Sudden Oak Death Threatens Coastal Oak Forests

Oaks ‘n’ Folks – Volume 17, Issue 1 – February 2001

As reported in two previous issues of Oaks ’n’ Folks (August 1999 and March 2000), a new type of mortality in tanoaks (Lithocarpus densiflorus) was first observed in Marin County in 1995. Since those first sightings by University of California Cooperative Extension Farm Advisor Pavel Svihra, this problem, named “Sudden Oak Death’ or SOD, has also been reported to affect both coast live oak (Quercus agrifolia) and California black oak (Quercus kelloggii). While initially limited to a few areas, in the last year and a half the range of Sudden Oak Death has expanded substantially, especially for coast live oak, and there is genuine concern that SOD could decimate coastal oak forests. That is the bad news.

The good news is that UC scientists and others working on SOD have also made considerable progress in understanding what the underlying cause of this problem is, and in mapping where it is occurring and how it has moved over time. In summer 2000, UC Davis forest pathologist Dr. David Rizzo isolated a new species of Phytophthora from a number of infected trees. Previous attempts to identify this pathogen were unsuccessful since it turns out that positive identification requires closely controlled laboratory conditions and samples must be cultured soon after collection. To date, this new disease organism has been isolated from trees exhibiting SOD symptoms in Monterey, Santa Cruz, San Mateo, Marin, Sonoma and Napa Counties.

Most recently, Dr. Rizzo and UC Berkeley forest pathologist Dr. Matteo Garbelotto, aided by Santa Cruz County Farm Adviser Steve Tjosvold, and European researchers Dr. Sabine Werres (Institute fur Pflanzenschutz in Gartenbau, Braunschweig, Germany), Dr. Peter Bonants (Plant Research International, Wageningen, The Netherlands), and Professor Clive Brasier ( Forest Research, Alice Holt Lodge, UK) have determined that a new species of Phytophthora isolated as early as 1993 from ornamental rhododendrons in Germany and The Netherlands closely matches the newly-discovered species found in California. However, the Phytophthora has not yet been officially described and given a scientific name.

Although side-by-side comparisons have not been performed yet, the morphological descriptions of both Phytophthoras match, indicating the isolates are likely to belong to the same species. Supporting evidence has been provided by DNA sequence comparisons conducted at Dr. Garbelotto’s lab at UC Berkeley of the internal transcribed spacer (ITS) of the ribosomal DNA. Additional evidence linking cultivated rhododendrons with SOD came when UC researchers isolated a Phytophthora from rhododendrons at in a nursery in Santa Cruz County and found it to have the same morphology and DNA signature as the Phytophthora associated with SOD.

This new Phytophthora appears to be genetically distant to most of the other 60 species within the genus Phytophthora. The closest relative in the genus is Phytophthora lateralis, a virulent pathogen of Port-Orford-cedar (Chamaecyparis lawsoniana) known to be present in natural stands in the Pacific Northwest and occasionally on Port-Orford-cedar stock in nurseries in Europe.

In California, there is a substantial overlap between areas where rhododendrons grow and areas where the oak species known to be susceptible to the new disease are found. Rhododendrons are also extremely common ornamental species throughout the state. The UC team is now intensifying efforts to examine natural populations of rhododendron and other non-oak hosts. Further host-range studies are ongoing in the laboratory and greenhouse.

Typical symptoms of infected trees include seeping of dark brown viscous sap from lower portions of the main stem, dead discolored patches beneath the bark, extensive tunneling by small insects (ambrosia and oak bark beetles), and the appearance of dark circular fruiting bodies of Hypoxylon fungi on the bole. Eventually, the leaves on the trees die and the crowns turn brown. While the name Sudden Oak Death implies that trees are killed rapidly, scientists believe that it can take months or even a year or two from the time of the initial infection for death to occur. The boring insects and the Hypoxylon fungus are believed to play a secondary role, attacking trees that have already been weakened by the new Phytophthora, thus hastening their demise.

There is also an extensive effort to identify and map locations where the disease has been positively identified and where trees with similar symptoms have been observed (see Community Involvement this issue). This information will be particularly important in determining how rapidly SOD is spreading, tracking areas of new infection, developing regulations, and managing fire prevention and hazard tree removal programs. It is also important for quantifying economic and ecological impacts. Another critical area of research is determining how the pathogen moves from infested to non-infested areas. Similar pathogens are often spread by soil and/or water, but it is possible that this one is transmitted through the air or vectored by the insects that attack infected trees. All of these pathways are being investigated.

Even though trees with similar symptoms have been observed from as far north as Humboldt County and as far south as Santa Barbara County, this new Phytophthora has not been isolated at those locations. In these instances it is believed that other diseases, such as Phytophthora cinnamomi (crown rot) and Armillaria mellea (oak root fungus), which can cause similar symptoms, are probably responsible for many of those tree deaths. These organisms may also be responsible for much of the mortality in areas where SOD has been detected. It is also important to point out that at this time, it appears that members of the white oak sub-genus of Quercus, including both blue oak (Q. douglassii) and valley oak (Q. lobata), do not appear to be susceptible. To date, this disease has also been limited to fairly cool and wet coastal locations.

The potential consequences of high levels of oak tree mortality from SOD are severe and far-reaching. First, the visual landscape, so characteristic of much of California, could be altered dramatically. There could also be significant impacts to the many wildlife species that are so dependent on coastal oak forests for food and shelter. Deer, turkeys, jays, quail, squirrels, and acorn woodpeckers are just a few of the many species that rely heavily on acorns as a food source. And there are countless other animals that use oak woodlands for breeding or as stopover points during migration. Ecological processes such as nutrient cycling, storage and release of water, and moderation of soil temperatures could also be affected. Of more immediate concern, however, is the greatly increased risk of fire resulting from the addition of large quantities of highly combustible fuels. This risk is particularly serious because so much of the coastal forest contains urban interface areas where homes and businesses are nestled among trees.

So what is being done? In August 2000, the California Oak Mortality Task Force (COMTF) was established to provide a comprehensive and unified approach to address Sudden Oak Death. This Task Force has six board members representing a wide range of interests and backgrounds and oversees committees on research, education, fire prevention, management, monitoring, funding, and regulation. It was immediately recognized by the Task Force that there was an urgent need for additional funding. Until recently, the research undertaken and described above has mainly been funded by the USDA Forest Service and short-term grants from the University of California’s Integrated Hardwood Range Management Program. Educational efforts have mainly been funded by Marin County. Clearly much more money was needed to effectively address this problem. Recently, Governor Gray Davis allocated $100,000 to the California Department of Forestry and Fire Protection to fight SOD, much of which will be used to fund and staff a full time statewide SOD Coordinator. The USDA Forest Service has proposed providing $3.5 million for research and monitoring. Finally, legislation has been introduced to the California State Legislature that would provide as much as $10,000,000 over 3 years for research, education, monitoring, management, fire prevention, and tree removal. Approximately half of this money would be provided to affected counties through grants to assist private landowners to remove dead trees on their property. While the threat of SOD is very serious and should be of concern to all Californians, it is encouraging to know that there is broad consensus that resources need to be allocated to minimize the impacts and find a solution. We urge the political agencies making these funding decisions to act swiftly and forcefully. Hopefully these efforts will succeed and our majestic oaks, which we sometimes take for granted, will continue to survive and prosper.

For additional information, see the following web sites:

prepared and edited by Adina Merenlender and Emily Heaton