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What is Sudden Oak Death?

Sudden oak death is responsible for killing tens of thousands of trees in the forests of coastal California.
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  • Written By: S. Mithra
  • Edited By: Lindsay D.
  • Last Modified Date: 15 August 2014
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Sudden Oak Death is a phenomenon that has killed tens of thousands of trees in the coastal forests of California. A fungal infection takes hold in many species of oaks and rapidly spreads through a wide variety of hosts, resulting in a very high mortality rate. Tree scientists are desperately researching the responsible pathogen, Phytophthora ramorum, before this disease spreads beyond California and Oregon to threaten all oaks on the West Coast.

Sudden Oak Death is caused by a fungus of unknown origin that was somehow introduced to nurseries on California's coastline. Most scientists believe that the fungus came from outside of the region. Soon, this fungus, P. ramorum, was attacking droves of trees, creating an epidemic that is alarming ecologists.

As far as we know, the pathogen that causes Sudden Oak Death can kill Black Oak, Tan Oak, Coast Live Oaks, Shreve Oak, Poison Oak, and Canyon Live Oak, but may possibly affect others. The trunk of these trees is affected, but another remarkable strength of the infection is its ability to use other "host" plants that aren't trees, like rhododendrons. This means the fungus widely circulates through the air and foliage, never losing strength.

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The symptoms of Sudden Oak Death begin with horrific "bleeding" of the trunk. A substance that looks like sap, a blackish red the color of animal blood, dribbles from the bark. This can be seen around eye-level, from 6 feet (2 m) downwards. This stage is the primary infection of P. ramorum, but gives way to a secondary infestation of bark beetles. Originally, experts thought that bark beetles were killing trees, but this has been amended.

The small, brown bark beetles bore into the weakened bark and their activities leave dust on the trunk. Once the insects take hold, the affected tree is certain to die soon, so the fungus and beetles are free to circulate among surrounding trees. Sudden Oak Death has felled so many trees that ecologists are concerned about other plants, animals, birds, and insects that share their environment, as well as erosion. The infections are spreading beyond California into Oregon. There are even some indications that it is in Europe.

Sudden Oak Death is especially alarming because of the fungus' strength, versatility, and assured mortality for oaks. If P. ramorum was native to the coast, arborists believe that the vast deaths of trees may be part of a natural cycle, and they might not be as concerned. But the fact that the epidemic may be due to human intervention urges them to examine and control the epidemic.

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