All of A Sudden... Oak Death.

12/17/2024

In the 1990's, a mysterious illness was discovered both along the western coast of North America and in Europe. The sickness affected foliage, trunks, and overall health of many genera and species of woody plants, resulting in death or serious damage. Since its first official reporting in 1995, much research has been undertaken to identify its source, spread, and effective treatments, and there is now cause to believe there will be a sizeable resurgence in the disease due to the emergence of a more evolved strain.

Because this illness primarily affected Oak trees in large numbers, the disease was given the moniker Sudden Oak Death, or SOD. However, it can affect other plants as well, and even more plants can be hosts to the disease without suffering any serious symptoms. It is caused by the fungus-like oomycete Phytophthora ramorum (pictured).

This organism develops spores on leaves of primary host species, including the California Bay Laurel, which then are carried by rain and/or air currents to the leaves of the new host, where they continue to develop. Other host genera or species include Rhododendron, Madrone, Huckleberry, Buckeye, Bigleaf Maple, Toyon, Manzanita, Coast Redwood, Douglas Fir, Coffeeberry, and Honeysuckle. Once in contact with the host plant, the organism then infects the inner bark and sapwood of affected plants, resulting in bleeding cankers. Affected trees and shrubs typically die as a result, either from the disease itself or from other pests or diseases that attack the weakened plant. After infected plant matter dies and decomposes, the organism is transferred to the soil, where it germinates until another host species takes up spores. Phytophthora ramorum can overwinter in soil, and will also take advantage of already wounded trees and shrubs.

The disease progression varies depending on the species affected. The first symptoms typically noticed by gardeners are cankers and discolored leaves. Foliage dieback is also a common complaint in situations involving Sudden Oak Death. Affected trees can be treated, though treatment will only prolong the life of the tree, not cure the disease.

A phosphonate compound is the chosen preventative, life-extending treatment for high-value trees. Trees and shrubs that are not highly valued, however, are best left to their own devices. If kept otherwise healthy, they may survive for a while before succumbing to the pathogen. The best way to deal with this issue, as with most landscape diseases, is to prevent it from occurring in the first place.

Preventing and Managing Sudden Oak Death in the Garden

The best way to prevent Sudden Oak Death from killing landscape plants is to prevent it from entering the garden, because once it is there, it is not possible to eradicate it. The first step in preventing SOD is to inspect nursery plants before bringing them home. Many commonly grown ornamental plants available at nurseries can be carriers of the pathogen, and the state-established regulations we have do not always catch infections. Carefully inspect leaves of host plants for symptoms, which include browning and spotting of foliage. Do not purchase any plants with these symptoms. After bringing plants home, quarantine them away from the garden for eight weeks before installing. Some nurseries will use general fungicides on their plants to prevent spread of many diseases, and the symptoms of SOD can be masked. This quarantine time will increase the likelihood that an infection will be caught before the new plant is added to the landscape.

Removal of infected trees is not necessary. If the pathogen is in the garden, it is there to stay, and removing an infected tree will not remove it from the landscape, especially if there are other host plants nearby that are likely also infected. The decision to remove an Oak is dependent on whether it is creating a hazardous situation on the property.

Trees affected by Phytophthora ramorum are prone to rapid decay and unpredictable failure. If an infected tree is growing close to homes or other structures, sometimes the best option is removal. Always consult local regulatory officials before deciding to remove trees, as sometimes permits are required. If there is an acute emergency, contact the city arborist or your local fire or police department.

Although there are many host plants for Sudden Oak Death, removal of these plants to prevent spread is not recommended at a residential level. If there is a healthy, susceptible-species Oak tree in the landscape, removal may be helpful to prevent spread, especially if there are few other host species planted nearby. Otherwise, leave host plants where they are.

Responsible disposal of infected plant debris and subsequent transportation of infected plant parts can be a cause of more areas in our state becoming affected by the pathogen. Even after being removed from the host plant, branches, leaves and twigs can still harbor Phytophthora ramorum. The best disposal method is to leave debris on-site, chipping small material for use as groundcover and using larger pieces for firewood. Removal of plant debris from the infection site is only recommended if it is the first infected tree in the area, or if the presence of debris on the property is a potential fire hazard. If infected tree parts are removed from the property, make sure whoever disposes of them does so inside the same quarantine area.

Cultural practices in the garden can help to lessen the spread of this pathogen. Keeping pruning and digging tools clean and sanitized can go a long way in preventing spread from plant to plant within the garden. Make sure to clean off any vehicles, shoes, or other gardening gear before leaving an infected site and before entering a site with susceptible hosts.

Finally, if infected trees or host plants are removed, they can be replaced with trees and shrubs that adapt well to the site. If another Oak tree is desired, make sure to choose a species that is not as affected by Phytophthora ramorum, such as those in the White Oak group (Valley Oak, Garry Oak, or Blue Oak).

