- Author: Steven Swain
Sudden oak death (SOD) is a disease syndrome that has killed millions of native oak trees (Figure 1) along the west coast of the United States, from Big Sur in California up to Southern Oregon. The disease may involve several organisms, but its main driver is the fungus-like organism (known as water mold), Phytophthora ramorum. This plant pathogen is spread in the springtime by windy rainstorms. It infects the bark of oak trees, frequently creating bleeding trunk cankers that interfere with water uptake and sugar transport.
Death of SOD-infected trees can be accelerated by attacks from bark and ambrosia beetles. In the absence of beetle attacks, infected oaks may take years to die.
Many common disorders (other than P. ramorum infections) can cause damage that resembles SOD, so laboratory testing is needed to confirm the diagnosis.
Sudden Oak Death Host Range
Sudden oak death isn't always sudden, nor does it infect just oaks. The potential host list of P. ramorum includes hundreds of plant species, many of which are natives of California's woodlands and forests. In most of these host species, the pathogen simply causes small necrotic spots on the leaves (Figure 2). In a few instances, notably in certain susceptible Rhododendron cultivars, the disease may progress from the leaves down into the stem and kill the plant.
Phytophthora ramorum has evolved as a foliar pathogen, primarily spreading from the leaves of infected hosts such as bay laurel (Umbellularia californica), tanoak (Notholithocarpus densiflorus), and rhododendrons. Therefore, it does not spread easily to other plants from oak trunk cankers. Thus, when oaks become infected, they are referred to as terminal hosts. Tanoak acts as both a foliar host, spreading spores from its leaves in wet and windy weather conditions, and a terminal host, developing cankers that are almost always lethal to the infected plant (Figure 3). Perhaps because infected tanoak leaves rain spores onto their own trunks, their SOD survival rates are among the lowest of all trees that may become afflicted by the disease in North America.
Despite its fearsome reputation, SOD doesn't always kill infected oaks. Valley oak (Quercus lobata), blue oak (Q. douglasii), and Oregon white oak (Q. garryana) are not known to develop cankers in nature, while coast live oak (Q. agrifolia), black oak (Q. kelloggii), Shreve oak (Q. parvula var. shrevei), and interior live oak (Q. wislizeni) are considered susceptible. Recent studies have demonstrated that there may be considerable variance in some coast live oaks, with measured resistance varying from about 16-40%. Many of these resistant or tolerant trees do become infected but are able to defeat the pathogen before trunk cankers enlarge to life-threatening sizes. Casual observations suggest that resistance levels seen in coast live oaks are likely similar to those in Shreve oak and black oak populations. Because interior live oak trees grow in comparatively hotter, dryer, environments than the other susceptible oaks, they almost never become infected, so resistance levels have not been studied. Tree age seems to play a role too. Oak trees under four inches in trunk diameter at chest height are not typically susceptible to infection by P. ramorum.
Treatment Approaches
Many different treatment approaches have been trialed, a few of which have shown promise.
Prevention
Potassium phosphite compounds (AgriFos, Reliant, Garden Phos, etc.) work best as preventive treatments. Most of the efficacy achieved by these compounds appears to be the result of stimulating the trees' natural defensive systems, although individual oaks vary widely in their immune responses. Thus, the application of potassium phosphite compounds is a bit of a gamble as to whether it will actually help an individual tree. Trees that are already showing symptoms of infection when treated have a significantly lower survival probability than trees that appear healthy during treatment.
There are two different recommended application methods for potassium phosphites—surface sprays made directly to the bark (using a surfactant such as Pentrabark) and trunk injections. Some applicators have claimed increased efficacy from the trunk injections, albeit at the cost of potential damage to the tree from wounding. Others have claimed equivalent efficacy with repeated bark sprays, when carefully timed. Application of calcium to the root zones of oaks treated with potassium phosphite has been shown to further improve resistance rates.
Potassium phosphite treatments should be made in the spring and fall, regardless of the application method used. This is because the uptake of potassium phosphite by the tree is dependent on high transpiration rates. High transpiration rates in turn depend on both adequate available soil moisture, and warm and sunny weather, preferably with a light breeze.
Another preventive treatment option that has proven to be effective is removal of foliar hosts that are near highly valued oak trees (Figure 4). Removing bay laurels that have foliage within about 30 feet of an oak trunk decreases the chances of that oak becoming infected. Removal is especially effective for small, understory foliar hosts, such as young bay laurels, poison oak, and rhododendron. This is not a recommendation for the wholesale removal of bay laurel trees, which are important parts of the California forest ecosystem and should be retained where appropriate. This management approach should only be adopted after careful consideration of the ecological function of the forest or woodland as a whole.
