From the samples, the researchers identified 35 unique strains of the pathogen. A computer analysis further revealed how those strains were related to each other. The study found that all strains were originally derived from three basal strains that were most prevalent in the samples and common in all sites.
Armed with that information, the researchers then plotted out which regions were linked to the basal strains to create a history of the epidemic.
Two sites emerged as the origin of the basal strains: 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, only 5 kilometers from the site where the disease was first observed in California. The basal strains also matched those found in the nursery samples.
"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," said Garbelotto.
Garbelotto noted that previous research has already pointed to the international plant trade as having introduced the pathogen to the United States. The basal strains from the Santa Cruz and Mt. Tamalpais sites also matched the strains from the nursery samples in the study, providing further support for the key role nurseries played in the spread of the epidemic, he said.
The most likely scenario, said Garbelotto, is that the pathogen arrived in California through the nursery trade, and that it then spread from the nursery in Santa Cruz to trees bordering the facility. While the site at Mt. Tamalpais is not adjacent to a nursery, there is anecdotal evidence of frequ
|Contact: Sarah Yang|
University of California - Berkeley