SPECIAL TOPICS: SUDDEN OAK DEATH

Sudden oak death is a new disease, caused by Phytophthora ramorum, 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. It was first described in Europe on ornamental Rhododendron sp. and Viburnum sp. in 2001. In 2002 it was reported in California and Oregon and has been found in western Canada.

Various symptoms have been produced on more than 40 species of native and cultivated ornamental plants infected with the pathogen. 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. Symptoms of this disease vary from host to host and may progress rapidly after infection or may not be visible for significant periods of time.

Symptom progression is favored by temperatures near 20°C and are as follow:


Leaf lesions typically begin where moisture accumulates on a leaf surface, such as at the leaf tip, 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. Blighted leaf tissue typically turns tan to brown and may have a reddish tinge. Lesions commonly expand from the midvein in an angular fashion.
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. 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. Vascular discoloration on mature tan oak is characteristically bright red.
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 result in death of attached leaves.
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.

Fungus can be spread by movement of infected host material, infested soil, irrigation water, and wind-blown rain. Unintentional movement of infected 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 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.

For more information and charts go to: http://edis.ifas.ufl.edu/PP118
Sudden Oak Death. Philip F. Harmon and Carrie L. Harmon.Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date March 2004.