This article summarizes the information presented at the Sudden Oak Death Management Meeting at the Salmon Creek School that was held on March 22, 2014. This event was sponsored by the Gold Ridge Resource Conservation District, the University of California Cooperative Extension with support from the USDA Forest Service.
This article does not by itself contain sufficient information for SOD forest management which should only be done in consultation with a professional arborist and the Sonoma County Extension Service; it is only intended to provide an introductory overview with links to sources of additional information. Nor does it necessarily reflect the opinions of Forest Unlimited, but contains information from several sources, especially the SOD Management Meeting on 3/22. Positive identification of SOD always requires sending tissue samples to a qualified lab for analysis.
Sudden Oak Death (SOD) is caused by Phytophthora ramorum, an oomycete plant pathogen. The disease kills oak and other species and has had devastating effects on the oak populations in California, Oregon and Europe but especially Sonoma County. Symptoms include bleeding cankers on the tree’s trunk and dieback of the foliage, in many cases eventually leading to the death of the tree. The survival of an individual tree is difficult to predict since lesions may heal spontaneously only to possibly reappear later, giving the temporary appearance of a “cure”.
P. ramorum infects a great number of other plant species, significantly woody ornamentals such as Tanoak, Rhododendron, Viburnum, and Pieris. Such plants can act as a source of inoculum for new infections, with the pathogen producing spores that can be transmitted by rain splash and rainwater. Wet years are the worst for transmission of the disease and dry conditions such as we currently have restrict the spread.
P. ramorum was first reported in 1995 and most evidence suggests it was repeatedly introduced into the US as an exotic species from Asia.
Impact in Sonoma County
SOD is of particular concern to Sonoma because it attacks and kills several species of hardwoods that are an important part of the forest ecosystem. Especially vulnerable is tanoak (Notholithocarpus densiflorus, formerly and also known as Lithocarpus densiflorus). The nuts from this tree are an important food source for animals and were a traditional staple in the local Native American diet.
The lifecycle of P. ramorum is complex and impacts directly on Sonoma County forests. Plants can either be a. hosts actively spreading the disease, b. susceptible or c. immune.
Bay Laurel, Rhododendron and probably Poison Oak are hosts that harbor the fungus without dying, but serve as repositories for the spores that can infect nearby trees. Tanoak is especially hard hit because it is both susceptible and is a host in which P. ramorum can complete its life cycle while it kills the tree.
Other oaks which are susceptible but not hosts are Black Oak (Quercus kelloggii) and Coast Live Oak (Quercus agrifolia). [Note that until the 1960s the US Forest Service maintained a systematic policy of eradicating Black Oak to favor conifers].
Valley Oak (Quercus lobata), Blue Oak (Quercus douglasii) and the White or Garry Oak (Quercus garryana) are immune to SOD. Conifers, Redwood, Douglas Fir, etc. are also immune to SOD.
To Summarize the roles of Sonoma species:
Poison Oak (probably)
Tanoak – the dual role for tanoak
Coast Live Oak
White or Garry Oak
Unfortunately, there is no known treatment to preserve or save tanoak, thus the focus is on management of SOD. There also has been no natural occurring immunity yet detected in any susceptible oak species. Ted Swiecki’s presentation highlighted that phosphite-based fungicide treatments have not been effective on tanoaks, either of the spray-on or injector types. The photos shown in the presentation of dead trunk cross sections treated with injectors looked awful with extensive
necrosis at all the injector sites. Potassium phosphite solutions were chosen because they have been effective against other Phytophthora diseases and have minimal environmental impact. Although some of these products are marketed as general treatment approaches, there is no scientific evidence that they are effective on tanoaks.
There have been anecdotal reports that practices to enhance tree vigor can counteract or prevent SOD. This seems intuitive but Ted Swiecki’s presentation and his research in Ref  do not support this. He pointed out that enhanced growth leads to more bark fissures in tanoak from natural tree growth and these provide a pathway into the living part of the tree. Some of the tanoak that have survived longest with SOD infection are those that are most stressed with smoothest bark and growing in a poor environment.
The future of tanoak as a forest tree is very grim. Bay Laurel is
plentiful and often found near tanoak in local forests, as in the
photograph. Control of host species such as California Bay Laurel (Umbellularia californica) in the forest is not possible and is highly undesirable since the bay laurel is a very important forest tree itself. Elimination of living impacted tanoak itself is also a poor general practice because it precludes observation of an individual tree that exhibits resistance.
According to the suddenoakdeath.org website, an additional measure to protect oaks is to reduce the pathogen population by pruning overstory California bay laurels or removing small, understory bays in close proximity to an oak or tanoak. These are preventative measures only and not to be undertaken if oaks are already infected. If oaks are still healthy, removing bay leaves from a 15-foot area around an oak trunk may reduce the chance that the oak will get infected. There is no treatment available for infections on bay leaves or other non-oak/tanoak hosts
Late season wind driven rains are the most important way for SOD to propagate from tree to tree. Other secondary mechanisms are forest traffic (foot, bicycle, animal, vehicle) and firewood transportation. Transportation of infected foliage can also potentially propagate SOD. It was stressed that transportation of firewood will potentially propagate SOD but there are even worse tree diseases that are very readily propagated like this, so firewood should always be burned close to the source.
Obviously a lot of trees dying in the forest has many impacts
including danger to buildings, firefighters and others from both fire and falling trees, reduction of food supply for animals, changes to the forest ecology, loss of timber resource, degradation of the landscape and increased erosion. There was some evidence that eliminating competition from oaks is good for the redwoods and other conifers.
Susan Anderson spoke about her experiences preserving a black oak forest on part of her property. She removed laurel and other hosts and applied a phosphite based fungicide (Agri-Fos) to the black oaks on a regular basis as a preventative. The black oaks on the treated plot have survived for about 5 years while those in the untreated, unmanaged plots have not. She and several other speakers stressed the importance of developing a realistic plan to manage SOD impacts on a specific property.
List of Speakers 3/22/14
SOD in Sonoma County, Lisa Bell – SOD Program Coordinator UCCE, Sonoma County.
Forest Ecology – Managing Forests with SOD, Geg Giusti – Farm Advisor, Forest and Wildland Ecology in Mendocino County.
Managing SOD: What Works, What Doesn’t and What We Need to Find Out,
Ted Swiecki, Phytospere Research
The influence of SOD on fire behavior, Steven Swain – Farm Advisor, Environmental Horticulture, Marin and Sonoma Counties
Farm Bill Programs and Local Partners in Conservation: NRCS and RCD, William Hart – Gold Ridge Conservation District
How We Manage Our 46 Acre SOD-infested West County Forest, Susan Anderson – Landowner, Valley Ford, Ca.
A Native American Perspective on Managing SOD, Nina Harper
 A Reference Manual for Managing Sudden Oak Death in California,
Tedmund J. Swieki and Elizabeth A. Bernhardt, USDA Forest Service,
General Technical Report PSW-GTR-242, December 2013, 129 pgs.
This excellent document provides an excellent resource with the latest
information about SOD and is available for free from the Forest
Service either online here or copies may be obtained from:
Rocky Mountain Research Station
240 West Prospect Road
Fort Collins, CO 80526-2098