Sudden Oak Death (SOD)

Stefan P. Parnay, Agriculture, Weights and Measures - Marin County

Sudden Oak Death (SOD) is a disease caused by the plant pathogen Phytophthora ramorum, and has caused widespread oak and tanoak dieback in native forests of northern and central coastal California and southwestern Oregon. The pathogen prefers areas that are naturally cool, moist, and foggy, and spread of the disease can vary with yearly changes in the weather.

Disease symptoms in susceptible trees include oozing trunk cankers and foliage dieback, often resulting in mortality in tanoaks and some oaks. In addition to affecting trees, the pathogen may also cause a foliar disease, often called Ramorum Blight, which results in leaf spotting and/or twig dieback, but rarely causes mortality.

Currently there are over a 100 native and ornamental hosts, and new hosts continue to be found and added to the state and federal quarantines which regulate their movement. Hosts include forest trees, shrubs, and understory plants, and ornamental nursery plants such as rhododendrons and camellias. One host, California bay laurel, is a primary source of pathogen inoculum. The disease is commonly spread through natural or artificial means (rainwater, soil, infested nursery stock) under moist conditions.

Tree mortality in wild land and urban/wild land interface areas can cause dramatic changes in the landscape, negatively impact ecosystems, increase fire and safety hazards, and decrease property values. Cooperation between government agencies, researchers, private landowners, and the nursery industry has been essential to managing the spread of the disease.

Marin County is generally infested with the disease, and it is primarily found in the native woodland and rural areas within the county. Our department is responsible for helping to prevent the artificial movement of the disease out of the regulated area. Production shipping nurseries are inspected regularly for the presence of the disease, and for compliance with best management practices. Anyone moving host plant material outside of the regulated area must contact our office to have the plants inspected and certified prior to shipment. We can be reached at (415) 473-6700.

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