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An Alien in US Forests

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In the mid 1990s, oaks and tanoaks across central California began dying by the score. Once-green forest canopies developed ragged brown holes as a new disease dubbed Sudden Oak Death worked its way through 14 California counties. The contagion eventually turned up in nurseries from Oregon, Washington, and nine European Union nations. The culprit was Phytophthora ramorum, a microbe new to science. Theories about its origins flew like spores through the air. Was it a native species, lain dormant until primed to emerge by weather, pollution, or drought? Or an alien that hitchhiked here on soil, other plants, or produce?

Now, plant pathologist Matteo Garbelotto, associate Cooperative Extension specialist in ecosystem sciences, has found evidence that P. ramorum is indeed a stranger to these shores.

Garbelotto and colleagues analyzed the DNA of 151 samples of the pathogen taken from U.S. forests and nurseries as well as nurseries in the European Union. They reported in the journal Molecular Ecology that all of the samples from U.S. forests consist of a strain found in U.S. nurseries. EU nurseries were infected by a second lineage also present in U.S. nurseries. And in samples from a Washington nursery, they also identified a third, cold-tolerant lineage never seen before.

As a rule, indigenous species harbor greater genetic diversity than introduced populations. Yet the researchers found the widest array of P. ramorum genotypes in nurseries, not the wild, and all of the samples from U.S. forests appear to be clones, reinforcing the idea that it is an exotic species. The bottom line, according to Garbelotto: “There is no way this organism is native.”

The discovery has major implications for controlling the pathogen’s spread. Federal regulations already restrict shipments of plant species susceptible to the disease from nurseries in Oregon and Washington. These plants must be inspected and found healthy before they can be transported to other states. But if imported plants are indeed responsible for the epidemic, it suggests more stringent scrutiny is needed to prevent devastating plant pathogens from entering the country in the first place.

-Kathleen M. Wong


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