Sudden Oak Death (SOD)


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.

Causal Agent

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”.

Plants Impacted

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, Doug Fir, etc. are also immune to SOD.

To Summarize the roles of Sonoma species:

Bay Laurel
Poison Oak (probably)

Tanoak – the dual role for tanoak
Black Oak
Coast Live Oak

Valley Oak
Blue Oak
White or Garry Oak

SOD Management

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 [1] 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 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.

SOD Impacts

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

Black Oak

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

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


Wikipedia Article

Organization devoted to SOD

A Reference Manual for Managing Sudden Oak Death in California

Many Resources and links UC Cooperative Extension

Pest notes from UC

SOD pest notes from UC

A Field Guide to Insects and Diseases of California Oaks

Living with Fire in Sonoma County

Sonoma County Forest Conservation Working Group

SOD Blitz 4/19 Sign up (Sebastopol, Santa Rosa, Cloverdale, Sonoma)

SOD Maps

US Forest Service




[1] 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:

Richard Schneider
Rocky Mountain Research Station
240 West Prospect Road
Fort Collins, CO 80526-2098

(970) 498-1392

Dead tanoak next to Bay Laurel on Willow Creek Road
Dead tanoak next to Bay Laurel on Willow Creek Road