- 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.
Many different treatment approaches have been trialed, a few of which have shown promise.
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.
Valley Oak (Querus lobata):
The acorns were a staple food, which was leached (rinsed with water) to remove the bitterness, and ground into flour with mortar and pestles. The ground acorns were used in stews/soups, pancakes/tortillas, mush, or layered into pits and cooked with other plants and meats. Oak galls were squeezed to make a blue-black ink for tattoos and tannins were used to make dyes and decorate animal skins.
Deer Grass (Muhlenbergia rigens):
A major grass for creating beautiful, sometimes water-tight baskets to cook food, to carry and store food and other items. Stalks were generally harvested in the spring when easy to pick, then wrapped to keep straight and allowed to cure for a year. They were often soaked prior to weaving into basket. About 1600 stalks would be needed to make one basket.
Santa Barbara Sedge (Carex barbarae):
The rhizomes (underground stems which generally grow horizontally) provided the strongest threads for basket making. The people would manage the rhizome growth by cleaning the soil of anything that might obstruct the growth (i.e., rocks) to allow the rhizomes to grow long and straight. An evergreen grass, the summer flowers range from cream, red and purple colors, which attract butterflies.
Toyon, aka California Holly (Heteromeles arbutifolia):
The wood from this sturdy shrub had many uses including tools, games/toys, fuel for smoking fish, and religious ceremonies. The red berries produced in the fall which were eaten after roasting over coals or dried in the sun.
An evergreen shrub, the summer white flowers attract bees and butterflies. Birds love the berries.
Blue Elderberry (Sambucus cerulea):
This tough, easy-to-grow shrub or tree is dormant in the winter. The spring and summer blooming cream or yellow flowers attract bees and butterflies, with their berries being an important food source to many birds.
Manzanita (Arctostaphylos manzanita):
Its distinctive red wood which was sometimes used to dry and smoke fish. The fruit was gathered in summer, then dried and ground to make coarse meal which would be mixed with a little water during winter months or made biscuits. They would make tea with the berries and tips of the branches, which apparently was a pleasant drink.
Sticky Monkey Flower (Mimulus aurantiacus):
The seeds were used as a food source. They were gathered, parched, ground, and added to foods or eaten by the handful. Flowers were used as décor after drying, made into wreaths, and used in religious ceremonies. The roots and leaves were used for medicinal purposes.
This drought-tolerant, evergreen shrub blooms in the spring, summer, and fall. The bright yellow tubular flowers attract hummingbirds, bees, and butterflies. Autumn seeds attract small birds.
Whenever I see any of these California native plants, I think of how the indigenous people of California used these plants over thousands of years. By growing them in our gardens, we honor that history, help the survival of these plants which provide food sources for so many birds, bees, and butterflies, reduce water usage, bring variety to our gardens, and joy to our spirits with their beauty.
Learn more at the Library - Take a free class!
This September, our UC Master Gardeners will present on the topic, "CA Native Plants" at 9 Stanislaus County Library locations. Visit our Calendar at https://ucanr.edu/sites/stancountymg/Calendar/ for dates, times, and locations.
On Saturday, October 7, 2023, we are offering our "The New Front Yard" workshop. Topics include drip irrigation, converting your yard to native plants, and how to garden for year-round bloom! Stay tuned for the registration announcement.
- Enough For All: Foods of My Dry Creek Pomo and Bodega Miwuk People by Kathleen Rose Smith
- The Real California Cuisine: A Treatise on California Native-Plant Foods by Judith Larner Lowry
- Tending the Wild: Native American Knowledge and the Management of California's Natural Resources by M. Kat Anderson
- Indian Summer: A True Account of Traditional Life Among the Choinumni Indians of California's San Joaquin Valley
- Great Valley Museum of Natural History at Modesto Junior College's exhibit on Yokuts
- California Native Plant Society: https://www.calscape.org/
Acknowledgment: Lillian Vallee, English professor emeritus, Modesto Junior College, who has shared her passion and knowledge with me over the years of California native plants and their historical uses by the California native people.
Denise Godbout-Avant has been a UC Cooperative Extension Master Gardener in Stanislaus County since 2020.
There are several flatheaded boring beetles in California, however only a few are of particular concern. The goldspotted oak borer (GSOB), Agrilus auroguttatus, is a metallic wood-boring beetle that threatens our native trees. Since it was introduced to Southern California on contaminated firewood in the early 2000s, this pest has caused extensive damage to woodlands and native oaks.
What does the goldspotted oak borer look like?
Adult GSOB are 0.4 inch long and 0.08 inch wide with bullet-shaped bodies. They are black with an iridescent green sheen and six distinct gold spots on their back.
What damage does the goldspotted oak borer cause?
Goldspotted oak borers only attack oaks (Quercus spp.). They are particularly damaging to coast live oak and California black oak. Adult beetles lay eggs on host trees and the larvae bore into the wood, feeding on the tree's vascular tissue. The larvae feed on the tree until they pupate into adults and exit the tree, leaving D-shaped exit holes in the bark. Extensive GSOB feeding can girdle trees, disrupting water and nutrient uptake and eventually causing the tree to die.
