An oak-tree-killing microbe that has frightened arborists and naturalists tracking its inexorable spread across the United States was probably introduced to the New World in commercial plants, according to a new study by UC Berkeley researchers.
The genetic analysis of 151 isolates of the sudden oak death pathogen gave scientists the first clear evidence that the deadly funguslike scourge did not originate in California, where it has killed hundreds of thousands of oak trees since first detected in 1995.
The study, which will be published in the May issue of the journal Molecular Ecology, shows a lack of genetic diversity in the disease that is characteristic of an introduced species. The known lineages in this country, concludes the study, are the result of the introduction of at least three strains. Lacking mates from the opposite sex, the strains apparently spread largely by cloning themselves.
"This study provides evidence that the pathogen was introduced to forested areas on the West Coast and suggests that U.S. nurseries may have been a stepping stone for the introduction," said Matteo Garbelotto, an adjunct professor of forest pathology at UC Berkeley and the principal investigator for the study.
The findings raise serious questions about how one can stop a scourge that can so easily be transported and whose origin is unknown. Scientists suspect that the pathogen, known scientifically as Phytophthora ramorum, was an exotic species introduced to America from some far off place, most likely Southeast Asia, but years of research have yet to find the source.
And the damage is spreading, albeit more slowly than in the first few years after discovery when thousands of oaks in California suddenly withered and died, inspiring the pathogen's name.
Sudden oak death is known to have more than 100 susceptible host plants and has been detected in nurseries or retail garden stores in 21 U.S. states, in British Columbia and throughout Europe. Surveys of forests and wildland areas around the nation, however, have yet to find the pathogen outside of California and Oregon.San Francisco Chronicle
