Sudden Oak Death
Knowledge on Sudden Oak Death (SOD) is evolving. Please email our web master if you find inaccuracies or believe information should be added to this page (firstname.lastname@example.org).
SOD has been killing oak trees in the Big Sur Valley for years. Pfeiffer Big Sur State Park has lost thousands of oaks to the disease and SOD has now been confirmed at various locations in the Big Sur area from the Palo Corona Ranch on the north to the San Luis Obispo County line. SOD seems to move inland up the lower / cooler portions of canyons, then uphill on the forested / north slopes of ridges.
If they are present, Tan oaks are usually first to be affected in an area. Coast Live Oaks, California Black Oaks, and Shreve's Oaks are also susceptible to being killed by SOD. Oaks in the white oak group appear to not be affected (e.g., Valley Oaks, Blue Oaks).
There are a number of trees and shrubs that host SOD but are not killed by it. For example, the leaves of Bay Laurel trees may be infected with SOD without killing the tree. It appears that Bay Laurel aids in the spread of SOD to oaks by hosting the pathogen on its leaves where spores are produced that then infect nearby oaks. Madrone is also a host, and it is now suspected that SOD not only infects the leaves and branches of Madrones but is killing them as well. Toyon is another known host, as are a growing list of about twenty other plants.
The active pathogen is a fungus, Phytophthora ramorum. Oak trees are killed when the fungus encircles their trunk under the bark, effectively girdling the tree. Some oaks succumb to SOD within a few months after first symptoms appear and others may live several years before dying. It appears the fungus's spores are spread by air, water, and contact.
The University of California has a web site with maps showing where SOD has been reported (http://kellylab.berkeley.edu/SODmonitoring). You can use the same web site to report the location of confirmed cases of SOD.
A diagnosis of SOD may be confirmed only by culturing the fungus under controlled conditions or by DNA analysis. Many of the symptoms described below may be mimicked by pathogens other than Phythophthora ramorum. However, symptoms have a high probability of indicating SOD if they occur on trees that are within several miles of an area confirmed with SOD. Samples to be cultured may be taken from suspect leaves or from under the bark near a canker on the trunk of trees. If you suspect SOD in a location where it has not been confirmed and want to have a sample tested, contact Brad Oliver for instructions on how to collect and submit a sample (biologist at the Monterey County Agricultural Commissioners Office in Salinas; (831) 759-7325; email@example.com). More contact information may be found on the California Oak Mortality Task Force contact web page.
The primary SOD symptom on Coast Live Oaks is often (though not always) one or more visible cankers on their trunk (see figures 2 and 3). The canker is usually low on the trunk, often close to ground level, and is rarely higher than eight feet. The canker typically oozes a viscous dark brown liquid and smells like it is fermenting (smells like a wine cork or a used wine barrel). Before the canker develops some trees exhibit weeping of dark brown viscous liquid through healthy-appearing bark. Live Oaks are not known to display symptoms on their leaves other than a general lack of vigor as the tree declines. The ultimate indication that SOD may be in an area is a cluster of dead oaks with no other apparent cause of death.
Initial symptoms on Tan oaks are varied, and may include brown blotches on leaves and wilting of new-growth shoots (giving the appearance of a shepherd's crook at the growing tip). While Tan oaks may have a canker similar to Live Oak, they may also display only a faint brown stain on a portion of their trunk, or no stain.
Bay Laurel displays symptoms on its leaves. There is typically a characteristic black line and yellow halo between a darkened portion of the leaf and healthy green tissue (figure 4). The darkened part of the leaf is usually oriented so it is the part of the leaf where water droplets linger after moisture has collected.
Other pathogens can cause symptoms that look like SOD symptoms. Follow this link for more complete information on recognizing SOD symptoms in about 15 different hosts, a paper prepared by University of California and USDA researchers in July of 2003. The paper is a 1.7MB pdf file and requires free Adobe Acrobat Reader software to read (a 9MB+ download from Adobe).
The treatment generally accepted as effective on SOD is application of potassium phosphonate (PP), also known as potassium phosphite. Other potential methods of reducing the likely hood of attack by SOD are being investigated, including treatment by smoke.
PP seems to act as both a nutrient and a fungicide. PP is used by trees to strengthen cellular membranes, making them resistant to the fungus. PP also seems to be converted by trees into chemicals that are toxic to the fungus. Controlled tests by University of California researchers have shown that PP can help control SOD, and PP has been used for many years to treat other strains of Phytophthora that attack other kinds of trees. It is not yet known how effective PP will prove to be treating SOD in real-world conditions over time. An Australian company called Agrichem has obtained a special registration for fungicidal use of PP so it may be used on oaks to combat SOD.
