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Welcome to our tree column, “Ask your local arborist.” Each month, arborists from the Davey Tree Expert Company will discuss general tree care and planting tips. If you have a tree-related question, email lifestyle@EmbarcaderoMedia or post in the comments section at the end of this column. Arborists may make house calls, when needed, to assess tree questions on your property.
From now through the end of December is the time of year to inspect your trees for Sudden Oak Death. The mold pathogen that causes Sudden Oak Death, phytophthora ramorum, is most active during wet periods and is at the greatest risk of spreading in rainy and muddy conditions.
More than one million trees along California’s coast have died from Sudden Oak Death since its discovery in 1994. Santa Clara and San Mateo are among the 14 counties where the disease is present. In the Peninsula region, it was first detected in Long Ridge Open Space Preserve in La Honda in 2000, and now poses a serious threat to trees in the Santa Cruz foothills, according to the Midpeninsula Regional Open Space District.
While Sudden Oak Death is a forest disease that primarily affects oaks – including the California coast live oak, black oak, Shreve’s oak, canyon live oak and tanoak trees – it also can affect landscaped yards near wooded areas and beyond. The fungus-like mold can be found in soil, water and infected plant material, and it can be transported to new areas when infected plants or soil are moved.
The pathogen has been identified on more than 100 diverse species of plants; hosts include hardwood and conifer trees, shrubs, herbaceous plants and ferns. On most plant species, symptoms are limited to leaf spots and shoot dieback and are not lethal, according to the California Oak Mortality task force.
While Sudden Oak Death poses serious threats to oak and tanoak trees , with proactive care, you can protect your trees.
Here are some tips on how to identify, treat and prevent the spread of the disease:
Look for symptoms
Sudden Oak Death can be difficult to detect until significant damage has already occurred. The symptoms are very similar to those caused by other fungi, insects or environmental conditions.
The infection can cause the rapid browning of leaves without an apparent, prolonged period of visible decline – thus the name “Sudden Oak Death.”
Early detection is key to managing Sudden Oak Death. Take a walk around your property and inspect your oaks for signs of trouble. Look for wilting leaves, sudden leaf drop, oozing cankers or mushrooms growing at the base of the tree and twig dieback.
Get a laboratory diagnosis
If you spot any symptoms, it’s important to get a diagnosis by a trained professional. The only way to be certain that a plant has phytophthora ramorum is to have a tissue sample laboratory tested.
If you believe your plant may be infected, check the USDA’s list of plants that are susceptible to Sudden Oak Death to see if your plant type is listed. Because this is a relatively new disease, the host list continues to grow as further research is conducted.
Compare your tree’s symptoms to the California Oak Mortality Task Force Symptom Gallery.
If you think your plant needs a sample taken, contact your local county agricultural commissioner.
Take preventative action
While some trees may require removal, many can be saved with the right treatments. Phosphonate treatments, or fungicides, can be particularly effective against Sudden Oak Death when applied at the right time. Prevention, however, is the best defense.
To prevent the spread of the disease, the USDA recommends taking the following precautions:
- Prune and remove any infected limbs
- Only buy healthy plants from reputable sources
- Don’t move plants that appear sick or dying
- Disinfect pruning and other gardening tools between plants.
- Reduce standing water in your yard or garden and avoid overhead irrigation
Help track the disease
To mitigate the spread of Sudden Oak Disease in the region, the Midpeninsula Open Space District relies on volunteers to help track the disease at its local open space preserves. Each year after the rainy season, the organization partners with UC Berkeley for a community science event called the SOD Blitz Project. During the event, volunteers help gather potentially infected bay laurel leaves, which are indicators as to whether the disease is in the area before oaks and tanoaks are infected. They collect samples, log their data and submit their samples to be tested in a lab. UC Berkeley publishes the results the following year. Click here to see the results from 2023.
George Reno is a local arborist at The Davey Tree Expert Company in Menlo Park, which serves communities on the Peninsula, including Menlo Park, Atherton, Palo Alto and Redwood City. Reno can be reached at 866-923-5658.