Management of Active Infections
Bark scribing, or cutting away the outer portion of infected bark to let the infection site dry out, was previously touted as an effective treatment for infected oaks. However, rigorous testing has since shown that it does not significantly increase the odds of an oak surviving. Similarly, the application of whitewash to tree trunks has not been shown to make measurable improvements to survival of treated oaks as compared to no treatment at all.
Mefanoxam (Subdue Maxx, Stergo MX, etc.) is a fungicide with a proven record of suppressing Phytophthora activity. It has been used to successfully treat infected plants, keeping them alive as long as treatment continues, typically over the span of a year or two. However, in most cases, once treatment stops, Phytophthora begins growing again within several months, and the infected plants will eventually succumb to the disease. While mefanoxam fungicides may be effectively used in certain limited situations, they should not be considered effective curative treatments. Resistance to this active ingredient has developed repeatedly, rendering it unacceptable for long-term use.
In summary, no silver bullet exists for preventing SOD, and there are no effective tools that will reliably save a tree that is already showing symptoms of infection. This does not mean that any oak that exhibits SOD trunk cankers or bleeding symptoms will die. Bleeding from the bark is a normal response to substantial damage, whether from a pest, pathogen, or, sometimes, even mechanical damage. Even if bleeding is the result of infection by P. ramorum, symptomatic trees have recovered with no intervention, usually in association with a few dry years. The most effective tools for managing SOD are preventive, in nature. For more information and best management practices, please see the California Oak Mortality Task Force website at: https://www.suddenoakdeath.org.
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In California, P. ramorum is primarily a leaf pathogen and thrives in coastal tanoak/redwood forests and oak woodlands. There are two categories of hosts for P. ramorum. In trunk hosts, the pathogen infects the bark and can cause bark cankers that may ooze a thick, sticky sap. Trunk hosts of the pathogen include many species including tanoak, coast live oak, California black oak, and Shreve's oak.
On leaves, foliar symptoms range from leaf spots to twig dieback. Sudden oak death can be lethal to trees in both forest settings and residential landscapes. Foliar hosts include California bay laurel, rhododendron and camellia.
To learn more about this disease, read Pest Notes: Sudden Oak Death and visit the UCCE Sonoma County Sudden Oak Death website.
- Author: Stephanie Larson
- Contributor: Karen Giovannini
Here is a summary of some of the work our office is doing during shelter in place.
Fire & Resiliency
UCCE Sonoma is building on the foundational work of other county departments such as Sonoma Water and Permit Sonoma, by providing outreach to private landowners to address forest health, vegetation management and fire fuel reduction.
Lake Sonoma Decision Support System: Development of an online geo spatial reporting tool to help landowners assess:
Match.Graze: development of an online database that connects land owners and grazers.
- Filmed educational videos
- Creating website
- Current status: roll out in early June.
Good Fire Alliance: partnership with Audubon Canyon Ranch to assist landowners in managing fire fuels through prescribed burning on private and publicly owned lands. The following prescribed fires are in the active planning stage:
- 100+ acre burn unit at Cooley Ranch near Lake Sonoma; late May.
- Sonoma Ecology Center is planning a burn at Van Hoosear Wildflower Preserve; late May - early June.
- Bodega Pastures spanning several weekends in October/November.
Resilient Landscapes: Master Gardener collaborative project to:
- Host Firewise webinars
- Develop materials for Fire Safe Sonoma's Living with Fire brochure and webpage resource.
- Post Fire Survival/Mortality: research project to develop a quick and simple post-fire tree survival reference tool to aid with triage of burned landscapes.
- Working with local fire departments to homogenize fire-resilient landscape standards.
Oak Tree Health: organized, hosted and presented:
- California Oak Workshop with science based oak health information. Over 500 participants.
- Sudden Oak Death Blitz pivoted to online, educating and distributing 93 test kits to the public.
Food Systems & Security
Master Gardeners:
- “Stay Home Grow Food” series has reached over 350 people with videos plus resources via an extensive social media campaign.
- Gardener Sense program delivered by video conference to help homeowners reduce water use.
- Master Gardener's are pivoting their classes to webinars.
Food Distribution
The value of a strong, connected local food system to sustain the resiliency of our communities has never been more clear.
Coordinating with Sonoma County Food System Alliance and strategizing for a series of video conferences on longer term emergency food response planning & strengthening the local food supply chain looking to local production & distribution as part of meeting food need.
- Meeting on March 23 with over 50 emergency food responders to strategize on coronavirus response.