Agrichem's trade name for the product is Agri-Fos. Agri-Fos may be injected into or sprayed onto oak-tree trunks. Until recently, injection was the only method recommended for Tan oaks, but it now appears that spray application on Tan oaks is also effective, though it takes longer for the PP to penetrate Tan oak bark than with other oaks. When sprayed on the trunk of oaks Agri-Fos must be mixed with another Agrichem product called Pentra-Bark to help the Agri-Fos penetrate through the bark and into the tree's tissues. Once inside the tree, the tree distributes the chemical through its system. One quart of Pentra-Bark is enough to mix with about 5 gallons of undiluted Agri-Fos. Agri-Fos is typically sold in 2 1/2 gallon containers (other sizes are available).
When spraying the trunks of oaks with a mixture of Agri-Fos and Pentra-Bark, the tree is sprayed from ground level to about 6 to 9 feet above ground level with enough solution to soak the bark just to run-off. Ensure that cracks and crevices receive plenty of solution as they are the shortest pathway into the tree's system, both for the treatment and the pathogen. An ordinary garden or back-pack sprayer may be used, set for a course droplet size to avoid excessive spray drift. To avoid waste of Agri-Foss, portions of trunks with moss that is so heavy as to interfere with spray reaching the tree's bark may be brushed to remove excessive moss, or if a relatively small area, may be sprayed around. The spray should not be applied when it is raining, but rain after application apparently does not affect results. The solution will likely discolor or kill moss and lichen. Avoid spraying leaves with solutions containing significant amounts of Pentra-Bark as it will likely injure or kill leaves it contacts. Ask your supplier for a copy of Agrichem's spray application directions and follow them.
Injecting trees with Agri-Fos uses the least amount of the fungicide, but is labor intensive. Small holes are drilled through the bark deep enough to penetrate into sap-wood and diluted Agri-Fos is immediately injected. Various devices are available for injecting trees, from inexpensive plastic hypodermic-like devices to sophisticated drill and injector combination tools with a significant price tag (e.g., www.treeinjectors.com). There is disagreement over the desirability of sealing injection holes after use. One SOD researcher said he has never seen re infection at an unsealed injection hole site, even when holes receive splash from SOD infected soil. Ask your Agri-Fos supplier for a copy of Agrichem's injection application directions and follow them to determine the correct dilution, number of holes, and quantity to inject per hole.
The manufacturer's recommendation is to apply Agri-Fos twice a year. Agri-Fos should not be applied when a tree is dormant as the tree will then not effectively move the chemical through its tissues. Coast Live Oaks are dormant in summer, generally July to October. Black Oaks are deciduous and are dormant in winter when they lose their leaves.
Ideally, trees should be treated before they show symptoms so they will be strengthened by the nutrient properties of PP to resist initial attack by the fungus. Treatment has a better chance of success if applied within a couple of months of when symptoms first appear. The time and expense involved in treatment will often mean choosing between trees you want to try to protect and those you will leave on their own. A paper has been prepared by University of California researchers that provides guidelines for analyzing which trees are most at risk for SOD, and which are most likely to benefit from treatment (34K pdf file).
The only local dealer we know of that sells Agri-Fos and Pentra-Bark is HealthySoil in Gonzales in the Salinas Valley; www.healthysoil.com; firstname.lastname@example.org, 831-675-3144 x102. HealthySoil is located at 425 Alta Street, Building 16, Gonzales. Call before you make the trip to be sure someone will be available to help you (sometimes everyone is in the field). Agri-Fos and Pentra-Bark are also available from Bioscape in Petaluma, California; www.bioscape.com; (877) 246-7227.
Please let us know if you learn of another source for Agri-Fos so we can post it. Also, please let us know if you have had a good experience hiring an arborist or other service to treat your trees so we may post their contact info.
Our understanding is that the active ingredients in Agri-Fos and Pentra-Bark are relatively benign, and the Monterey County Agricultural Commissioner's office assures us you are not required to have a certificate or license to apply Agri-Fos so long as you are not doing so for hire. However, some dealers are apparently confused about requirements and are mistakenly asking for an applicator license. If you have a problem, have the dealer call the Monterey County Agricultural Commissioner's office at (831) 647-7629 or 759-7325.
This web page is intended as a readily accessible summary of more detailed information. Much more SOD information is available on the Internet and we suggest you search the Internet to learn more. A good place to start is the web site of University of California SOD researchers. Another is www.suddenoakdeath.org, which has links to numerous other sites. Agrichem literature should guide your use of their products and may be obtained from Agrichem's dealers. Agrichem's U.S. representative, William Stringfellow, is very helpful and is another source for information on use of Agrichem's products; toll free (866) 309-8600.