A tanoak tree in Northern California's Humboldt County, killed by sudden oak death. The disease continues to spread in Curry County in Southern Oregon.
Courtesy University of California Extension
Sudden oak death, a tree disease caused by a fungal-like pathogen, can lead to bleeding cankers, leaf blight and rapid dieoff in a number of plant and tree species. The disease was first detected in northern California in the mid-1990s, and has been slowly spreading in Southern Oregon since 2001. Newly infected trees were recently found in Humbug Mountain State Park near Port Orford, leading to renewed treatment efforts by state agencies.
Gabriela Ritokova is a forest pathologist with the Oregon Department of Forestry. She joins us with more details about the disease and efforts to contain it.
This transcript was created by a computer and edited by a volunteer.
Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. Sudden oak death is a tree disease caused by a fungus-like pathogen. It can lead to bleeding cankers, leaf blight and rapid die off in a number of tree and plant species. The disease was first detected in Northern California in the mid-1990s. It was found in Curry County in Southern Oregon in 2001 and has been slowly spreading ever since. Newly infected trees were recently discovered in Humbug Mountain State Park near Port Orford, leading to renewed treatment efforts by state agencies. Gabriela Ritokova is a forest pathologist with the Oregon Department of Forestry. She joins us now with more details about this disease and efforts to contain it. Welcome to the show.
Gabriela Ritokova: Hello. Thank you for having me on your show.
Miller: Yeah, thanks for joining us. Can you give us a better understanding of what sudden oak death is?
Ritokova: Well, sudden oak death is an invasive disease that is caused by a fungal-like pathogen called Phytophthora ramorum. And it kills highly susceptible trees in the oak family, most notably tanoaks, but also California black oaks and Canyon live oaks by causing cankers on the main stem that girdle the trunks of the trees.
It can also cause leaf blights of shoot dieback on a number of other hosts including native rhododendrons, Evergreen huckleberry, Douglas fir, Grand fir, and Myrtle in Oregon. These other hosts are only infected when they grow in close proximity to infected tanoaks.
Miller: But that does sound even more serious when you include all those other species, some of which are very common in Oregon. I mean, Douglas fir, for example. We’re talking about a disease that could have widespread effects.
Ritokova: Yes, that is correct. However, in Oregon, this particular disease is driven by tanoaks.
So I want to get into the mode of how it spreads. And first of all, this disease has to have species that it can infect. And in Oregon, it is tanoak. And second, this organism thrives in wet and cool conditions, which is what we have along the coast in Southwestern Oregon. And it can survive in infested plant material or infested soil or water such as rain or fog drip. Now, the pathogen spreads during rainy periods but also with fog when spores produced on infected tail leaves and twigs are released into the air and they’re either washed down onto tree trunks and nearby vegetation on the forest floor or they can be carried by wind and rain across the landscape.
And there is another culprit. People can also spread spores by moving infected plants such as nursery stock or infected plant parts such as firewood or infected soil.
Miller: How much has this disease spread in Oregon since it was first detected a little over two decades ago?
Ritokova: Well, in 2001, when the effort to combat the pathogen began, the original goal was to eradicate it from Curry County where it was found originally. And since 2001, the sudden oak death has spread north as far as Port Orford and expanded eight miles to the east. Now, despite major efforts to eradicate the disease - this effort has been a joint project of the Oregon Department of Forestry, US Forest Service, Oregon Department of Agriculture, Oregon State University, Bureau of Land Management and the Oregon Sudden Oak Death Force - it was eventually recognized that eradication wasn’t realistic. And in 2010, the effort switched to slowing the spread.
Miller: A kind of acknowledgement that it was impossible to stop it. And so the definition of victory really changed.
Ritokova: It did. It was a weak victory of some kind because we were able to slow the spread and we were able to so far in one county, in Curry County, and it has not spread beyond the borders of Curry County. The most recent infestation that you mentioned in Humboldt County is only south of Port Orford where it was found in 2021.
Miller: How much do we know about what caused that new infestation?
Ritokova: Well, we are still in the beginning stages of understanding what happened there. But we do believe that…because as I mentioned, the way this disease spreads, it spreads by wind and rain, that it came from the Port Orford infestation from somewhere a few years ago or so. But we’re still trying to understand what exactly happened.
Miller: How do you quarantine an area when a pathogen can be airborne?