- Chairing the Community Organizations Active in Disaster (COAD) Food group. Continue to collaborate with the group to meet the needs of emergency food organizations.
- Sonoma County Food Recovery Coalition
UCCE used connections to secure donations of over 12,000 wine boxes to
Redwood Empire Food Bank for boxing and distributing food.
Local Meat Supplies: Mobile Slaughter Unit (MSU)
UCCE is working with local livestock producers in Marin/Sonoma to create a business plan for an MSU which can assist local livestock producers with a local, safe option for processing livestock.
- Applying for USDA grant to assist these producers to determine the functionality of keeping it in production.
- Will develop educational and management strategies to ensure economic security for small-scale livestock producers.
Integrated Pest Management
- Collaborating with UC Davis researchers to continue projects that address soil and fungal pests that shorten the lifespan of vineyards.
Critical research on the newly detected invasive Mediterranean oak borer (pictured) found in valley oak in eastern Sonoma County. Collaborators include Cal Fire, US Forest Service, and CDFA.
Youth
4-H youth educational programs have continued to engage youth and adults with online technologies.
- Developed fact sheets to support volunteer educators in delivering online programs available at Youth Development Resources
- Short-term educational programs have been implemented reaching elementary-aged children with science and art content.
- Ongoing programming has been transitioning online focused for teens around college and career readiness (Juntos 4-H) and youth participatory action research.
Annual Sonoma 4-H Open House and ChickenQue transitioned from a full-day chicken BBQ lunch fundraiser to a radiothon.
- Partnered with local radio The Bull 93.7 to do a radiothon.
- The station promoted 4-H with interviews of staff, volunteers, and youth.
- The event also served as a public awareness campaign showcasing the program's legacy helping youth reach their full potential.
Economic Viability
Climate Change
Working with local dairy and livestock producers to apply for grants from CDFA to reduce greenhouse gases:
- Alternative Manure Management Program
- Healthy Solis Program
If funded, these grants would bring over $5 million to reduce GHG by 4,154 MTeCO2
Support Local Producers
- Working with local creameries and FEED Sonoma, to develop a dairy CSA box option.
- Revisit the County Lands for Food Production program initiated by UCCE to increase the availability of county owned land to communities, farmers and ranchers.
- Find Local Food & Aid the Community
Outreach
Providing information and updates.
Coronavirus Resources webpages provides information for agricultural enterprises focusing on financial resources, Ag worker safety and food safety and includes resources for where to find food from local farms and for opportunities to volunteer.
Subscribe to our monthly newsletter and alerts.
Follow our social media posts about coronavirus grant opportunities, resources, workshops and more on Twitter, Facebook and Instagram @UCCESonoma.
/span>/h3>/h3>/h3>/h2>/h2>/h2>/h3>/h3>/h3>/h2>/h2>- Author: Kerry Wininger
- Editor: Karen Giovannini
Sonoma county residents love their oak trees, and with good reason: Oak woodlands are a source of immense value not just to the more than 330 types of animals and hundreds of other organisms they support, but also to the cultural, economic, and social fabric of our society. Sudden oak death (SOD) threatens to unhinge these systems, imperil biodiversity, create hazard trees or fuel for fires, and potentially infect agriculturally or horticulturally important plants. So whether it means adapting educational events to virtual spaces, delivering materials by mail, training online, answering home phones instead of staffing the Master Gardener desk, or collecting leaves for the SOD blitz by bicycle while wearing a mask and social distancing, the UCCE Sonoma SOD Program is not letting COVID-19 slow us down.
A key endeavor of this group is the yearly Sudden Oak Death Blitz in partnership with UC Berkeley's Forest Pathology and Mycology Lab. This state-wide citizen science project allows landowners to receive free testing of California bay laurel and tanoak leaves, contribute to long-term scientific research, and update the SOD Map Mobile app used by landowners and professionals to assess disease risk in their area. As SOD Blitz creator Dr. Matteo Garbelotto wrote in a letter addressed to all Californians about this year's program,
“The SOD Blitzes have become a tradition for many, while providing key information to help us save our oaks from a devastating disease.”
UCCE Sonoma hosts six blitz events annually whereas most counties hold just one, and we were eager to test new areas of tree mortality that were discovered after last-year's blitz had already passed. Here's how we made it work:
- Training took place online.
- Three Zoom sessions were set up with SOD team members for participant questions.
- Collection packets were mailed to most participants, and the rest picked up following strict safety guidelines supervised by staff.
- Leaves were collected at homes and on public land by walkers, cyclists, and others engaging in outdoor exercise.
