Here's the December update on Sudden Oak Death-related research and management affecting the North Coast. Remember, your questions and comments are welcome. Happy reading!
Severe Impacts in Sonoma: Are We Next?
Tanoak trees have been dying in very large numbers in Sonoma County for several years now. As we’ve mentioned in this newsletter before, residents who had been watching trees die by the dozens in their yards and in steep canyons behind their homes finally got together in 2006 to attract some money from the county and from the USDA Forest Service to address the issue. That initial work soon sprouted into a grant from the Bureau of Land Management to fund removals of dead tanoaks near homes and in neighborhoods to provide clearance, breaks in fuel continuity, and hazard mitigation.
Now that program is getting underway, according to Caerleon Safford of the county Department of Emergency Services and Fire Safe Sonoma, with the first projects set to begin any day now. Awards have gone to individual landowners as well as coordinated neighborhood groups, and the intensity of the work to be done varies according to the topography, the nature of the vegetation, and the amount of mortality on each property. For more information about the program, call Safford at (707) 206-5467 or email her at csafford@mcn.org. You can also visit the program web site at http://www.firesafesonoma.org/whats_new.htm.
Mendocino and Humboldt County residents would do well to examine Sonoma County’s experiences. Although the wave of tanoak mortality has slowed in much of Humboldt County over the past two years because of the dryness of the late winter/early spring periods, the next wet winter could set off another wave. Parts of Humboldt County look a lot like the heavily infested areas of Sonoma County, with many people living in steep canyons that are covered by forests very similar to the ones in Sonoma County. These areas are heavily vegetated with tanoak and have difficult access and egress.
If things go the way they have down south, within a year or two after the next wet winter/spring period, Humboldt County residents could see increased numbers of dead tanoaks near their houses and along the roads. For example, one Sonoma County resident reported 137 dead tanoak trees within 150 feet of his house, according to Safford.
For a sense of the magnitude of the problem in Sonoma County, you can check out the Sonoma County Sudden Oak Death Strategic Response Plan prepared by the county outreach coordinators with help from their colleagues in the county and presented to the Board of Supervisors early this year. Altogether, the plan estimates that the county faces over $4 million in immediate needs across various county departments related to mitigating Sudden Oak Death impacts, with over $3 million in costs over subsequent years. You can access the plan at http://cesonoma.ucdavis.edu/Custom_Program193/Sonoma_County_SOD_Strategic_Response_Plan_-_January_2008.htm.
Tanoak mortality at one Sonoma County property (Photo courtesy of Caerleon Safford)

More Sonoma County tanoak mortality (Photo courtesy of Caerleon Safford
Banana Slugs and Sudden Oak Death
For a long time, people have suggested that wildlife may carry P. ramorum, serving as vectors for Sudden Oak Death and P. ramorum infection of non-oak host species. But it’s hard to figure out how to study that. Look for spores on turkey feet? Detain some deer and send them through an infected forest?
Researchers from Oregon State University and UC Davis have done pretty much exactly that—on a miniature scale. Observing that the spores of some other species of Phytophthora were known to have improved germination rates after passing through the digestive systems of snails, these researchers decided to feed two species of slugs common in the Pacific Northwest—the banana slug, Ariolimax Columbiana, and the gray garden slug, Deroceras reticulatum—chlamydospores of P. ramorum.
Chlamydospores are the “resting spore” stage of P. ramorum. They are big, relative to the other spore stages, and tough. They presumably take a lot of energy for the organism to produce, and they appear to function as one possible “oversummering” mechanism, allowing the pathogen to survive our long, dry summers and autumns. In the laboratory, they germinate at unpredictable times, with no well-known, specific triggers to make them do so.
After the researchers fed the slugs pure cultures of P. ramorum or strawberries infected with P. ramorum, they observed numerous chlamydospores in the slugs’ feces. Many of these chlamydospores germinated to form new germ tubes, and others germinated to form structures bearing other spore types. The scientists observed the same phenomenon when they let the slugs loose on a prepared “litter” of infected bay, tanoak, and rhododendron leaves: the slugs excreted lots and lots of chlamydospores in their feces.
