Phytophthora ramorum evolves in California

A University of California-Berkeley study found Phytophthora ramorum is evolving in California forests, with unique genotypes appearing in newly infested areas.

Samples from the wild were compared with 15 P. ramorum isolates collected from nurseries in 12 states. Matteo Garbelotto, University of California-Berkeley associate extension specialist, identified 35 unique strains of the pathogen. He discovered how those strains were related to each other and that all strains were originally derived from three basal strains.

“The Santa Cruz and Marin County locations, as well as nurseries, had the most ancestral populations of the pathogen in our samples,” Garbelotto said. “It is possible that the pathogen arrived in California through the nursery trade, and then spread to trees bordering infested facilities. Based upon the genetic analysis of this study, the disease could have then progressed to other parts of California’s coast, including Marin County, Sonoma County, and Big Sur.”

The study also found strains from areas of recent forest infestations are more genetically distant from current nursery strains, suggesting that regulations controlling the spread of the pathogen from nurseries to the wild are working. However, moving plant material within the quarantine area allows different strains of the pathogen to freely move within the 14-county quarantine area, he said.

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For more: Matteo Garbelotto, University of California-Berkeley, (510) 643-4282; matteo@nature.berkeley.edu.

July 2008