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, according to the study.
"Interestingly, areas where the Sudden Oak Death infestation are particularly severe are not necessarily the first ones to have become infected," said Garbelotto. "Big Sur was hit particularly hard by Sudden Oak Death, with a proportionally larger amount of trees killed by the disease. That is why this historical reconstruction based upon genetic analysis is so important; it does not always match the picture of the epidemic one would develop based solely upon observations."
The fact that there are more symptoms and higher mortality in areas where the pathogen didn't originate suggests that some factor other than length of infestation, such as climate or environmental conditions, is needed for the pathogen to thrive, the researchers said.
To better understand how different areas ended up with matching strains, the researchers analyzed how far the pathogen can spread naturally, such as through airborne spores and wind gusts. To do this, GPS coordinates were recorded for every plant sampled, and the distance between plants with identical strains was calculated.
The researchers found that the vast majority of movement occurred within 200 to 300 meters. However, spores were occasionally found moving 1 to 5 kilometers, possibly aided by strong winds, they said.
"That means if we find a perfect match in strains at sites more than 10 kilometers apart, we can safely assume that the spread did not happen naturally, and that the pathogen was introduced to the site by some other means," said Garbelotto. "The Santa Cruz site is about 100 kilometers away from Mt. Tam, so it's clear that natural spread did not occur between the sites."
|Contact: Sarah Yang|
University of California - Berkeley