Banana slug fecal samples caused lesions, indicative of P. ramorum infection, on tanoak and rhododendron leaves to which they were applied. Also, when slugs that had grazed on infected litter crawled over intact tanoak logs (blemish-free bark), two of nine logs became diseased.
Although the researchers heavily cautioned readers not to read too much into the experiment, noting that field conditions can be quite different from laboratory conditions, they have apparently uncovered one possible animal vector (albeit an awfully slow-moving one) for Sudden Oak Death and related diseases. Slugs, of course, are very active in the wet months when P. ramorum inoculum is being produced and transported in water and wind. The researchers are seeking funding for more long-term studies of slugs and P. ramorum. Stay tuned.

Banana slug (photo courtesy Joseph O'Brien, USDA Forest Service, bugwood.org)
New Research on Fire and Sudden Oak Death in the North Coast
Many people have speculated that increased oak and tanoak mortality from Sudden Oak Death might contribute to an increase in extreme fire behavior in the coastal ranges. Especially worrisome to fire behavior analysts is the time period from tree death to the shedding of brown, dead leaves. This period can last from one to three years (more in some cases) depending on the weather and exposure of the tree. Analysts wonder whether this persisting fine dead material can increase ember production and spotting behavior, potentially contributing greatly to fire spread, given the right conditions. Other concerns center on the buildup of surface fuels in infested stands.
Studies have begun to test these speculations. For example, a new research partnership plans to visit sites in Sonoma, Mendocino, Humboldt, and Del Norte Counties to inventory ground fuels in Douglas-fir-dominated stands that contain large amounts of tanoak and that show varying levels of impact from Sudden Oak Death. They also plan to visit stands in which the tanoak was killed with herbicides varying numbers of years ago, to see if these stands are viable surrogates for stands that have been impacted by Sudden Oak Death over the long term. Since we have only observed the effects of the disease in California forests for about a decade, it is important to try to get a sense of what the impacts will be over a broader time horizon. Will surface fuels decay quickly? Will they build up and subside in waves, as the disease cycles through in wet years and then dies down again? This group consists of partners from UCCE Humboldt-Del Norte, CAL FIRE Humboldt-Del Norte, the Department of Forestry and Wildland Resources at Humboldt State University, and the Rizzo Laboratory at UC Davis. The project was funded by a grant from the USDA Forest Service Pacific Southwest Research Station.
Another study related to fire and Sudden Oak Death is ongoing right now in Humboldt County, where Humboldt State University graduate student Howard Kuljian, advised by Morgan Varner from the Department of Forestry and Wildland Resources, is studying foliar moisture in the leaves of tanoaks that are declining or dead from Sudden Oak Death. Howard presented preliminary results from his moisture measurements, and they were surprising: although foliar moisture levels did not decline significantly in the initial stages of P. ramorum infection, foliar moisture was much lower than expected in dead tanoak leaves and twigs. Howard will continue his measurements monthly through March 2009, completing a year of observations in this infested stand.
Additional fire-related studies are underway in the large areas burned by fires in 2008 in Big Sur, where the Rizzo Lab from UC Davis had already established hundreds of plots in P. ramorum-infested forests. Altogether, these studies should help fill in some large gaps in our understanding of the relationship between fire and Sudden Oak Death and enable us to better plan for managing the impacts of this disease.
!--[if>/w:View>/w:Zoom>/w:SaveIfXMLInvalid>/w:IgnoreMixedContent>/w:AlwaysShowPlaceholderText>/w:LidThemeOther>/w:LidThemeAsian>/w:LidThemeComplexScript>/w:Compatibility>/w:BrowserLevel>/m:mathPr>/w:WordDocument>![endif]-->!--[if>/w:LatentStyles>![endif]-->!-- /* Font Definitions */ @font-face {font-family:"Cambria Math"; panose-1:2 4 5 3 5 4 6 3 2 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-1610611985 1107304683 0 0 159 0;} @font-face {font-family:Calibri; panose-1:2 15 5 2 2 2 4 3 2 4; mso-font-charset:0; mso-generic-font-family:swiss; mso-font-pitch:variable; mso-font-signature:-1610611985 1073750139 0 0 159 0;} /* Style Definitions */ p.MsoNormal li.MsoNormal div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin-top:0in; margin-right:0in; margin-bottom:10.0pt; margin-left:0in; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri""sans-serif"; mso-fareast-font-family:Calibri; mso-bidi-font-family:"Times New Roman";} a:link span.MsoHyperlink {mso-style-priority:99; color:blue; text-decoration:underline; text-underline:single;} a:visited>!--[if>![endif]-->/o:p>/o:p>/o:p>/o:p>/o:p>/o:p>We're moving our quarterly updates about Sudden Oak Death in the north coast to this space. This will be easier to access and will also enable you to interact electronically with us here at UCCE Humboldt-Del Norte by sending us your questions and comments. (We will be able to view your comments in advance so that if it's something sensitive, we can call you rather than posting the answer on this blog.)
