SOD UPDATE 3/17/04 |
Sudden Oak Death (SOD) & Ramorum Blight
Dr. Tim Schubert, Timothy.Schubert@freshfromflorida.com, and
Robert Leahy, Robert.Leahy@freshfromflorida.com, Florida Department of Agriculture & Consumer Services, Division of Plant Industry
INTRODUCTION: In the early to mid 1990s, a dramatic, lethal disease was recognized for the first time on tanoak (Lithocarpus densiflorus) and coastal live oaks and California black oaks (Quercus agrifolia and Q. kelloggii) in Marin County, California. Investigation into the cause of the decline began in late 1999, and a novel isolate of Phytophthora was identified as the pathogen. The disease has since spread into nine other counties in California and one in Oregon. Recently, this Phytophthora has been declared a new species, P. ramorum. The pathogen causes progressive tip dieback and extensive bleeding cankers on the stems of its major hosts, usually killing them. Additionally, foliar infections that can progress to stem infections are possible on less susceptible hosts (Figs 1-6). Infections are strictly above-ground.
 Fig. 1. Death of hosts in a mixed stand. |
 Fig. 2. Wilted tanoak shoot. |
 Fig. 3. Bleeding from trunk canker on tanoak. |
 Fig. 4. Early canker exposed under peeled bark of infected oak. |
 Fig. 5. Bleeding from older trunk canker on coastal live oak. |
 Fig. 6. Foliar lesions on Rhododendron. |
BIOLOGY: Inoculum of the pathogen takes the form of thick-walled chlamydospores, zoosporangia with zoospores, and (in culture) oospores. Inoculum is presumably wind blown and could conceivably be moved on infected / exposed plants and associated growing media, in soil and debris attached to equipment, clothing, etc., and in runoff water. Ongoing investigations into the full host range of the pathogen have helped explain how the pathogen is spreading into new territory. Foliar and/or stem infections have been demonstrated on both cultivated and native rhododendrons (Rhododendron spp.), madrone (Arbutus menziesii), evergreen huckleberry (Vaccinium ovatum), California-laurel or bay-laurel (Umbellularia californica), California buckeye (Aesculus californica), manzanita (Arctostaphylos spp.), Shreve oak (Quercus parvula var. shrevei), Viburnum bodnantense, Christmasberry (Heteromeles arbutifolia), and big-leaf maple (Acer macrophyllum). In 2002, the host range was expanded to include redwood (Sequoia sempervirens) and Douglas-fir (Pseudotsuga menziesii). Cultivated rhododendrons and Viburnum bodnantense are known hosts of P. ramorum in Europe, and may be the original source of the pathogen now present in California and Oregon. Artificial inoculations on seedlings of several red oak species not native to California have been successful, indicating the risk may extend through the native range of certain red oaks, which would include the entire continent. The pathogen appears to have a strict requirement for cool, moist environmental conditions for successful infection.
REGULATORY: Susceptible plant material from counties where SOD has been confirmed (Alameda, Contra Costa, Humboldt, Marin, Mendocino, Monterey, Napa, San Mateo, Santa Clara, Santa Cruz, Solano, and Sonoma in counties in California) and from extreme southwest Oregon (Curry County) should be considered high risk. A Federal Quarantine has been put in place. Ericaceous plants, viburnums, and perhaps some oaks and oak relatives would be the most likely commodities to be shipped from the infested areas.
All illustrations and map are provided courtesy of the California Oak Mortality Task Force www.suddenoakdeath.org
In addition to these sites in California, SOD was detected in 8/2001 in Curry Co. in extreme southwest Oregon.
LITERATURE CITED:
Werres, S., R. Marwitz, W. A. Man In't Veld, A. W. A. M. De Cock, P. J. M. Bonants, M. De Weerdt, K. Thiemann, E. Ilieva, and R. P. Baayen. 2001. Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycol. Res. 105: 1155-1165.
SOD Distribution in California