- Kashia Pomo staff continued to sample on their land.
- In collaboration with SSU's Center for Environmental Inquiry, we hosted the only SOD blitz in the state that was fully bilingual in Spanish and English.
Despite limitations posed by COVID-19 regarding how to train volunteers, access areas where leaves could be sampled, and safely pass materials and samples between participants and blitz organizers, this year's campaign is on track to be even more successful than 2019. Returning leaves by mail instead of in-person allowed residents on the coast and other remote areas to participate easily, leading to a better geographic distribution of sampling. Online trainings and Q&A sessions, though less personal and lacking physical demonstrations, encouraged people with schedule constraints to give the blitz a try. One participant with extra time on his hands reported that he thought this was a great way to give back to the community in a time of need. Based on the number of participants and collection packets requested, we estimate a 15% increase in sampling from last year, including all major areas of Sonoma county and most of Mendocino county.
And things are just getting started. Though a SOD-themed Plant Walk with the local Milo Baker chapter of the California Native Plant Society was postponed, discussions are underway about creating a live virtual field trip in its place, complete with 360 degree photos of highlighted spots along the trail. Similarly, SOD Specialist Master Gardeners have been presenting at public library series' for years, and are now hoping to create a recorded version to be posted on UCCE Sonoma's Sudden Oak Death webpage. Check back for announcements on these and other happenings.
Our gratitude goes out to every volunteer who has invested their time in staying educated, spreading the word, and participating in citizen science with us over the years, especially during the current shelter-in-place, with special recognition for our dedicated team of SOD Specialist Master Gardeners. We couldn't help our oaks without you!
/span>Sudden Oak Death (SOD), a serious exotic tree disease, threatens the survival of tanoak and several oak species in California. Currently, SOD infects trees in 14 coastal California counties, from Monterey to Humboldt. The disease, which was estimated to have killed over 50 million oaks and tanoaks over two decades, has changed the coastal forests composition in Northern California and Southern Oregon.
Though SOD occurs in patches, the overall infection area continues to grow with each passing year. Researchers had previously discovered that Phytophthora ramorum, the pathogen that causes SOD, most often spreads on the leaves of infected California bay laurel and tanoak. Forest managers can use proactive methods for controlling the disease — including sanitation, chemical treatments, and the targeted removal of bay trees — but such tools are preemptive in nature, only useful before oaks and tanoaks are infected. Timely detection of the disease in these species is therefore critical to slowing the epidemic.
With this in mind, UC Berkeley joined with over 30 local organizers in 2019 to assemble 25 SOD Blitzes staffed by trained volunteers. More than 400 volunteers were taught to identify SOD symptoms and to carefully collect symptomatic leaves from California bay laurels and tanoaks. Armed with this training, the teams surveyed 16,227 trees across 16 California counties, collecting approximately 9,000 leaves from 1,732 symptomatic trees. Samples were sent to the UC Berkeley Forest Pathology and Mycology Laboratory for processing and analysis.
The benefits of SOD Blitzes are becoming increasingly clear. Importantly, they educate the community about Sudden Oak Death, getting locals involved in detecting the disease while creating detailed maps of infected areas. Analyses of these maps help forest managers figure out where proactive measures, such as chemical treatments or tree removal, most effectively stop an infestation. All results from the data collection are made publicly available on SODBlitz.org, SODMap.org, and on the SODmap mobile app available at the Apple Store and at Google Play. (Results can also be accessed directly on the UC Berkeley Forest Pathology and Mycology Laboratory website.)
Using the app allows anyone to estimate the risk of oak infection, specific to their current location. While standing next to a tree of interest, a person can tap the app's “RISK” button and determine whether an oak is in danger of contracting SOD. In addition, results meetings and treatment training workshops will be held for the public in various Bay Area locations during fall 2019.
Matteo Garbelotto, UC Cooperative Extension specialist and adjunct professor in the Department of Environmental Science, Policy, and Management, has led the SOD Blitzes for 12 years. The program enrolls approximately 500 volunteers yearly to canvas California woodlands for symptoms of SOD. Results of the surveys are made available nearly in real-time, immediately after completion of the lab analysis.
“We estimate that as many as two to three million people may have accessed the SOD Blitz results to determine whether trees on their property may be at risk,” Garbelotto said. “This is a huge success and shows the great societal value that citizen science has, and it highlights the importance of collaborating with volunteers on issues that are relevant to safeguard trees that are an important part of California's natural heritage.”
