garbelotto, associate extension specialist and adjunct professor of forest pathology, provides evidence of how the epidemic unfolded in this state.

The study shows that the pathogen is currently evolving in California, with mutant genotypes appearing as new areas are infested. These findings suggest that movement of infected plants between different regions where Sudden Oak Death is established should be minimized, said Garbelotto.

From nearly 300 samples of the fungus-like pathogen, the researchers identified 35 unique strains. A computer analysis further revealed how those strains were related to each other. Armed with that information, the researchers were able to create a history of the epidemic.

Two sites emerged as the origin of Sudden Oak Death in California: Bean Creek in Santa Cruz County, located just outside a nursery that had been shut down because of its Sudden Oak Death infestation, and the Bolinas Ridge site in Marin County’s Golden Gate National Recreation Area on Mt. Tamalpais, near the site where the disease was first observed in California.

“Although our study identified two locations from which the Sudden Oak Death pathogen spread to other parts of California, the close proximity of the site in Santa Cruz to a nursery makes it highly probable that the epidemic began there rather than at Mt. Tam,” says Garbelotto.

The most likely scenario, says Garbelotto, is that the pathogen arrived in California through the nursery trade, and then spread from the nursery in Santa Cruz to trees bordering the facility. The site at Mt. Tamalpais is not adjacent to a nursery; however, there is anecdotal evidence of frequent use of ornamental plants from nurseries in landscaping projects in the area, said Garbelotto.

Based upon the genetic analysis of this study, the disease could have then progressed to other parts of California’s coast, including Sonoma County and Big Sur.

The study also found that strains from areas of recent infestations are more genetically distant from current nursery strains, which suggests that regulations controlling the spread of the pathogen from the nursery to the wild are working. “Nurseries do not seem to be the source of current outbreaks of Sudden Oak Death in the wild,” says Garbelotto. “Unfortunately, evidence shows that the pathogen is spreading on its own in the wild. New strains are evolving.”

-Sarah Yang

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