Sudden Oak Death and Ramorum Blight1
Philip F. Harmon and Carrie L. Harmon2
Sudden oak death and ramorum blight are emerging diseases capable of causing a range of symptoms, from leaf spots to plant death, on many woody hosts. Because these diseases are emerging, much about the pathogen, host range, and disease epidemiology is currently being researched. The pathogen was introduced into the state through ornamental plant commerce, but eradication efforts have reduced the pathogen to below detectable levels. Only the foliar dieback disease ramorum blight has occurred in Florida in the past; no oak trees have died in Florida from sudden oak death. Currently, neither sudden oak death nor ramorum blight is established in Florida, but state agencies and university educators continue to work together to monitor for these diseases. If you suspect ramorum blight or sudden oak death in Florida, please contact your local UF/IFAS Extension agent or submit a sample through a Florida Extension Plant Diagnostic Center (http://edis.ifas.ufl.edu/sr007).
Causal Agent and Geographic Distribution
Sudden oak death is caused by Phytophthora ramorum. The historical origin of the pathogen is unknown, but it was first described in Europe on ornamental Rhododendron spp. and Viburnum spp. in 2001. In 2002, the pathogen was reported in California and Oregon and has since been found in western Canada. In 2004, the pathogen was inadvertently shipped on infected ornamental plant material throughout the United States.
Host Range
Various symptoms have occurred on more than 100 species of native and cultivated ornamental plants infected with the pathogen (http://www.aphis.usda.gov/plant_health/plant_pest_info/pram/downloads/pdf_files/usdaprlist.pdf). The various symptoms are not exclusive because many plant species have not yet been tested. It is likely that species closely related to susceptible hosts also could be infected by P. ramorum.
Disease Symptoms
Symptoms of these diseases vary from host to host; however, roots of plants infected with P. ramorum typically appear healthy. Symptoms may progress rapidly after infection or may not be visible for significant periods of time. Symptom progression is favored by temperatures near 20°C. Ramorum blight is a foliar blight and dieback disease, and sudden oak death is a trunk canker-causing disease of certain species of oak and related trees.
Leaf Lesions
depending on the orientation on the plant), along the midvein, or around the margin. Lesions may first appear water soaked, and a water-soaked margin may be visible on rapidly expanding lesions (Fig. 1). Blighted leaf tissue typically turns tan to brown and may have a reddish tinge (Fig. 2). Lesions commonly expand from the midvein in an angular fashion (Fig.3).
Leaf lesions on Kalmia latifolia (moutain laurel). Note the symptom occurs at the leaf tip, and the lesion has a water-soaked border.
Credit: Plant Management Network
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Tan to reddish brown leaf lesions typical of sudden oak death on Camellia spp.
Credit: Plant Management Network
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Angular lesions caused by P. ramorum on leaves of Camellia spp.
Credit: Plant Management Network
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Bleeding Cankers
Cankers are sunken or swollen lesions on the branches and trunks of woody plants. Cankers may occur beneath the bark and can be difficult to distinguish. Cankers caused by P. ramorum often ooze red, sticky sap (Fig. 4). Some bacterial pathogens can cause sap to bleed from cankers, but sap from these cankers has a foul odor. Vascular discoloration is revealed when the bark and outer cambial layer are removed (Fig. 5). Vascular discoloration on mature Lithocarpus densiflorus (tan oak) is characteristically bright red (Fig. 6).
Bleeding canker symptom on coast live oak caused by P. ramorum.
Credit: Plant Management Network
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Vascular discoloration of Rhododendron spp. caused by P. ramorum.
Credit: Plant Management Network
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Canker and vascular discoloration on a mature tan oak.
Credit: Plant Management Network
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Shoot Tip and Stem Blight
Stem blight often begins at a shoot tip and progresses toward the base of the plant, but infection may occur at any point on a stem and move up or down the plant. Blighted stems appear brown to black and may be killed, resulting in death of attached leaves (Fig. 7). Early symptoms of shoot tip blight may include the formation of a "shepherd's crook" (Fig. 8).
Stem dieback of Rhododendron spp. caused by P. ramorum.
Credit: Plant Management Network
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Shepherds crook symptom on tan oak sapling.
Credit: Plant Management Network
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General Wilt
Viburnum spp. are among the most susceptible hosts and typically exhibit wilting symptoms that can mimic drought stress. As symptoms progress, individual branches and eventually the entire plant may collapse and die.
Spread of the Pathogen and Control Options
The fungus can be spread by movement of infected host material, infested soil, irrigation water, and wind-blown rain. Unintentional movement of infected but asymptomatic nursery stock is a potential means of pathogen dissemination. Because this is an emerging pathogen, the best option for controlling spread of these diseases is preventing the introduction and establishment of the pathogen in new areas. Quarantines and eradication programs in conjunction with extensive surveys are the most effective way to deal with potential introductions. Infected plant material should be destroyed by burning or deep burial in a landfill. Composting infected plant material is not recommended because it is difficult to reach temperatures that are high enough to destroy the fungus.
Fungicides are recommended in nurseries where these diseases occur. Fungicides can prevent disease caused by this and other Phytophthora spp., but fungicide resistance in the pathogen has been observed. See "Additional Resources" below for more information.
Additional Resources
Parke, J., J. Pscheidt, and R. Linderman. 2003. Phytophthora ramorum: A Guide for Oregon Nurseries. Corvallis: Oregon State University. http://www.suddenoakdeath.org/pdf/OSUP.ramorum.pdf.
Parke, J. L., and S. Lucas. 2008. Sudden Oak Death and Ramorum Blight. Corvallis: Oregon State University http://www.apsnet.org/edcenter/intropp/lessons/fungi/Oomycetes/Pages/SuddenOakDeath.aspx.
USDA APHIS. "Sudden Oak Death." http://www.aphis.usda.gov/wps/portal/aphis/ourfocus/importexport?1dmy&urile=wcm%3apath%3a%2FAPHIS_Content_Library%2FSA_Our_Focus%2FSA_Plant_Health%2FSA_Domestic_Pests_And_Diseases%2FSA_Pests_And_Diseases%2FSA_Plant_Disease%2FSA_PRAM%2F.
Goheen, E. M., Hansen, E., Kanaskie, A., Osterbauer, N., Parke, J., Pscheidt, J., and G. Chastagner. 2006. Sudden Oak Death and Phytophthora ramorum: A guide for forest managers, Christmas tree growers, and forest-tree nursery operators in Oregon and Washington. EM 8877, Oregon State University Extension Service, Corvallis, OR. http://www.oregon.gov/odf/privateforests/docs/fh/sodguide.pdf.
Footnotes
This document is PP197, one of a series of the Plant Pathology Department, UF/IFAS Extension. Original publication date March 2004. Revised August 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.
Philip F. Harmon, associate professor, Plant Pathology Department; and Carrie Harmon, associate director, Southern Plant Diagnostic Network, Plant Pathology Department, UF/IFAS Extension, Gainesville, FL 32611.
Images included in this document were adapted with permission from Davidson, J. M., S. Werres, M. Garbelotto, E. M. Hansen, and D. M. Rizzo. 2003. "Sudden Oak Death and Associated Diseases Caused by Phytophthora ramorum. Plant Health Progress doi:10.1094/PHP-2003-0707-01-DG.
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U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.
