Highly contagious disease named for the amazing speed at which it can kill healthy oak trees-- as little as four months.

While visitors to California may picture in their minds the coastal redwoods, the orange groves, or the giant sequoia of the Sierra Nevada (all of which are disappearing), residents of the state who notice the landscape at all know that it's mostly defined by the native oaks (there are some 2 dozen or so endemic species). With its combination of interest from homeowners worried about property values, land managers worried about fire danger, and weekend hikers worried about losing their parks, the disease has made front page news and turned plant pathologists into minor media celebrities.

So far, Sudden Oak Death has killed tens of thousands of California oak trees since it was first noticed in 1995, in 14 counties (on the northen and central coast, with cool, moist climates) where it is killing off Coast live oak (Quercus agrifolia), Tanoak (Lithocarpus densiflorus), Black oak (Quercus kelloggii), and Shreve's oak (Quercus parvula var. shrevei). The pathogen has also been found in

Bay laurel and rhododendron seem to be the primary hosts for spores-- oak trees near them don't stand much of a chance. Infections that don't kill the trees right away have made them susceptible other fungal infections as well as to beetle infestation.

The primary cause of Sudden Oak Death (SOD) was isolated from bleeding cankers on the bark of trees by UC Davis plant pathologist David Rizzo— it has been identified as a fungus-like organism* in the genus Phytophthora. Various species of Phytophthora caused the Irish potato famine of 1845–50 and the modern deaths of eucalyptus forests in Australia and oak forests in Mexico, Spain and Portugal. Acting on a tip from UK researcher Clive Brasier, UC Berkeley forest pathologist Matteo Garbelotto confirmed through DNA testing that the organism was identical to a species first noted in European rhododendrons in 1993. (However, it has not been found in European oaks. Yet.) The variety in the Western United States, type A2, is a distinct genotype from Europe's type A1 (which has been found in In Europe, P. ramorum has been identified on plants in Germany, Spain, France, Poland, Belgium, Sweden, Italy, the Netherlands, and the United Kingdom.)

Politicians have been quick to jump on the issue, finding funding where they can for research, treatment, and fire suppression. (Dead trees increase the flammability of a forest, so resource managers prefer to remove them). And while Garboletto has found promising chemical treatments for saving individual trees, there is no solution to saving forests at the moment.

The U.S. Department of Agriculture has quarantined the 14 counties with SOD, regulating the shipment of not only California oaks, but other host and potential host plants. They also instigated a national survey of nurseries, to detect the pathogen on plants that were shipped from nurseries in the affected areas.

It should also be noted, that though the disease makes headlines, the number one cause of oak mortality in California is agriculture, notably vineyards. (While pasture land for grazing keeps its oaks, when the land is sold to another owner, as long as the use remains agricultural, no environmental impact report is required, so a new owner could remove trees at will.)

September 2002: 9 isolated sites containing SOD have been confirmed in Oregon.

December 2002: Reports of genetic immunity of some infected oaks raise hope of eventually replenishing forests. The state Department of Pesticide Regulation has approved the application of one company's phosphonate product as a treatment, but for tan oaks only.


Sources:
Eric Brazil, "The Grapes of Wrath," San Francisco Chronicle, 17 March 2002.
California Oak Report, April 2002, <http://www.californiaoaks.org/html/current_issues.html> 8 April 2002.
Marin Independent Journal, http://www.marinij.com/
Sudden Oak Death, http://camfer.cnr.berkeley.edu/oaks/
David M. Rizzo, Matteo Garboletto, Pavel Svirha, "Sudden Oak Death Fells 3 Oak Species, " California Agriculture, January-Febraury 2001
Fimrite, Peter. "Unusual form of oak disease found in Tahoe area." San Francisco Chronicle, December 18, 2002.

 


*(Formerly a fungus, but recently the entire class of Oomycetes has been banished from the Fungi Kingdom. As Rizzo puts it, Phytophthera is not so much a fungus as it is a "killer brown algae.")

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