Sudden Oak Death

Publication Number 450-801, Posted February 2003

Sudden oak death (SOD), caused by Phytophthora ramorum, is a new disease in the United States. The outbreak has prompted the USDA Animal and Plant Health Inspection Service (APHIS) to quarantine some counties in California and Oregon and prohibit the interstate movement of host-plant material from those counties. Phytophthora ramorum is a new species of Phytophthora that was first described as a pathogen of Rhododendron and Viburnum spp. in Germany and the Netherlands in 2001. It has a broad host range and is a very destructive pathogen. The spread of this pathogen would cause devastating economic losses to the nursery and forest industries as well as severe ecological impact to the natural landscape and forest ecosystems of America. This publication summarizes up-to-date research on the host range, symptoms, basic biology and epidemiology, geographic distribution, and risk assessment for dissemination of this pathogen. The primary objective of the publication is to alert the industry and the public to this disease to help prevent the pathogen from entering Virginia.

Host Range
The current list of host plants susceptible to P. ramorum spans at least 12 families. This list is growing rapidly. Lithocarpus densiflorus, the tanoak, is the most susceptible tree species and Rhododendron and Vaccinium are among the most susceptible shrubs. Following is a list of known and potential host plants as of December 18, 2002.

Known host plants:
Acer macrophyllum - Big leaf maple
Aesculus californica - California Buckeye
Arbutus menziesii - Madrone
Arctostaphylos manzanita - Manzanita
Chrysolepis chrysophylla - Golden chinquapin
Heteromeles arbutifolia - Toyon
Lithocarpus densiflorus - Tanoak
Lonicera hispidula - California honeysuckle
Pittosporum undulatum - Victoria box
Pseudotsuga menziesii - Douglas fir
Quercus agrifolia - Coast live oak
Quercus chrysolepsis - Canyon live oak
Quercus kelloggii - Black oak
Quercus parvula var. shrevei - Shreve's oak
Rhamnus californica - California coffeeberry
Rhamnus purshiana - Cascara
Rhododendron macrophyllum - Rhododendron
Rhododendron spp. - Rhododendron
Rubus spectabilis - Salmon berry
Rhus diversiloba - Poison oak
Sequoia sempervirens - Coast redwood
Trientalis latifolia - Western starflower
Umbellularia californica - California bay laurel
Vaccinium ovatum - Huckleberry
Viburnum x bodnantense - Arrowwood
Viburnum tinus

Potential host plants (susceptible when inoculated, but not yet found in nature):
Arctostaphylos uvaursi - Kinnikinnick
Camellia spp. - Camellia
Carpinus spp. - Hornbeam
Castanopsis chrysolepsis - Chinquapin
Cotoneaster spp. - Cotoneaster
Gaultheria procumbens - Wintergreen
Gaultheria shallon - Salal
Hedera spp. - Ivy
Ilex aquifolium - English holly
Kalmia latifolia - Mountain laurel
Leucothoe spp. - Leucothoe
Nandina spp. - Heavenly bamboo
Pachysandra spp. - Pachysandra
Pieris spp. - Pieris
Prunus spp. - Cherry laurel
Quercus garryana - Oregon white oak
Quercus palustris - Pin oak
Quercus petraea - Durmast oak
Quercus robur - English oak
Quercus rubra - Northern red oak
Rhododendron maximum

Rhododendron 'Cunningham's White'
Rhododendron 'P.J.M'
Rhododendron 'Unique'
Rhododendron 'Nova zembla'

Symptoms

The most consistent and diagnostic symptoms of the disease on large trees such as tanoak are cankers on the lower trunk that have brown or black discolored outer bark and bleed dark red sap. These cankers develop before foliar symptoms become evident. However, the whole crown of affected trees often appears to die rapidly. The name 'Sudden Oak Death' reflects this rapid symptom development on foliage.

The most common symptoms on shrubs such as rhododendron are twig dieback and leaf spotting. Blackened shoots with or without foliage attached are typical of twig dieback caused by P. ramorum. Leaf spots are brown in color with diffuse, fuzzy margins. P. ramorum usually does not kill shrub hosts with the exception of Viburnum spp. P. ramorum causes wilting of arrowwood in Europe, but this has not been found in the United States. The pathogen infects arrowwood at the base of the stem, killing the outer tissues and causing wilting of entire plants.

These symptoms are not unique for sudden oak death. Other Phytophthora species such as P. cinnamomi, P. cactorum, and P. citricola can cause similar cankers on the trunks of oak trees and other forest species. P. nicotianae, P. citrophthora, and P. heveae also may cause foliar symptoms similar to those of SOD. Differentiation of symptoms among SOD and other Phytophthora diseases may be accomplished by examining the progress of the disease.

P. ramorum appears to attack only aerial plant parts. Disease symptoms have not been detected below the soil-line. This contrasts with other Phytophthora diseases. Cankers caused by P. cinnamomi, P. citricola, or P. cactorum usually start with root rot then develop into cankers on the main stem and move upwards. This is a useful characteristic for distinguishing cankers caused by P. ramorum from those caused by other Phytophthora spp.

