Sudden Oak Death: A Potential Threat to Eastern Oaks

Robert P. Long, OSAF Forest Science Coordinator

You’ve probably heard about the Asian Longhorned Beetle, hemlock woolly adelgid and, of course, gypsy moth, but now there is another serious non-native organism that could threaten Eastern forests, and especially oaks. Sudden oak death was first identified in California in 1994 and 1995 along central coastal areas where tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), Shreve oak (Q. parvula var. shreveii), and madrone (Arbutus menziesii) have been killed by this newly identified pathogen, Phytophthora ramorum. The pathogen has also been found on rhododendron species, huckleberry (Vaccinium ovatum), bay laurel (Umbellularia californica), California buckeye (Aesculus californica), bigleaf maple (Acer macrophyllum) and manzanita (Arctostaphylos manzanita). However, on these hosts the pathogen only causes leaf spots and twig dieback. As of September 2001, the pathogen has been found in 10 California coastal counties and Curry County, near Brookings, Oregon. The furthest inland extent of the pathogen is about 50 miles in Solano County, California.

The most alarming new information about the host range of this pathogen was presented by Dr. Dave Rizzo, forest pathologist at the University of California at Davis, at the American Phytopathological Society meetings held in Salt Lake City this past August. Dr. Rizzo reported that seedlings of two eastern oak species, northern red oak (Q. rubra) and pin oak (Q. palustris) developed stem cankers after inoculations with P. ramorum. While seedlings of both species appear to be highly susceptible to this pathogen, Dr. Rizzo cautioned that this does not necessarily mean that mature trees will be susceptible. There have been difficulties in testing the host range of the pathogen due to restrictions on importing oak seedlings or saplings that may harbor the oak wilt pathogen. In the eastern US, a preliminary survey of declining rhododendrons at 10 locations in Virginia, Tennessee and North Carolina did not reveal the presence of the P. ramorum pathogen or any other Phytophthora species. Currently, there is only one quarantine laboratory in the east at Fort Detrick, Maryland, that has capabilities for conducting research on non-native plant pathogens.

On tanoak, California live oak, and California black oak, the first symptom is black or reddish ooze or “bleeding” from cankers. All size classes can be affected. Mature tress with cankers will survive for 1-3 years, but it is unknown how long the trees were infected before these symptoms became evident. Cankering is accompanied by crown dieback and discoloration of leaves that turn pale yellow to brown within several weeks. Black zone lines surround necrotic bark tissues and are evident under the affected bark. Cankers and necrosis typically occur on the lower 10 feet of the stem, but some cankers have been found as high as 60 feet. Cankers do not occur below the soil line. Cankering likely kills the tree by girdling the main stem. Since other Phytophthora pathogens may cause similar symptoms it is necessary to isolate and culture the pathogen for laboratory identification. To date, no white oaks have been shown to be susceptible to the pathogen. On these oaks the pathogen causes a lesion, but the hosts are able to contain the infection and prevent it from further colonization of new tissues.

For the shrub hosts such as rhododendron and huckleberry, the pathogen produces only leaf spots and some twig dieback, but does not cause mortality. These hosts may be very important in spreading the pathogen since the leaf spots may produce spores. Researchers have not found the fungus sporulating on the oak hosts, but abundant spores have been recovered in the winter during rain events. These spores may come from leaf spots on the shrub hosts. The fungus produces asexual spores, sporangia, or if conditions are adverse, thick-walled resting spores known as chlamydospores. Research has shown chlamydospores can only be killed by heating them to 55 C for one hour. European scientists in Germany and the Netherlands have recovered P. ramorum from rhododendrons. Ornamental rhododendrons in California have also been found with the fungus. This research suggests that the fungus was introduced to California on ornamental rhododendrons, but it is not clear where the fungus originated.

Presently, efforts are focused on preventing the spread of this pathogen. Quarantines on movement of plant parts of oak and other host species have been instituted in California. Restrictions on importing ornamental rhododendrons are still being debated. The ease of spread of this pathogen, on shoes or car and bicycle tires, means it will be difficult to contain. The jump in the range of the disease this summer from California to southern Oregon in an area remote from development, roads, or trails is particularly alarming. We don’t know enough about this pathogen to say whether it could survive and spread in our Eastern forests. Lets hope that there are major new efforts to limit the spread and keep the pathogen contained. Given the abundance of oak in our eastern and central hardwood forests, this pathogen could be serious threat to many of our forests. There is considerable information available on the internet. Below are a few web page addresses, and you can likely find even more.

SOD Web Sites:

http://cemarin.ucdavis.edu/index2.html

http://www.cnr.berkeley.edu/comtf/

http://www.calflora.org/calflora/SOD/index.html

http://camfer.cnr.berkeley.edu/oaks/