This article was written by Sarah Reith on behalf of the Mendocino County Fire Safe Council. The Mendocino Voice retains editorial oversight.
MENDOCINO CO., 12/17/25 — On a bright, crisp winter morning outside of Willits, the Mendocino County Fire Safe Council hosted a training by scientists from the UC Cooperative Extension and Cal Fire on that scourge of oaks that made its first California appearance in Marin County in 1994. Since then, 14 counties, including Humboldt, Mendocino, Lake, and Sonoma counties are all on federal and state quarantine regulations to combat Sudden Oak Death, more commonly called SOD.
The training on Dec. 11 covered quarantine regulations, including how to recognize the pathogen and most importantly, how not to spread it during fuel reduction activities. Crews from the Fire Safe council came, but the training was also open to other fuel reduction contractors and environmental stewards. The council’s fuels management crew and crew members from Hybrid Indigenous Stewardship took advantage of the opportunity to ask questions, review best management practices, and learn to identify some infected specimens.
Sudden Oak Death is caused by an invasive pathogen called Phytophthora ramorum, a fungus-like water mold that attacks a variety of plants, not just oak trees. The pathogen has been present in Mendocino County since 2008, from Yorkville to MacKerricher State Park.
UC Forest Advisor Mike Jones reported that, “Doing driving surveys, I’ve found highly suspect trees in every watershed in the west side of the county, all west of 101.” The eastern end, he added, is dryer and not as inviting to the organism, which needs moisture to travel and thrive.
Still, added Chris Lee, a forester and forest pest specialist with Cal Fire, “it’s coming down slowly from southern Humboldt County. It’s coming from that MacKerricher Fort Bragg area toward Jackson Demonstration State Forest, so it’s slowly starting to spread in that southern Mendocino area.”
Unfortunately, the disease is also mutating into varieties that can withstand dry conditions better than the original version. There is no cure and no vaccine. Dead standing trees provide fuel for wildfires, and the loss of acorn-bearing trees has a devastating effect on the wildlife and the larger ecosystem.
Jones led the group to the edge of a draw and invited them to peer through the shadows at a tanoak that died of SOD. Before plunging over the edge for a closer look, he pointed out a few smaller tanoaks, their leaves stiff and brown from the blight. These trees were surrounded by California bay laurels, a native plant that carries the disease but typically does not succumb to it. If an ailing oak tree is surrounded by bay laurels, the chances that it’s suffering from SOD are pretty high. SOD is not an opportunistic infection, meaning it attacks healthy trees as well as those that are already doing poorly for other reasons.
Identifying the disease is not a slam-dunk
The illness manifests in a variety of ways and can be difficult to identify, even for experts. Wallis Robinson, a UC researcher working in Humboldt and Del Norte counties, noted that, “It’s important to understand that the symptoms are different on different plants. Depending on what those symptoms are, they’re [easy] to miss.”
On bay laurels, the pathogen makes tiny spots on the leaves. Robinson said the bay laurels are able to shed those leaves, so the disease has little impact. Tanoaks, black oaks, and live oaks, on the other hand, are susceptible because the pathogen gets into their vascular system.
The disease is an inveterate hitchhiker. For this reason, Jones urged crews that work around the county to take measures to prevent spreading it. Leaves and twigs are most likely to spread SOD, but it and other pathogens that also persist in the soil can get stuck in tire treads and boots, and thus establish themselves in a new site.
Minimizing the risk of spread
A few rules of thumb can minimize risk. If a crew is working at multiple locations throughout the day, it’s best to save the site that’s known to be infected for last. Parking on asphalt or rock instead of wet soil can simplify a cleaning routine at the end of the day. Brushing off boots with a scrub brush and then spraying with a disinfectant is a good habit to adopt. And using an air compressor to blow the dirt, leaves, and twigs from tires before leaving the site is a good way to discourage pathogens from finding a new home in another stand of trees.
In spite of its slow creep, its ambiguity, and its capacity to become even more devastating, Cal Fire pest specialist Lee has hope over the advance of Sudden Oak Death.
“In California, oaks and tanoaks are a keystone species in terms of ecological function,” he said. “When we start to lose those trees, it has a cascading effect on animals, on invertebrate species, on herbaceous and shrub species, on hydrology and all sorts of other things. I think that most land managers recognize that, and they are working to monitor this pathogen and control it wherever they can. I think people are being responsible about it.”