Biology and Epidemiology

Viable zoospores have been detected in streams and other surface waters from areas near diseased trees. The species has also been detected in soil collected from a hiking trail and hikers' shoes in a popular California state park. As a precaution, anyone entering the quarantine areas in California is required to wear special shoes and relinquish them on-site before leaving. Similarly, vehicles must be washed, before leaving the quarantine areas to prevent this pathogen from spreading. Long distance transmission of P. ramorum through eastward movement of nursery stock is a serious concern due to the broad host range of this species, and the large number of shipments of nursery stock.

Geographic Distribution and Dissemination Risk Assessment

In California, P. ramorum causes diseases on a variety of plant species along approximately 200 miles (325 kilometers) of the Pacific Coast from Big Sur in Monterey County (36°5'N, 121°47'W) to southern Mendocino County (38°56'N, 123°19'W). The sites furthest inland are approximately 43 miles (70 kilometers) from the coast in Solano County (38°18'N, 122°12'W). The most severe sudden oak death occurs within approximately 18 miles (30 kilometers) of San Francisco Bay. Plant pathologists have recovered the pathogen from forests at sea level to an elevation of approximately 2,625 feet (800 meters). Both the California Department of Food and Agriculture and APHIS have very strict quarantine measures in place to prevent the pathogen from spreading.

Sudden oak death also was detected at nine sites in Southwestern Oregon. P. ramorum was recovered from tanoak stem canker and from foliage and shoots of native Rhododendron spp. and Vaccinium spp. All sites were confined to a 9-square-mile (23-square-kilometer) area in Curry County. All lands within 1 mile (1.6 km) of individual disease centers are subject to Oregon and APHIS quarantines, which bar the transport of host materials. Eradication efforts followed by intensive monitoring of treated and perimeter areas are underway.

According to a comparative study conducted by USDA Forest Service, Washington state is a high-risk area for sudden oak death (Figure 2). Virginia, along with North Carolina, South Carolina, Tennessee, and Georgia, also were ranked as high-risk regions for this disease (Figure 2). Pilot surveys for this pathogen at nurseries and in surrounding forests in these five states are underway.

Precautions Taken to Prevent This Pathogen from Entering Virginia

1. Avoid purchasing plant materials, including stock plants and plant products from the quarantine areas of California and Oregon, since the full host range of P. ramorum is still unknown.
2. Take precautions when purchasing plant materials, especially host materials from the high-risk areas on the Pacific Coast, including the non-quarantine areas of California and Oregon and part of Washington state (Figure 2).
3. Local inspectors should check all incoming plant materials for contamination by P. ramorum prior to shipment into Virginia.
4. Report any incoming plant materials you suspect with contamination from P. ramorum to the Virginia Department of Agriculture and Consumer Services at (804) 786-3515.

References

1. Anonymous. 2002. The State of Our Knowledge on Sudden Oak Death. Sudden Oak Death Science Symposium. Monterey, Calif., December 17-18, 2002.
2. Davidson, J. M., Garbelotto, M., Koike, S. T., and Rizzo, D. M. 2002. First report of Phytophthora ramorum on Douglas fir in California. Plant Dis. 86:1274.
3. Goheen, E. M., Hansen, E. M., Kanaskie, A., McWilliams, M. G., Osterbauer, N., and Sutton W. 2002. Sudden oak death caused by Phytophthora ramorum in Oregon. Plant Dis. 86: 441.
4. Goheen, E. M., Hansen, E. M., Kanaskie, A., McWilliams, M. G., Osterbauer, N., and Sutton W. 2002. Eradication of sudden oak death in Oregon. Phytopathology 92:S30.
5. Hansen, E.M., and Sutton, W. 2002. Log inoculations to assess tree susceptibility to sudden oak death. Phytopathology 92:S33.
6. Linderman, R.G., Parke, J.L., and Hansen, E.M. 2002. Relative virulence of Phytophthora species, including the sudden oak death pathogen, P. ramorum, on leaves of several ornamentals. Phytopathology 92:S47.
7. Maloney, P.E., Rizzo, D. M., Koike, S. T., Harnik, T. Y., and Garbelotto, M. 2002. First report of Phytophthora ramorum on coast redwood in California. Plant Dis. 86:1274.
8. McWilliams, M.G., Kanaskie, A., Osterbauer, N., Goheen, E.M., Hansen, E.M., Sutton, W., and Mair, J. 2002. Sudden oak death surveys in Oregon -2001. Phytopathology 92:S54.
9. Parke, J.L., Linderman, R.G., and Hansen, E.M. 2002. Susceptibility of Vaccinium to Phytophthora ramorum, cause of sudden oak death. Phytopathology 92:S63.
10. Rizzo, D.M., Garbelotto, M., Davidson, J.M., Slaughter, G.W., and Koike, S.T. 2002. Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflorus in California. Plant Dis. 86:205-214.
11. Tooley, P.W., and Englander, L. 2002. Infectivity of Phytophthora ramorum on selected Ericaceous host species. Phytopathology 92:S81.
12. Werres, S., Marwitz, R., Man In'T Veld, W.A., De Cock, A.W.A.M., Bonants, P.J.M., De Weerdt, M., Themann, K. Ilieva, E., and Baayen, R.P. 2001. Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycol. Res. 105:1155-1165.

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