Death Still Stalks the Oak

Scientists have been sprinting to understand sudden oak death since its discovery in 1995. At a recent meeting, they clarified the biology of this disease, which has stricken trees in the Western United States while raising mysteries about its origin. The outbreak is still growing, but some hope has emerged from control efforts in Oregon.

Sudden oak death (SOD) is caused by the funguslike microbe Phytophthora ramorum and has felled tens of thousands of oaks in California and Oregon. Scientists recognized it as a member of a common disease genus but knew little about where it originated or how it reproduces (ScienceNOW, 8 August 2000). They also struggle with how to stop it. Researchers filled in a few of the blanks at the American Phytopathological Society meeting from 27 to 31 July in Milwaukee, Wisconsin.

To understand the organism's history, Matteo Garbelotto of the University of California, Berkeley, and colleagues analyzed the genetic makeup of samples from across the disease's range. The analysis shows that 82% of the samples are genetically identical, suggesting a single clone is doing most of the damage. What's more, the version prevalent in the U.S. differs significantly from that found in Germany, suggesting the outbreak is not the result of recently imported German plants, as was previously thought. Now researchers are searching for an alternate explanation for the sudden outbreak.

Everett Hansen, of Oregon State University in Corvallis, and colleagues presented more bad news: The SOD organism can infect almost all major forest species, including Douglas fir and American chestnut. That's almost as alarming as the unconfirmed infection of a coastal redwood tree in January (ScienceNOW, 10 January), even though infections of trees outside the oak family are minor. A more hopeful finding comes from eradication efforts by Hansen's team in Oregon. They cut and burned about 16 hectares in nine infected areas, and so far all signs point to a virtual halt in the disease's spread.

Garbelotto, however, says the approach may not work, especially in heavily infested areas. "Because [SOD] has an aerial phase and it has such a broad host range, instead of eradication we should be thinking of suppression." Hansen says other conferees indeed see the results of research so far, on the whole, as gloomy. "The mood that I sensed among them was one of great concern and almost resignation."

Related sites
Garbelotto's site
Hansen's site

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