As a researcher studying sudden oak death, Ebba Peterson keeps a close watch on the landscape when driving through Southwest Oregon.
“You always keep your eye open for dead tanoak,” a tree species vulnerable to the disease, said Peterson, a research associate at Oregon State University.
During a trip this spring, Peterson noticed some dead tanoaks near Port Orford, Ore., and decided to get a better look.
Among them, she found a sprout with disease symptoms, which laboratory tests later confirmed were caused by phytophthora ramorum, the fungal-like pathogen responsible for sudden oak death.
Her discovery was distinctive for two reasons.
The outbreak was discovered 21 miles outside the sudden oak death quarantine area in southern Curry County — farther than expected for a new infestation.
The disease was also caused by a strain of sudden oak death, NA2, that previously wasn’t detected in the wild, though it’s been found in West Coast nurseries since 2004.
It stands to reason the strain moved from nursery stock to forest plants at some point, though it’s unknown whether that’s how it got to the site in Southwest Oregon, Peterson said.
“This whole area is susceptible to phytophthora ramorum,” she said.
The infestation marks the second discovery of sudden oak death outside Oregon’s 515-square-mile quarantine area this year, after an outbreak was confirmed in the Rogue River-Siskiyou National Forest earlier this spring.
That site was infected with another strain, EU1, so the two infestations are not related, said Sarah Navarro, sudden oak death pathologist with the U.S. Forest Service.
The first discovery was less surprising because it was about six miles from the quarantine area, which is in line with the distance the pathogen naturally spreads, she said.
It’s concerning that three sudden oak death strains — NA1, NA2 and EU1 — have now been found in Oregon, though the state’s quarantine area is still in one county as opposed to 16 counties in California, Navarro said.
At the initial site found outside the quarantine area this year, infected trees and those within a 600-foot buffer have already been cut down, she said. The second site will also undergo eradication treatments and the surrounding area is being monitored to see if the pathogen is more widespread.
“We do have boots on the ground looking every day where this disease could be,” Navarro said.
Douglas firs and other commercial conifers can be infected with sudden oak death, though it’s not known to kill them in Oregon, she said. However, the disease has been found to kill conifers in the United Kingdom.
The EU1 strain spreads more aggressively than the NA1 strain, but further testing will be needed to assess the threat from NA2 in the wild, Navarro said. “We don’t know the aggressiveness or how it’s going to spread in Oregon forests.”
More information is also needed to figure out if the 2021 discoveries are isolated incidents or part of a trajectory of spread outside the quarantine area, said Chris Benemann, nursery and Christmas tree program manager at the Oregon Department of Agriculture.
It’s possible that additional information could lead to an expansion of the quarantine area, with discussions about that decision expected to occur this summer, she said. The discovery of NA2 in the wild won’t bear on that analysis.
“How we treat it from a regulatory standpoint is not impacted by that information,” Benemann said.