What can you do about the goldspotted oak borer?
Don't move firewood! The most important way to prevent the spread of invasive wood borers like GSOB, is to buy firewood where you're going to burn it. If you live in an area of Southern California where GSOB is present, avoid planting susceptible trees. If you need to remove an infested tree, keep the cut wood away from healthy oaks and tarp or grind the wood to kill any larvae present. To report possible sightings, fill out the Goldspotted Oak Borer Symptoms Reporting Form at https://ucanr.edu/sites/gsobinfo/What_You_Can_Do/Report_GSOB_Symptoms/
By Julie Clark, Community Education Specialist III
Goldspotted borer (GSOB) is a beetle invasive to oaks in California. Infestations have ravaged oak woodlands in San Diego and Riverside counties the last 12 years and in Anaheim Hills, Angeles and San Bernardino National Forests the last few years. Preferred hosts are black oak, canyon oak, coast live oak, and occasionally Engelmann oak.
Oak woodlands are highly valued ecosystems that support numerous species of fauna. Oak trees serve as the anchor for these systems and support over 5000 insect species, over 105 bird species, 105 mammal species, 58 species of amphibians and reptiles during their respective life cycles. Many beneficial insects rely on oaks to complete their life cycles and do not damage the trees in doing so.
The insects damage the water and food transfer structures (xylem and phloem) of the tree, causing crown die-back and eventual death in heavily infested (amplifier) trees. Widespread loss of oaks from GSOB has occurred in Idyllwild and San Diego County mountain areas.
Although GSOB are winged, they do not fly long distance. UC researchers, partners with CAL FIRE and the California Firewood Task Force discovered that several of the infestations throughout Southern California were caused by introduction of firewood imported from infestations in other areas.
D-shaped GSOB exit holes. Credit: UC ANR
Ventura County is vulnerable to attack by the beetle and other invasive tree pests that are on watchlists for the area. Best ways of being assured your firewood is safe include purchasing locally source material or selecting kiln-dried or certified firewood.
For more information:
Report suspected GSOB infestation:
- Author: Mike Hsu
Cal OAK Network to build on, grow connections between UC and organic community
After pioneering the organic movement in the 1970s, California now leads the nation in number of organic farms, total organic acreage and overall organic crop value. Attaining this status was no small feat, and largely driven by resourceful growers who developed and refined the wide range of novel organic farming practices seen in California today.
Now, with the creation of the Organic Agriculture Institute, the University of California will be able to leverage its vast capacity for research, extension and education to further improve the sustainability, resilience and profitability of organic agriculture in the state.
In its first major public initiative, the Organic Agriculture Institute – a program of UC Agriculture and Natural Resources – is conducting a statewide needs assessment for organic agriculture, as well as forming a knowledge-sharing network that connects UC experts with growers, processors, producer organizations, certifiers, crop consultants, community groups and state agencies.
“This network will be a sustainability partnership that enables learning, innovation and cooperation among organic agriculture stakeholders,” said Houston Wilson, director of the Organic Agriculture Institute, which was established in January 2020. “As facilitator of the Cal OAK Network, the Organic Ag Institute will serve as an intermediary that fosters communication among stakeholder groups, organizes discourse, forges new collaborations, and generally enhances coordination of stakeholder activities.”
By creating closer connections between the UC and the organic community, the Cal OAK Network will foster ongoing feedback cycles of knowledge and best practices, and in doing so help create the conditions and momentum to facilitate the development and adoption of organic production practices.
“The Cal OAK Network will better connect the organic community with UC technical and training resources, while at the same time providing a mechanism for that community to feed information back to the UC that helps us shape our programs at the Organic Ag Institute,” said Wilson, noting potential contributions in areas such as pest control, weed management and crop nutrition.
In the first 18 months of the Institute, Wilson has been charting the current landscape of organic agriculture in California, listening to the needs of stakeholders and working to position the Institute in a way that best uses the UC's unique research and extension capacities to support and augment existing efforts by growers and other groups.
The Institute recently received a planning grant from the National Institute of Food and Agriculture to conduct a formal needs assessment through summer 2022. In surveying growers and other stakeholders, the Institute seeks to identify their primary research and extension priorities, as well as gain a better understanding of the key people and organizations they currently rely on for information about organic production.
Through this process, a map of nodes and connections in the organic community will also take shape – and the roster of members for the Cal OAK Network will continue to grow. As Wilson points out, it is only through a diverse and robust information-sharing system that California organic agriculture will successfully adapt to challenges like climate change.
“We're trying to create a resilient infrastructure for engagement with organic stakeholders,” he said. “When we talk about sustainability in agriculture, part of that is having sustainable institutions that function well and can be flexible and dynamic as new challenges arise down the road.”