Sudden oak death is a disease caused by an invasive plant pathogen, Phytophthora ramorum. Phytophthora species are fungus-like organisms related to algae. They thrive in warm and moist conditions. Most are soil-dwelling root pathogens but P. ramorum acts primarily as a leaf pathogen.
While most non-oak hosts are not killed by the disease, they do play a key role in the spread of P. ramorum. Oaks are considered terminal hosts, since the pathogen does not readily spread from intact bark cankers but instead becomes infected only when exposed to spores produced on the leaves and twigs of neighboring plants. P. ramorum uses natural openings in the bark to colonize the bark tissues, killing cells and clogging water and nutrient transport vessels.
Infections caused by P. ramorum cannot be identified on symptoms alone and must be confirmed in the laboratory, but presence can be determined through a process called “Pear-baiting”. Characteristic symptoms include presence of bark cankers, bleeding of a thick, sticky sap, large sap stains, leaf spots (usually brown tips surrounded by a halo of yellow), and beneath the bark you may find brown tissue often separated from healthy bark by a distinct black line. Check online mapping resources for the most current data on pathogen distribution: http://www.suddenoakdeath.org/maps-media/maps/
Inspect Nursery Plants Before Making a Purchase
Nurseries not located in cool, moist areas often create microclimates that support P. ramorum and allow it to grow and spread. Many common horticultural plants are hosts for P. ramorum. Carefully inspect the leaves of host plants for symptoms before making a purchase. Nurseries often use general fungicides that can mask P. ramorum symptoms. Check the condition of the roots by testing the root sheath to see if it is soggy and comes off easily when rubbed in a downward motion. Also, look for nurseries that keep plants out of direct contact with soil and consider quarantining new plants for up to eight weeks to see if symptoms manifest before transplanting.
Disposing of Plant Debris
Branches, twigs, and leaves from California bay laurel, rhododendron, and other host plants can harbor P. ramorum, even after they are removed from the plant. Composting can successfully kill the pathogen but must reach temperatures that are not practical in residential composting sytems. Removing plant debris from the property is recommended only if it is the first infected tree detected in the area or if fire risk is high. If infected wood is removed from your property, make sure it is utilized or disposed of in a way that does not spread the disease. Avoid leaving wood debris next to roads where unauthorized parties may tamper with it. If you have infected trees cut down, make sure the wood and other tree parts are not moved outside of the quarantine area.
Sanitation Measures to Minimize Pathogen Spread
Clean and disinfect pruning tools with butane gas/flame and/or bleach after use on confirmed or suspected infested plants. Sanitize pruning tools before pruning healthy plants. Clean vehicles and shoes of mud, dirt, leaves, and woody debris before leaving a P. ramorum-infested site and before entering a site with susceptible hosts.
Using insecticides to treat or prevent P. ramorum infections provides no control. However, treating individual, high-value landscape trees displaying early bleeding symptoms of Sudden oak death might be justified to control damage from secondary bark beetle attacks. If using an insecticide, apply it only if the disease is not at an advanced stage.
For more information check out the UC Davis IPM Pest notes page: http://ipm.ucanr.edu/PMG/PESTNOTES/pn74151.html
Special thank you to Pavel Svihra, UCCE Marin County, for the use of these photos.