Ritokova: Well, I suppose the answer is you do your best, and we monitor the spread of the disease in numerous ways. It is the first component of our Slow the Spread Program. I’m going to mention surveying and detecting the disease. Most of the original discoveries of sudden oak death were by aerial surveys with ground verification. And the program now relies on multiple survey methods to detect new infestations. And that includes aerial surveys, ground bait transact, stream monitoring and utilizing orthoimagery to detect declining oaks.
Now, aerial surveys cover at least 700,000 acres of forest and are conducted from fixed wing aircraft and helicopter. These take place four times per year with the main survey occurring in July when current year tanoak mortality is most viable. And then we also monitor the spread of spores and high-risk streams, using stream baiting where we put uninfected host leaves and mesh bags into streams and then later test them to see if they’ve been infected by spores flowing within the water.
Miller: If I could just go back. So, in terms of the aerial surveys, if you’re up in a helicopter or an airplane, and you’re looking down, you can actually see evidence of this disease from hundreds of feet away?
Ritokova: That is correct. And we just conducted the helicopter and fixed wing aerial survey. And to identify the problem from the air, what we do is we look for discolored gray individual trees that indicate recent mortality. And I should mention that tanoak trees are evergreen hardwoods. So the gray tint or discoloration suggests that the tree is dying or dead and they look like they are cauliflower heads from the air. This year, we found approximately 60 suspect trees in the Humbug area just south of Port Orford as well as a few in the existing quarantine areas.
But before I go any further, I need to say that these trees need to be ground checked, sampled and then lab checked for the presence of Phytophthora ramorum because there are other agents that can cause similar symptoms. And therefore, it is essential not to make any assumptions that any of these suspect trees we see from the air are infected with the sudden oak death pathogen.
Miller: But what happens if those tests come back positive? What options do you have?
Ritokova: When a new infestation is detected, we go to the site and we treat the site by cutting, piling and burning all tanoaks within a specified buffer. And a 300 ft radius treatment buffer from infected tree is the standard treatment. But recently in these new sites, we have been using 600 ft buffer because it has been most effective. And on most of the treatment sites, we use herbicide before cutting to prevent stem sprouting.
Miller: So you mentioned earlier that in 2010, Oregon’s program to eradicate sudden oak death in forests moved to the more realistic goal of slowing down the spread. What is the realistic outlook now? I mean, if you look 10 or 20 years in the future, what’s your best understanding of how far this disease will have spread?
Ritokova: Well, I think this program has been very successful. We know that tanoak is the driver of the infection in Oregon and tanoak is limited to the southwest corner of the state. And I don’t expect that the efforts we’re making on the south coast will need to be applied to look further north. But we do want to keep this from venturing north into Coos County where it can affect forest product exports from the Port of Coos Bay.
However, as I mentioned, the host range includes more than 130 species which also includes larch. Larch is readily killed by the pathogen in the United Kingdom. And we don’t know what this can do to our native larch trees. And thus, we need to continue to monitor the disease progression. And the existing Sudden Oak Death Program.
Miller: Is the Sudden Oak Death, or SOD Program, affecting nurseries or nursery owners?
Ritokova: Yes, it is. But I do have to admit that I haven’t spoken with nursery owners and this is something that is really best asked of the Oregon Department of Agriculture. But certainly, this is an issue. For example, in 2004, some infected, but symptomless rhodies were found at two large nurseries in California and one in Oregon. These nurseries had recently shipped millions of potentially infected plants to 1,200 nurseries throughout 39 states. But in general, quarantine and inspection measures restrict markets and add to cost. So the stock has to be destroyed in infected nurseries and they are forced to close while investigative and cleanup actions are undertaken.
Miller: What should someone do? I mean, maybe the question is, is there something that lay people should do as opposed to forest managers or nursery owners if they think that they see an infected tree?
Ritokova: Well, definitely they should not dig it up, they should report it to ODA - Oregon Department of Agriculture - [and] call Oregon State University Extension in whatever county they’re in. And we all talk to each other. If it’s forestry, they can call us, Oregon Department of Forestry Sudden Oak Death Program in Brookings or call me and we will come out and take a look at the problem.
Miller: Gabriela Ritokova, thanks very much.
Ritokova: Thank you for having me.
Miller: Gabriela Ritokova is a forest pathologist with the Oregon Department of Forestry. She joined us to talk about sudden oak death, a tree disease that continues to spread, if slowly, in Southern Oregon.
Contact “Think Out Loud®”
If you’d like to comment on any of the topics in this show, or suggest a topic of your own, please get in touch with us on Facebook, send an email to thinkoutloud@opb.org, or you can leave a voicemail for us at 503-293-1983. The call-in phone number during the noon hour is 888-665-5865.