SUDDEN OAK DEATH
What the disease means to the PNW
By Gary Chastagner, WSU Plant Pathologist
This past September, researchers in California reported that coast redwood and Douglas-fir are susceptible to Phytophthora ramorum, which is a newly discovered fungus-like organism that causes sudden oak death (SOD). Given the importance of Douglas-fir and some other SOD hosts in the Pacific Northwest (PNW), there are many foresters, Christmas tree growers and nurserymen who are concerned about this newly reported disease and what effect it will have on these industries.
SOD was first detected in the United States on tanoak in Marin County, California in 1995. In 2001, Phytophthora ramorum was identified as the cause of this disease. Unlike most Phytophthora diseases, SOD appears to be limited to the above-ground portions of plants, i.e. the leaves, shoots and stems or trunks. With some hosts, such as oak and tanoak, it has killed thousands of very large trees in California. On other hosts, most infections appear to be limited to the foliage or small shoots. Based on natural infection under field conditions, there are currently 17 confirmed hosts of SOD (Table 1).
Since it’s initial discovery, SOD has spread to 12 counties in California and a small area in southwestern Oregon. This pathogen has also been reported in parts of Europe (Germany, The Netherlands, The United Kingdom, and Spain).
COASTAL AREAS HIGH RISK
The U.S. Forest Service recently completed a risk assessment relating to SOD. Based on the number of susceptible hosts and climatic conditions, the coastal areas of the PNW are considered to be at high risk for this disease. In an effort to limit its spread by human activity, APHIS has established a federal quarantine that prohibits the movement of known SOD hosts out of areas where this disease is already present unless they have been certified to be disease-free. The certification process for Douglas-fir nursery stock and Christmas trees involves a visual inspection of a sample of plants for SOD symptoms. Because of problems associated with the lack of SOD-specific symptoms on many of the hosts, including Douglas-fir, a number of samples also have to be collected for DNA-based testing for the pathogen. If no SOD is found, a certificate is issued that allows the host material to be shipped out of the quarantine area. If SOD is detected, none of the host material can be shipped outside of the quarantine area.
The Pacific Northwest (PNW) leads the nation in the production of Douglas-fir Christmas trees and nursery stock. Douglas-fir accounts for about 45% of the 11.6 million Christmas trees harvested each year. In addition, Douglas-fir seedlings are produced at a number of nurseries in the PNW. In 2001, nurseries in Oregon and Washington produced an estimated 179 million bareroot seedlings and 45 million container grown seedlings and Douglas-fir typically accounts for more than half of the production.
Since over 90% of the Christmas trees produced in the PNW are shipped to markets outside of the PNW and most of the Douglas-fir seedlings produced in the PNW are commonly shipped from one area to another, the quarantine relating to this disease has the potential to significantly impact these industries if SOD spreads throughout the PNW.
STILL QUESTION MARKS
There are a number of uncertainties regarding the potential for this disease to cause direct damage to Douglas-fir seedlings and trees. SOD has only been confirmed on Douglas-fir at one site in California where saplings were growing under an infected bay laurel tree. Infections were limited to the needles and young shoots. At this point it is unclear what conditions must occur for this disease to develop on Douglas-fir growing in timber stands, Christmas tree plantations or seedling nurseries. Research is in progress at Oregon State University to obtain a better understanding of the biology of P. ramorum and the development of SOD on Douglas-fir.
State and Federal regulatory agencies are currently monitoring the movement of host material coming into the PNW from quarantined areas in California, Oregon and Europe in an effort to prevent further spread of this disease. The Oregon Department of Agriculture has taken a very proactive stance in relation to SOD. In September, ODA reported that SOD remains limited to a relatively small, quarantined area of Curry County, Oregon, where it was discovered in 2001. In fact, there is an aggressive eradication program underway in an effort to eliminate this pathogen in Oregon. Extensive surveys throughout Oregon have not found any infected Douglas-fir trees. So far, P. ramorum has not been detected in Washington by the Washington State Department of Agriculture.
The APHIS restrictions related to known SOD hosts only apply to the movement of these hosts from areas known to have the disease. Although these restrictions have the potential to have a significant impact on the timber, Christmas tree and nursery industries, there should be very little impact on these industries unless this pathogen spreads to other areas in the PNW. If growers have questions relating to this process, they should talk directly with ODA or WSDA representatives.
Given the wide host range, the volume and diverse methods associated with the movement of host material, and the uncertain role of natural spread of P. ramorum, it is unclear how long these regulatory steps will be effective in preventing further spread of this pathogen in the PNW. If this pathogen does spread, the ability of growers to ship trees and seedlings will depend on inspections to show that they are disease free. Ultimately, it would be beneficial if growers were able to protect their seedlings and trees from infection by P. ramorum. This would provide them with a means of reducing the potential that SOD would disrupt crop shipment.
Extensive research concerning this pathogen and disease is underway at a number of University and USDA laboratories and there are a number of excellent websites dealing with SOD (Table 2). Although SOD has not been found on Douglas-fir in the PNW, the Oregon Department of Agriculture and Washington State Commission on Pesticide Registration have recently approved funding to support a Washington State University/Oregon State University joint project to identify fungicides that are effective in protecting Douglas-fir from this disease.
This project will determine the sensitivity of P. ramorum to a number of fungicides that are commonly used to manage Phytophthora diseases on various agricultural and nursery crops and determine their ability to control this pathogen on Douglas-fir. Drs. Everett Hansen at OSU and Gary Chastagner at WSU will lead this research, which will be conducted in ODA/APHIS approved quarantine research facilities in Corvallis. The goal of this project is to provide the Christmas tree and nursery industry with an effective way to protect Douglas-fir trees and seedlings from P. ramorum, which should minimize any potential disruption of shipment of Douglas-fir Christmas trees and nursery stock if this pathogen continues to spread throughout the PNW.
Hosts of SOD under field conditions
Coast live oak Quercus agrifolia
Shreve oak Quercus parvula var. shrevei
Tanoak Lithocarpus densiflorus
California rhododendron Rhododendron macrophyllum
California bay laurel Umbellularia californica
Big leaf maple Acer macrophyllum
Madrone Arbutus menziesii
Manzanita Arctostaphylos spp.
Evergreen Huckleberry Vaccinium ovatum
California honeysuckle Lonicera hispidula
Toyon Heteromeles arbutifolia
California buckeye Aesculus californica
California coffeeberry Rhamnus californica
Coast redwood Sequoia sempervirens
Douglas-fir Pseudotsugae menziesii
Arrow-wood (Europe only) Viburnum X bodnantense
Websites
dealing with SOD.
http://www.suddenoakdeath.org/
http://camfer.cnr.berkeley.edu/oaks/
http://www.oda.state.or.us/plant/ppd/path/SOD/index.html
http://www.oda.state.or.us/Information/news/sod_news.html
http://cemarin.ucdavis.edu/index2.html - Has especially nice pictures of all the hosts and symptoms, also the associated bark beetle and hypoxylon information.
http://www.na.fs.fed.us/SOD/contacts.htm - Has links to many of the other websites.
http://www.fs.fed.us/cgi-bin/texis/searchallsites/search.allsites/ - If you type in "sudden oak death", it has links to abstracts and news items
http://www.aphis.usda.gov/ppq/ispm/sod/ -This is the general APHIS website for SOD. Includes rules and regulations, etc.
http://www.aphis.usda.gov/ppq/ispm/sod/01-054-1.pdf - This is the PDF file with APHIS rules and regulations as of 2/14/02