• Topics: Plant Pathology  | Landscape Pest Management  |  Current Pest Topics  |  Nursery and Ornamental Plant Diseases  |  Lawn and Garden Plant Diseases  |  Harmon, Phil  |  Harmon, Carrie Lapaire  | Sudden Oak Death 

Sudden Oak Death¹

Sudden oak death is a new disease capable of causing a range of symptoms from leaf spots to plant death on many woody hosts. Because sudden oak death is a new disease, much about the pathogen, host range, and the disease epidemiology is unknown.

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 sp. and Viburnum sp. in 2001. In 2002 the pathogen was reported in California and Oregon and has since been found in western Canada.

Host Range

Various symptoms have been produced on more than 40 species of native and cultivated ornamental plants infected with the pathogen (Table 1). The list is not exclusive as 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. Additional hosts will be identified as the pathogen is spread to new areas.

Disease Symptoms

Symptoms of this disease vary from host to host; however, roots of plants infected with P. ramorum typically appear healthy. Known hosts and observed symptoms are listed in Table 1. 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.

Leaf Lesions

Leaf lesions typically begin where moisture accumulates on a leaf surface, such as at the leaf tip or petiole (depending on 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).

Figure 1. 

Leaf lesions on Kalmia latifolia (moutain laurel). Note the symptom occurs at the leaf tip, and the lesion has a water-soaked border.

Figure 2. 

Tan to reddish brown leaf lesions typical of sudden oak death on Camellia sp.

Bleeding Cankers

Cankers are sunken or swollen lesions on 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 tan oak is characteristically bright red (Fig. 6).

Figure 3. 

Angular lesions caused by P. ramorum on leaves of Camellia sp.

Figure 4. 

Bleeding canker symptom on coast live oak caused by P. ramorum.

Figure 5. 

Vascular discoloration of Rhododendron sp. caused by P. ramorum.

Figure 6. 

Canker and vascular discoloration on a mature tan oak.

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 formation of a “shepherds crook” (Fig. 8).

Figure 7. 

Stem dieback of Rhododendron sp. caused by P. ramorum.

Figure 8. 

Shepherds crook symptom on tan oak sapling.

General Wilt

Viburnum species 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 a new pathogen, the best option for controlling spread of the disease 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. Eradication efforts include burning and deep burial of infected plant material.

Fungicides have not been evaluated for management of this disease. It is possible that fungicides that prevent and control diseases caused by other Phytophthora spp. may be effective, but no data are available at this time.

Tables

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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