What is Sudden Oak Death?
Sudden Oak Death is caused by a pathogen called Phytophthora ramorum. While there are three known strains, the NA2 strain—previously less common—has now been detected in the Bay area is causing damage.
The pathogen thrives during rainy seasons and spreads through water droplets carried by wind and rain. This means wet winters, like the one anticipated for 2024–2025, may increase the risk of oak infections.
How Does It Spread?
The primary host of SOD in California is the California bay laurel (Umbellularia californica). The pathogen spreads from infected bay laurels to nearby oaks, including Coast Live Oaks, which are particularly vulnerable. Oaks can only be infected if the pathogen is within 200 yards of the tree.
Signs of Infection
SOD has been identified in over 100 plant species, but not all host plants die. To help identify SOD in bay laurels or other hosts, visit the resources at the end of this article or consult a certified arborist.
Managing SOD on Your Property
Proper management can help reduce the spread of SOD and protect valuable oak trees.
1. Remove Small Bay Laurels
· Perform this work in the fall, when weather conditions are ideal.
· Focus on removing bay laurels with a diameter of 4 inches or smaller that are within 30 feet of a Coast Live Oak tree.
- If using herbicide to treat stumps and prevent regrowth, hire a licensed professional to ensure safe and effective application.
Note: If you plan to remove a bay laurel with a diameter of 15 inches or greater, you will need a permit from the Town.
2. Preventive Treatments for Coast Live Oaks
· Apply phosphonate treatments every two years.
o For oaks with a trunk diameter of 25 inches or smaller, use a spray mixed with Pentra-Bark.
o For larger oaks, injections are recommended, along with adding 3–5 pounds of gypsum around the base of the trunk.
· Always hire a licensed contractor with arborist certification for applying treatments.
Testing and Resources
- For testing and treatment advice, contact a private certified arborist.
- For questions, email the Town Arborist, Sally Bentz, at sbentz@ci.atherton.ca.us.
Learn More
Here are some helpful resources for further information:
Proactive management is essential to protect our trees and reduce the impact of Sudden Oak Death in our community. Reach out with any concerns, and together, we can help preserve our urban forest.