Other Exotic Forest Threats - Sudden Oak Death

Sudden oak death was first observed in Marin County, California (north of San Francisco) in 1995, when thousands of oaks were found to be dead or dying from an unidentified cause. It was not until 2000 that Phytophthora ramorum, a poorly understood fungus-like organism, was determined to be the pathogen that causes sudden oak death. (A pathogen is an organism that causes a disease.) In the United States, the disease is presently known to occur along the coast of California and southern Oregon.

Disease Symptoms

On susceptible oaks such as tanoak, California black oak, and California live oak, the most apparent initial symptoms are stem cankers that ooze a dark sap (a canker is a region of dead tissue). The outer bark is discolored, and beneath the bark, a dark line usually surrounds patches of dead tissue. Cankered trees may live for several years, but the tree dies quickly once branch mortality begins in the canopy of the tree. The term 'sudden' is misleading in it refers to a rapid death of the tree canopy. However, an infected tree may live for a prolonged period of time before dying, or it may not die at all.

Thumbnails link to larger images.

Coast live oak dying from P. ramorum infection in California, Photo Credit: Joseph O'Brien, USDA Forest Service, from www.forestryimages.org

Patch of dead tissue beneath bark, Photo Credit: Joseph O'Brien, USDA Forest Service, from www.forestryimages.org

Bleeding stem canker, Photo Credit: Joseph O'Brien, USDA Forest Service, from www.forestryimages.org

P. ramorum leaf spot on rhododendron, Photo Credit: Joseph O'Brien, USDA Forest Service, from www.forestryimages.org

The pathogen also infects many other non-oak species (currently at least 60 species) such as azaleas, camelias, rhododendrons, madrone, California buckeye, and bigleaf maple, but on these plants it causes only a leaf lesion and/or twig dieback. Because a number of other pathogens and environmental conditions can cause similar symptoms on oaks and other plant species, laboratory testing is required to confirm that P. ramorum is present.

Susceptible Species

Greenhouse studies have shown that some oak species in uninfected areas of the U.S. are highly susceptible to P. ramorum. These include species that are found in Wisconsin, such as red oak, black oak and northern pin oak. The pathogen grows best in moist climates, and at present the disease has not been found more than about 40 miles inland. The impact on oaks in the upper Midwest is predicted to be low due to temperature and moisture conditions that inhibit the growth of P. ramorum, even though there are susceptible tree species present.

Phytophthora ramorum is also found in a number of European countries, but until 2003 the pathogen had only been found on ornamental plants such as rhododendrons and viburnums. That year, infected oaks (including southern red oak and northern red oak, both native to North America) were found in southwest England and The Netherlands. All of these infected trees were in the vicinity of infected rhododendrons.

The pathogen may be spread by water droplets, runoff, and by the movement of infected soil or plant materials. The movement of nursery plants is suspected as the main cause of spread in North America and Europe. However, research studies have found that the North American and European forms of the pathogen are genetically different. The European strain has also been detected at nurseries in the Pacific Northwest and in British Columbia, Canada. The origin of the pathogen remains unknown.

Over the past few years, P. ramorum was accidentally transported from nurseries in California to at least 21 other states through the shipping of infected plants. However, surveys did not find the pathogen in nature in these states, and the infected plants were destroyed. Quarantine restrictions are in place to prevent future shipments of infected plants.

If P. ramorum were to become established in additional states, there would be significant impacts on their forest and nursery industries due to quarantine restrictions and lost markets. For example, plant nurseries would find it much more difficult to sell host plants to buyers in states where the pathogen was not present.

In Wisconsin

Phytophthora ramorum has not been detected in Wisconsin nurseries or forests, and surveys to detect the pathogen are ongoing.

There are several oak problems in Wisconsin that may be mistaken for sudden oak death. Oak wilt is a fungal disease that kills species in the black and red oak groups very quickly, and typically causes a circular 'disease center' as the fungus spreads through a tree's root system into the roots of adjacent trees. In addition, a beetle called the 'two-lined chestnut borer' attacks weakened oaks, causing branch dieback and tree mortality that could be mistaken for sudden oak death.

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