New Strain, Who Dis?

A recent project undertaken by Dr. Matteo Garbellotto at the University of California Berkeley campus in conjunction with other contributors fueled SOD Blitzes, information from which was used to create a comprehensive Google Earth map overlay. The map overlay is available for download on the project's website. This 2024 survey found that infection levels are on the rise in the state of California, and have the potential to rise even more in early 2025 due to our wet winter weather. It also concluded that Sudden Oak Death is spreading quickly due to the emergence of a new strain of the pathogen.

The original pathogen, which was of the N1 lineage, has been surpassed in infectiousness and aggressiveness by the new N2 lineage. This new lineage has the ability to be more successful in warmer temperatures, which is a looming issue in California due to both climate change and the warmer weather of our Central Valley. Given these outlooks, it is of utmost importance that the public become aware of this disease and learn all they can about its spread, including how property owners can discourage it from entering their landscape.

Frequently Asked Questions about Sudden Oak Death

To learn more about Sudden Oak Death, review the following questions:

What is the connection between Sudden Oak Death (SOD) and nursery plants?
As with many other pests and diseases that affect landscape plants, SOD is spread by human activity via the transport and sale of nursery specimens. All host plants for the disease are strictly regulated in California, and must be inspected and approved for transport and sale prior to shipment out of the regulated area they originate in. Purchasing of host plants at nurseries should be done carefully; all should be inspected before a sale is made. Inspect the leaves of host plants for symptoms, and refrain from planting host species near susceptible Oaks in the landscape. It is recommended that no host species be planted within 150-1,000 feet of susceptible Oak species. For an updated, comprehensive list of SOD host species, visit suddenoakdeath.org.

How do I determine if my tree has Sudden Oak Death?
It is a bit difficult to determine whether a tree is affected by SOD or another disease or pest. Homeowners are not equipped to diagnose trees on their own with absolute certainty. However, there are some steps that can be taken to determine whether it is a likely cause of tree decline:

Find out what type of tree you have, and whether it is a susceptible species. Although the disease is called Sudden Oak Death, it does not affect all Oak tree species equally. In California, SOD mainly affects Tan Oak, Coast Live Oak, Shreve Oak, Canyon Live Oak, and California Black Oak.
Determine if you are in an infested zone. There are many maps available to the public, including one on the Board of Forestry and Fire Protection website. Keep in mind that even if your property is not located within a designated infested zone, it is still possible, though a bit less likely, that your tree is infested with Phytophthora ramorum.
Compare the symptoms of SOD with those exhibited by your tree. Make sure to check any host species nearby for symptoms, as well. If plants located close to your Oak are showing SOD symptomology, it is more likely that your tree could be suffering from it as well. Symptoms range from browning or dead foliage to spots or discoloration on leaves, cankers on trunks, and general dieback.

How do I pronounce Phytophthora ramorum?
The fungus-like organism that causes SOD, Phytophthora ramorum, is pronounced fye-TOFF-thor-rah rah-MOR-um.

If my Oak has SOD, what are the chances that it will die?
There isn't a great way to determine whether a tree will die as a result of the disease, as each tree responds differently to infection. Even trees of the same species, planted in the same yard can exhibit different symptoms. For the most part, however, once infected, Tan Oaks have a relatively high probability of being killed. Coast Live Oaks have a somewhat lower probability of being killed. Most of the time, infections are a death sentence for susceptible species.

If I suspect that my tree has SOD, who should I hire to treat it?
The California Oak Mortality Task Force (COMTF) regularly carries out trainings for tree care professionals in the state. A list of training participants can be found on their website (suddenoakdeath.org). Hiring a professional who is familiar with the progression and treatment of SOD is key to ongoing health in affected plants.

If I cut down my infected Oak tree, what should I do with the debris and wood?
The easiest way to deal with infested wood is to leave it on site. The wood can be chipped and used as mulch, and larger pieces can be used as firewood. Doing this will not significantly worsen the local disease conditions. Do not move any infected wood or plant parts out of the infected area; this will increase the spread of the disease to more locations in our state, and possibly lead to the destruction of large swaths of forest. As a general rule of thumb, it is not recommended to pile up wood next to established plants of any kind, as pests tend to 'jump' from wood piles to living trees and shrubs. Composting infected plant parts is not recommended, as most home-created compost piles do not reach high enough temperatures to kill the pathogen.

What plants can be installed to replace a tree killed by SOD?
If a new tree is desired to replace the lost one, choose a plat that suits your needs and that will adapt well to the planting site. If another Oak tree is the best choice, make sure to select a species that is not as affected by Phytophthora ramorum, such as those in the White Oak group (Valley Oak, Garry Oak, or Blue Oak). Also, take care when choosing new plantings for underneath Oak trees; many ornamental plants are carriers of the pathogen, and have the potential to affect nearby Oak trees. For a comprehensive list of host species, visit suddenoakdeath.org.