Phytophthora ramorum
Sudden Oak Death Syndrome - Phytophthora ramorum Werres et al.
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Sudden oak death syndrome
Since 1995, a newly described pathogen (Werres et al., 2001), Phytophthora ramorum, has been killing trees in the coastal hardwood forests of California and a small area in southern Oregon. The disease has killed more than 100,000 oak trees in 10 coastal Californian counties. Vulnerable oak species are distributed along 2400 km (1,500 miles) of the California and Oregon coasts; some are the dominant hardwoods in large areas, growing in nearly pure stands. Sudden oak death is now known to infect nearly all the major trees along the central and northern California coast (Lau, 2002). Hosts killed by the pathogen include tan oak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and Shreve's oak (Q. parvula), and sometimes rhododendrons and evergreen huckleberry (Vaccinium ovatum) accessed July 2004). Hosts which become infected but usually are not killed include manzanita (Arctostaphylos), madrone (Arbutus), and California toyon (Heteromeles arbutifolia) , accessed July 2004). California bay laurel (Umbellularia californica) and rhododendron (Rhododendron spp.) are known to be reservoirs of infection (S. Frankel, pers. comm., 2004). Several conifers have also been identified as hosts, although the impact of the pathogen on them remains unclear. These include coastal redwood (Sequoia sempervirens), Douglas-fir (Pseudotsuga menziesii), and grand fir (Abies grandis) , accessed July 2004).
Eastern forest species known or suspected to be hosts include red oak (Quercus rubra) and mountain laurel (Kalmia latifolia). A study carried out by USDA Forest Service scientists, where it was assumed that all species in the red oak and live oak groups in the east would be susceptible to the pathogen, determined that over half of the forests in the eastern U.S. contain some susceptible tree hosts. The greatest impact would be on forests in areas that contain large percentages of red or live oak species, including the Ozark-Ouachita Highlands, pin oak sand flats in the Lake States, and live oak areas in Florida. Understory hosts must also be considered. (Gottschalk et al., undated).
To date, no solution to sudden oak death in forest ecosystems has been found. A fungicide, AGRI-FOS®, has been found that is effective in preventing healthy trees from becoming infected. They are suitable for treating individual landscape trees but not trees in forests (Yang, 2003).
In order to prevent spread of the disease to potentially vulnerable locations outside California and southern Oregon, USDA APHIS has imposed quarantines on the movement of living plants, cut greens, and other materials taken from host plants (Code of Federal Regulations: 7 CFR 301.92 - ). Concern has focused on the nursery trade, since host species are such widely planted species as camellia, rhododendron, and various viburnums , accessed July 2004). In March 2004, officials discovered the disease on camellias in a California nursery that ships 30 million dollars worth of plants to out-of-state customers annually (C. Knighten, pers. comm., 2004). By the end of May 2004, diseased plants had been traced to nurseries in 17 states (USDA APHIS Sudden Oak Death SITUATION REPORT # 45 June 1, 2004). USDA APHIS, state departments of agriculture, the USDA Forest Service, and others are currently (i.e. in 2004) attempting to find and test the plants shipped from California. This work, already difficult, has been impeded by uncertainty over whether sufficient funds will be available. Scientists are also trying to establish the risk of spreading the disease with bark, mulch, and wood from redwood and Douglas-fir.
Numerous federal and state government agencies have posted on the web photographs of Phytophthora ramorum symptoms and written descriptions of how to survey for the disease in such common nursery or garden plants as camellias, rhododendrons, and viburnums. To find these resources, type "sudden oak death, camellia, symptoms" into your favorite search engine.
A different form of the Phytophthora ramorum pathogen is also found in Europe. The European and American forms of the pathogen have different mating strategies and apparently different levels of virulence (Anonymous, 2004). For the first several years it was confined to plants in nurseries in the Netherlands and Germany. By 2004, the disease was present in nurseries in at least 11 European countries ( , accessed July 2004). In autumn 2003, scientists detected the first cases of nearby trees becoming infected as a result of spores from nursery stock (Anonymous, 2004). In the United Kingdom, affected tree species have included two species native to North America, southern (Q. falcata) and northern red oak; as well as three European species: beech (Fagus sylvatica), Holm oak (Q. ilex), and horse chestnut trees (Aesculus hippocastanum) (Anonymous, 2004). Since autumn 2002, APHIS has strictly regulated imports of host plants from Europe. However, because many of the pathogen's plant hosts remain unidentified, the level of protection using this host list is unclear.
Sources
Anonymous. 2004. Felled by fungus. The New Scientist 182(2450): June 5, 2004, Vol. 182, No. 2450, pp. 40-43.
Frankel, S. October 2004. Plant Pathologist, USDA-Forest Service, personal communication.
Gottschalk, K. W., R. S. Morin, and A. M. Liebhold, Undated publication, USDA Forest Service, Northeastern Research Station, 180 Canfield St., Morgantown, WV 26505-3180.
Knighten, Claude R. March 2004. Public Affairs Specialist -USDA/APHIS Plant Protection and Quarantine, personal communication.
Lau, E. 2002, 5 September. Algae assault hits redwoods, firs. The Sacramento Bee: A1.
Werres, S., R. Marwitz, W. A. M. I. Veld, A. W. A. M. De Cock., P. J. M. Bonants, M. De Weerdt, K. Themann, E. Ilieva, and R. P. Baayen. 2001. Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycol. Re. 105: 1155-1165.
Yang, S. 2003. Frequently Asked Questions on Sudden Oak Death treatment.
