By Joe Doccola
Sudden Oak Death, (Phytophthora ramorum), or Phytophthora canker disease, is a fungus first identified in 1993 in Germany and the Netherlands on ornamental rhododendrons. First reported in California in 1995, it has killed tens of thousands of trees in California (primarily different species of oaks), but is now spreading to other areas of the country through the transportation of infected hosts.
The name Sudden Oak Death (SOD) is used because of the rapid color change of leaves from green to brown. Phytophthora attacks roots in poorly drained or anaerobic soils.
What does it look like?
The fungus causes a bleeding canker on the stem of certain hosts, staining the surface of the bark and the lichens that grow on it, accompanied by browning of leaves. Necrotic bark tissues surrounded by black zone lines are usually present under affected bark.
Infected trees may be infested with ambrosia beetles, bark beetles and sapwood rotting fungi. Infection of the foliage of susceptible hosts is indicated by dark-grey to brown lesions with indistinct edges. The lesions may occur on the leaf blade, in vascular tissue or on the petiole.
Host Material and Range
Phytophthora ramorum causes two types of diseases — bark cankers that may kill the host and foliar blights that may serve as a reservoir for the pathogen.
Regulated bark canker hosts include Tanoak, coast live oak, California black oak, Shreve oak, canyon live oak, coast redwood, Douglas fir for West Coast species and northern pin oak and northern red oak for East Coast species — suggesting the potential for establishing SOD beyond the West Coast.
Regulated foliar hosts include Manzanita, rhododendron, bay laurel, bigleaf maple, California buckeye, honeysuckle and others.
Sudden Oak Death has been confirmed in forests in California and Oregon and in nurseries in California, Oregon, Washington and British Columbia.
SOD is spread aerially, usually at short distances (100 yards or less), but occasionally up to 1-2 miles. Infectious airborne microscopic structures known as sporangia are produced during rain events on plant surfaces (leaves) and accumulate also in soil and stream water. Besides the natural spread of the pathogen, movement of infected plants or plant parts, soil and water may lead to new infestations. Soil and plant material on tools and equipment may vector the disease.*
SOD has occurred primarily in California and southwestern Oregon. The pathogen has the potential to infect oaks and other trees and shrubs elsewhere in the United States. It is not really known how SOD spreads. Most species of Phytophthora are spread by soil, rainwater and infected plant material. Some species are also airborne. A susceptible host and favorable climate conditions are necessary for infection to occur. Moist, cool, windy conditions are thought to spread the pathogen by dispersing spores from the leaves of foliar hosts. Transport of infected foliar host plants may aid the spread of SOD throughout the United States.
It is important to focus on maintaining tree health though proper cultural practices. Avoid disturbance of the root zone, prevent frequent irrigation, and minimize injuries to the trunk and large branches. Prune coastal oaks during the dry summer months when the beetles and causal pathogens are least active. Limit pruning to dead, dying or structurally unsound branches.
Symptoms on affected hosts may vary by species, and it is therefore hard to differentiate. You should contact your state’s university diagnostic laboratory or Department of Agriculture if you suspect trees or plants are infected.
Monitor oaks in urban settings for the bleeding symptoms. If bleeding occurs, systemic applications of insecticides or fungicides (such as Arborjet’s PHOSPHO-jet) registered for woody ornamentals may be recommended.
A severely infected plant should be destroyed, and surrounding trees should be treated preventively with a fungicide. The optimum time to treat is before trees are infected. When treating by tree injection, apply when temperatures are moderate and the soils have adequate moisture.
In landscape trees, apply a labeled systemic fungicide and incorporate high-quality organic matter (e.g., humic compounds) to the soil around the base of the tree. The addition of humates will help to increase soil aeration and make the root environment less conducive to disease development. Humic compounds also support the development of mycorrhizal associates, which further protect tree roots. Tree recovery depends on the severity of the Phytophthora infection at the time of injection.
What can you do?
As with most tree diseases and insect pests, early detection is the key to successful management. If SOD is detected in your area, treat early (i.e., when your trees still appear healthy), following the guidelines suggested here, for the best outcomes.
Joe Doccola is director of research and development at Arborjet, Inc. He is an ISA Certified Arborist and Plant Health Care specialist with more than 30 years experience in the horticultural and arboricultural field.
* “Sudden Oak Death (SOD): Biology and Control,” U.C. Berkeley Forest Pathology and Mycology SOD workshops.