2019 SOD Blitz Results
Across the state, the number of trees infected with SOD, along with the estimated SOD infection rate, has almost doubled since 2018. In some areas, infection rates were as much as 10 times higher than the previous year. These results suggest that the overall risk of oak infection by SOD is rising.
Spikes in the estimated SOD infection level at certain locations are particularly noteworthy; for example, from 1 to 12 percent in the western part of the East Bay between Richmond and San Leandro, including distinct outbreaks discovered in El Cerrito, Kensington and Berkeley. The western slopes of the Santa Cruz Mountains in San Mateo County have also seen an increase in SOD infection rate (from 6 to 18 percent), which may make safeguarding recreational venues and ecologically important forest sites more difficult. In Sonoma County, the 2019 SOD infection rate was approximately double that of 2018, and in the same timeframe, Napa saw a fourfold increase in detection. Peninsula towns between Redwood City and Los Altos Hills had a very high infection rate (21.6 percent) with an expansion of outbreaks to the east and north.
Some additional key results that came from the 2019 SOD Blitzes include:
- Two tanoaks were positive for SOD in a state park east of Crescent City in Del Norte County, marking the first report of SOD for the county. This was an important finding because, until 2019, Del Norte was the only county free of the disease in the area between the vast California infestation and the Southern Oregon outbreak. The SOD Blitz finding does not have immediate implications for regulations, but regulations will be imposed once CDFA can confirm the finding.
- An extensive survey of San Luis Obispo failed to identify SOD; however, multiple trees were found to be infected in the southernmost canyon of Monterey County. Previously, this canyon had only provided positive findings from water monitored by UC Davis scientists, but not from trees.
- Although Humboldt County has a few significant SOD outbreaks, and Trinity County has a marginal SOD outbreak in its southwestern border, SOD Blitzes in tribal lands in Humboldt and Trinity counties did not yield any SOD positives.
- San Francisco parks, including the Presidio, were negative for a second year in a row, suggesting that disease management practices have been successfully implemented in the area.
- Several cases were identified in the northern neighborhoods of the City of Napa. No positives had been found in this area since 2011.
- All isolates of the pathogen belonged to the NA1 lineage, which is more easily treated and common in the region. This is good news because of uncertainty about the potential virulence of the EU1 lineage recently discovered in Oregon forests.
A sampling of other noteworthy 2019 SOD Blitzes results follow. More results are available on the websites and app:
- The first infected Bay laurel was identified a few miles east of the town of Mendocino (Mendocino County) along the Comptche-Ukiah Road.
- Multiple SOD positive trees were identified between Guerneville and Duncan Mills in Sonoma County. The Russian River area has long been known to be affected by SOD.
- An outbreak was identified in southwestern Petaluma, while infestations were confirmed in Bennett Valley, Santa Rosa and east of Rohnert Park.
- Multiple infestations were identified in Marin County, including the ones northeast of San Rafael, Larkspur, Woodacre, Mount Tamalpais, Marin City and north of Inverness.
- In the Peninsula, SOD was identified in Burlingame Hills, northern Woodside, Emerald Hills, Palomar Park, Portola Valley. In Los Altos Hills and Loyola, SOD positive trees were detected both East and West of Interstate 280. An outbreak was detected west of Saratoga.
- High levels of infection were identified all along Skyline between Bear Gulch Road and Highway 17, and on the western slopes of the Santa Cruz Mountains between Skyline and Pescadero.
- SOD was confirmed between Felton and Santa Cruz and between Aptos and Santa Cruz.
- In Monterey County, the chronic Big Sur outbreak was once again confirmed to be active, while the disease appeared also in drier areas of the Carmel Valley, where it had been absent for a few years.
The SOD Blitz program was funded in part by the U.S. Forest Service's State and Private Forestry Organization and by the Midpeninsula Regional Open Space District. It was also made possible by a collaboration with the California Native Plant Society, CalFire, the Humboldt/Del Norte and Sonoma County UC Cooperative Extension, the Sonoma County UC Master Gardeners, the U.S. National Parks, California State Parks, the East Bay Regional Parks, the San Francisco Public Utility Commission, Mid-Pen Open Space, the UC Santa Cruz Arboretum, Santa Cruz Open Space, the Santa Lucia Conservancy, the Karuk and Hoopa Nations, the Mendocino Botanical Garden, the City and County of San Francisco Parks and Recs Office, and Strybing Arboretum. Many individuals, who have generously devoted their time and efforts, have been pivotal for the existence and success of the program.
READ MORE:
- Matteo Garbelotto: Leading the citizen science contagion
- First known cases of sudden oak death detected in Del Norte County
- SOD Blitz Project
- SOD Blitz map of results and summary table