"Nature Guide Journal"

18 October 2001

Almost handsome, the dull-copper cedars stand stark and brittle against their green forest neighbors.  That color doesn't signal a seasonal change; those cedars are dead.  A close relative of the disease that is killing our Port Orford cedars is now appearing in the oaks and other broad-leaved trees and shrubs in Southwestern Oregon.

The pathogen that causes Port Orford Cedar Root Rot, Phytophthora lateralis, has been joined in our corner of the state by Phytophthora ramorum, the organism that causes Sudden Oak Death Syndrome.  Trees have shown P. ramorum infection in four sites in Curry County, near Brookings.

(Of historical interest, another close relative, Phytophthora infestans, caused the Irish Potato Famine of the mid-1840s.  That epidemic blight resulted in the starvation of about one million people.)

Aptly, the genus name, Phytophthora, means "plant killer" or "plant destroyer" in Latin.

Phytophthora, like other downy mildews and water molds, have two stages in their life cycle.  In Port Orford Cedar Root Rot, the mobile "zoospores" move with water and mud to spread the infection.  (The zoospores are asexual; their sexual spores don't move.)  Finding a suitable host, P. lateralis zoospores develop into thread-like hyphae that grow up the host plant's roots and inner bark, eventually killing it.

In Sudden Oak Death Syndrome, the mobile spores of P. ramorum generally invade the host through the bark.  The oozing cankers that result weaken the host and make it vulnerable to boring insects.  It seems that P. ramorum invades through the leaves in some species.

As detailed earlier this month in news reports, Sudden Oak Death Syndrome rapidly infests tanoak, as well as rhododendron and evergreen huckleberry.  It has also infected madrone, myrtle, black oak, big-leaf maple, and manzanita in California.

In the normal course of events, as a pathogen (such as Phytophthora) evolves to become more virulent, it's hosts evolve to become more resistant.  The two populations tend to adapt to each other.  In ecosystems with established members, other organisms participate in the back-and-forth battle between evolving populations of hosts and infections, helping to keep the two in relative balance over time.

While it is still uncertain where they originated, the sudden appearance and completely devastating effects of these two Phytophthora infections clearly indicate they are not native to our forests.

The dry, coppery Port Orford cedars—and the other species that may follow—are almost certainly visual evidence of human intervention; they are testimony that we need to take more care as we travel the world with plants and animals in tow.

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Websites with more detailed information on Sudden Oak Death include the US Forest Service's Southwest Oregon Forest Insect and Disease Service Center; and the University of California at Berkeley's site.  

Websites with more detailed information on Port Orford Root Rot include the one managed by the US Forest Service's Region 6.

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