Additionally, we will post items and articles of interest here on a more regular basis, so check back here often. Remember that we are here to provide assistance and answer any questions you have about Sudden Oak Death or other forest health-related topics; just post a question to this blog or call Chris or Yana at (707) 445-7351.
Genetics and Pathogen Movement
Although no one knows exactly when the Sudden Oak Death epidemic began in
The Garbelotto lab’s study uses microsatellites, which are locations on the genome where base pairs of amino acids repeat many times. These base pairs often randomly mutate even in organisms that reproduce asexually (i.e., through cloning), as is the case with P. ramorum in North American forests. Strictly speaking, all the isolates of P. ramorum throughout
The Garbelotto lab collected 300 separate isolates of P. ramorum throughout
The sites at the base of the tree—in other words, the isolates that are probably the ancestors of all the others—contained the nursery isolates, the isolates from
With this information, and also looking at the relationships of isolates on a smaller scale by comparing the proportions of genotypes present in each collection locale, the lab determined that natural spread of P. ramorum probably occurs through two main modes: short-distance dispersal of less than 200 m in normal weather conditions, and occasional long-distance dispersal of 1-5 km in extraordinary storm events with extreme winds.
The one question that the lab has unfortunately been unable to answer so far involves how the pathogen got to
European Strain in McKinleyville
In 2006, surveyors from UC Cooperative Extension Humboldt-Del Norte (UCCE) and UC Davis were surprised to find P. ramorum in a small watercourse in McKinleyville. The pathogen showed up again in 2007 and has continually confounded every effort to trace its source.
The watercourse in question is Widow White Creek, which begins in the hills east of town and flows through residential neighborhoods before draining to the ocean just north of the mouth of the
Personnel from UCCE and collaborators have visited the watercourse numerous times searching for plants near the stream that may have been infected by the pathogen. The searchers have submitted numerous samples from planted ornamental hosts as well as from plants not known to be hosts. Most samples did not look highly suspicious, but the surveyors collected and sent everything they could find. All the samples turned up negative for the pathogen, leaving the surveyors scratching their heads and wondering where the inoculum in the stream could have originated.
To complicate the picture further, genetic analysis done at Oregon State University on all the McKinleyville stream samples collected in 2006 and 2007 revealed that two separate strains of P. ramorum—both the North American strain and the European strain—are present. Up until this point, only the North American strain has shown up in the natural environment in
Again, there is at least one bright spot in this picture—the absence of host trees along the stream and its short travel time to the ocean. No one knows whether P. ramorum can sustain itself within watercourses perennially or is delivered anew each year from the inoculum source, wherever it is. For the foreseeable future, surveyors will continue to beat the bushes around the stream in an attempt to find that source.
Similar stream finds have recently been recorded in
As searches for the source continue and our understanding of the behavior of P. ramorum in watercourses improves, we will hopefully come closer to knowing whether such infestations are an actual threat to the wildland environment or just minor points of interest useful mostly for detecting the organism in that environment.
Widow White Creek, McKinleyville
2007 Difficult for
The team of scientists and land managers working to eradicate P. ramorum from
Altogether, 720 acres were marked for treatment in 2007; only 420 of those acres were treated before money ran out. The
Tanoak Resistance
Here’s one question that comes up time and again when we talk to people about Sudden Oak Death in the north coast: Are any tanoaks naturally resistant to the disease? If scientists find this to be the case, then obviously we have some hope for restoring and reforesting areas where P. ramorum has caused massive tanoak mortality (such as in Point Reyes, where in some individual stands up to 95% of the tanoaks have died).
