Sudden Oak Death: A Brief Overview and a Call to Action for Citizen Scientists

Up next in our Science Committee’s Naturalist Segment is an over view of Sudden Oak Death written by Yvette Skinner.

Sudden Oak Death (SOD) is the common name for a plant disease that is devastating the native oak trees along the California coastal ranges. Although the pathogen that causes SOD can also cause disease in other plants this article is focused on the effects on oak species as well as the Tanoak, which is not part of the oaks’ genetic family, the genus Quercus. Oak tree species of concern include the California Coastal Live Oak, California Black Oak, Shreve Oak, and Canyon Live Oak. This brief article will present a brief overview of SOD’s cause, how it spreads, and what can be done to help in the battle against this potent tree disease. At the end readers will find several links to web pages on the topic which can provide references and additional reading.

Sudden Oak Death is caused by an invasive fungus, Phytophthora ramorum, which is believed to have been accidentally introduced more than 20 years ago from imported ornamental plants. Phytophthora translates to English as “plant killer” and this disease is a virulent killer of oak trees with mortality rates as high as 100%. Once the trees are weakened by SOD they are susceptible to increased attacks by other pests such as bark beetles which hasten the tree’s demise. A classic symptom of the disease are oozing “cankers” on the trunks which look like dark sap seeping from the bark.

SOD does not spread uniformly but rather occurs in many clusters or “hot spots” which extend as far north as Curry County in southern Oregon to the extreme southern border of Monterey County. The pathogen thrives in cool, moist climates which inhibits the disease from spreading to the hot, dry central valley. In fact, in the East Bay counties of Contra Costa and Alameda the disease can be found along the Front Range but the number of infected trees drops precipitously in the eastern half of these same counties due to slightly hotter and drier microclimates. Unfortunately the total extent of the infection has not been quantified as much of the forests in vulnerable areas have not yet been tested.

The California Bay Laurel is key vector for the disease. Bay trees will become infected but typically do not die. The pathogen can reproduce while living on the Bay trees and release their spores. Spores are spread through wind and water but will not travel far from the infected tree, often 30 yards or less. Outbreaks of SOD are higher in wet years with above average rainfall. Bay trees will become infected in an area before the oaks therefore scientists monitoring the spread of the disease have recruited citizen scientists to sample Bay trees and collect leaves which show signs of infection to be tested.

[caption id="attachment_11881" align="aligncenter" width="400"] Photo by Joseph O'Brien, USDA Forest Service[/caption]

SOD Blitz is the name given to the annual SOD tree surveys held each Spring when the temperature and moisture conditions are ideal for the spreading of the pathogen. Volunteers are trained to spot the symptoms of infected Bay trees and are given envelopes to collect leaves from trees suspected of SOD infection. The leaves are labelled with the tree’s GPS coordinates to pinpoint the tree’s location. Volunteers turn in their packets of envelopes and the leaves are sent to be tested at the UC Berkeley laboratories because the symptoms of SOD can appear similar to other unrelated tree diseases. The results are posted by October on the SODMAP web page and app (available on Apple and Google Play stores). In the 2019 SOD Blitz, the first infected trees in Del Norte County were identified. Previously the closest infection “hot spots” in that area were southern Curry County (Oregon) and southern Humboldt County.

Identification of infected Bay trees provides land managers information to assess the risk of nearby oaks becoming infected. It may be necessary to cut down Bay trees in proximity to an oak tree to prevent the spread of infection. Best Management Practices and Phosphite treatments have been shown to prevent the spread of SOD on individual trees using in landscaping, however, these methods are not practical solutions to save an entire forest. It is possible to prioritize and maintain healthy trees in a landscaped environment. Consultation with a licensed arborist is recommended for these situations. Unfortunately, by the time an oak tree is showing symptoms of SOD such as the oozing cankers and die-back the tree will almost certainly die.

This April the SOD Blitz is still scheduled to take place in mid-April, however, significant changes have been made in light of the current COVID-19 outbreak. Instead of classroom training, volunteers will be trained using online videos. A limited number of leaf collection packets will be available this year. The current “Shelter at Home” order will drastically limit the scope of this year’s Blitz. Public health and safety take priority. Nevertheless, the public’s support of SOD Blitzes in the past decade have been strong. Last year’s Blitz was the largest to date. More than 600 participants surveyed over 16,000 trees. Although this year’s Blitz will be small by comparison, readers are encouraged to watch the online videos, learn about SOD, and consider joining the 2021 Blitz.

[caption id="attachment_11882" align="aligncenter" width="448"] Photo by Douglas Schmidt, UC Berkeley[/caption]

These Blitzs provide an opportunity for mountain bikers to take action to protect the forests they love to ride in. Many of the parks and open spaces in the East and North Bay Area frequented by mountain bikers have seen little to no testing: Joaquin Miller Park, Crockett Hills, Fernandez Ranch, Rockville Regional Park, China Camp State Park, Skyline Wilderness Park and more. In the South Bay the rates of testing and infection are higher with SOD-positive trees stretching from Redwood City to Gilroy and south to Santa Cruz.

Download the SODMAP app, available for iPhone and Android, to see where infected trees have been identified. If sufficient data is available the app can estimate the level of risk for oak tree infection using the smartphone’s current location (GPS coordinates).

Online references for more information:

SODMAP for Windows and Mac: https://nature.berkeley.edu/matteolab/?page_id=755

Information on SOD: http://www.suddenoakdeath.org/

SOD Blitz: http://www.sodblitz.org/

Yvette Skinner is a member of MSOBC’s Science Committee, and avid mountain biker, and a lover of the Great Outdoors. She fell in love with mountain biking while riding the swampy dirt paths that passed for trails in southern Louisiana. From there she ventured farther to enjoy more challenging and adventurous trails starting in Southeast and then progressing to epic adventures in some of mountain biking’s meccas: Durango and Fruita, CO, and Moab, UT.

She moved to Vallejo, CA, in 2017 and immediately joined the BTCEB (Bicycle Trails Council of the East Bay) and became Vice President and a routine volunteer for trail maintenance at Crockett Hills Regional Park and China Camp State Park. She is an Apprentice K9 Search and Rescue handler with her German Shepherd Dog, Arnaud, and she loves to ride with him at her side to keep both of them in shape for the rigors of Search and Rescue.

Skinner joined the MSOBC’s Science Committee because as a chemical engineer she has a deep love for science and believes that fact-based decision-making will lead to a better outcome for outdoor recreation than emotional rhetoric. She has been active in MTB advocacy in the East Bay and is glad to join to the MSOBC in the quest for expanded trail access for mountain bikers.