Unfortunately, the answer to these questions involves more than a trip to a Sudden Oak Death-devastated area to see which tanoaks don’t seem to be affected. Although that certainly is an important first step, assessing resistance to P. ramorum is not so straightforward. The movement of P. ramorum, and thus the incidence of Sudden Oak Death, is extremely patchy and apparently random across the landscape. Which trees escape death may be more a function of “luck” than anything else.
It also may have to do with climate; in wet years, more P. ramorum spores are produced, and more trees are infected. Conversely, during dry years, even extremely susceptible tanoaks may escape infection.
If past experience is a guide, successful programs to identify and/or breed disease-resistant trees take many years and tremendous amounts of money to develop. At the USDA Forest Service’s
Now Bay Area researchers have taken the first steps toward building a similar resistance program for tanoaks to help address the growing problem of mortality caused by Sudden Oak Death. UC Berkeley Professors Richard Dodd and Matteo Garbelotto, graduate student Katy Hayden, and Forest Service conservation geneticist Jessica Wright are exploring some of the preliminary questions that must be answered to get such a program off the ground.
Some of these questions involve the history of tanoak and how tanoaks came to be distributed as they currently are. The answers to this question can help determine whether tanoak acorns taken from one site will be successful in other sites. The scientists have found that tanoaks exist in at least four discrete supergroups in
Other research efforts involve growing tanoaks from acorns in a common environment and inoculating them directly with P. ramorum to test for greatest resistance to the pathogen. These kinds of projects typically involve a great deal of start-up investment in figuring out how to germinate and grow the seedlings properly; they are now underway at
Such efforts will probably not lead to the discovery or development of genetic individuals that are completely immune to Sudden Oak Death; they are more likely to lead to individuals that are able to live with the disease—a desirable outcome, because it is not likely to stimulate the rapid counter-development of pathogen adaptations that would enable P. ramorum to overcome immune tanoaks. In any case, although resistance may provide a partial answer to the many dilemmas posed by Sudden Oak Death in the north coast, that answer is a long way off.
Bay Removal Information
Recently completed studies suggest that strategic removal of bay laurel (pepperwood) trees near coast live oaks may help prevent the oaks from being infected by P. ramorum. Ted Swiecki and Elizabeth Bernhardt of Phytosphere Research in
Swiecki and Bernhardt looked closely at the distances of bay trees from oak tree boles in various plots and compared those distances with the severity of symptoms caused by P. ramorum on the oaks (e.g., bleeding cankers on the stems). They concluded that the presence of bay trees within 2.5 m (8.2 ft) of oaks was most strongly correlated with infection. This strongly suggests that removal of bay trees within this distance might help protect susceptible oaks; removal of bay trees within 5 m (16.4 ft) can be expected to have even stronger protective effects.
This result is a straightforward, commonsense one given that bay trees have been shown to be the primary carrier of P. ramorum in
The researchers have now begun to test their recommendations about bay removal by cutting bay trees in their study plots. Because coast live oak is more resistant to P. ramorum infection than tanoak, it could be years before they have firm results. Meanwhile, similar studies concerning the latter species will most likely also commence in the near future, and UCCE Humboldt-Del Norte is working on similar studies on a larger scale by coordinating two bay removal projects. The first is at an experimental silvicultural management area southwest of Garberville where other kinds of P. ramorum management studies are ongoing. The other is an attempt to forestall the movement of P. ramorum up the South Fork Eel/Avenue of the Giants corridor between Myers Flat and Weott, which represents the northernmost natural extent of P. ramorum-infected trees in Humboldt County. In this area, only bay trees have been found to be infected, and in 2007 California State Parks, CALFIRE, and UCCE collaborated to remove all bay laurels in an approximately 2 1/2-mile strip between the river and the Avenue. The removal of the bay laurels did not affect habitat value or aesthetics in the treated forest, and so far in 2008, none of the remaining vegetation has been found to be infected.
For more information about this research, along with photos that illustrate the study plots where these observations were made, go to http://phytosphere.com/publications/SODmanagementstudy.htm.
