Courtesy of Matteo Garbelotto
Citizen scientists jot down the GPS coordinates of a tree featuring signs of Sudden Oak Death. UC Berkeley’s annual SOD Blitz is underway through June in California.
Los Altos Hills resident Sue Welch’s Edgerton Road property is surrounded by dense foliage and perched above a steep ravine cradling a creek. It’s an idyllic spot, but the cool, shaded habitat is prime territory for Sudden Oak Death, or SOD; in the past decade, the disease has claimed six of Welch’s majestic oak trees.
“I call it ‘SOD central’ in the town,” she said.
SOD can infect a number of plant species but is known to kill only four: coast live oaks, California black oaks, Shreve oaks and tanoaks. There is no treatment. Leaves turn brown and sap oozes from trunk bark. Eventually, the tree dies, leaving the decimated remains susceptible to both falls and wildfire.
“As soon as the leaves turn brown, it’s gone. You need to cut it down,” said George Clifford, a Hills resident who lives nearMoody Road, another SOD hot spot.
Both Welch and Clifford are members of the Los Altos Hills Open Space Committee (OSC), a group that has coordinated with the UC Berkeley Forest Pathology and Mycology Laboratory each year since 2009 to bring the lab’s annual SOD Blitz to town. Between April and June, coastal California communities from the Oregon border to the Santa Barbara County border are dispatching citizen scientists, volunteers who collect suspicious tree leaves and record the suspected presence of SOD for the lab. The Bay Area Peninsula effort, which includes residents from Los Altos Hills, Portola Valley and Woodside, begins Friday and ends Tuesday.
Most years, Blitz participation is limited to a handful of OSC members, but Welch and Clifford are hopeful about attracting more volunteers in 2021, perhaps even parents intent on making the activity fun and educational for their pandemic-weary, cooped-up kids. The OSC is particularly interested in recruiting anyone willing to survey the east side of Interstate 280, which has been underrepresented in past blitzes.
‘Plant destroyer’
Sudden Oak Death is a disease caused by the Phytophthora ramorum pathogen. The genus, Phytophthora, means “plant destroyer” in Greek, and it includes many other species of ecologically devastating species, like Phytophthora infestans, the same water mold that triggered the infamous Irish Potato Famine of the 19th century.
Courtesy of Matteo Garbelotto
The leaves of a California bay laurel display the signs of SOD.
Researchers believe SOD entered California in the 1980s through the importation of ornamentals such as rhododendrons and camellias to plant nurseries from Asia. Tanoaks and California bay laurels are the primary species known for spreading the disease, and they do so by shedding spores carried by wind and rain. So far, these vectors and others have managed to transmit SOD to approximately 20% of coastal California forests.
As SOD thrives in cool, wet environments with dense canopy cover, introduction along the Peninsula likely initiated within the forests around Skyline Boulevard, according to Matteo Garbelotto, leader of the UC Berkeley lab and of the SOD Blitzes. From there, the disease descended into the Santa Cruz Mountain canyons and gradually inched east.
“It’s getting worse in that, inevitably, every time we have a Blitz, we identify new outbreaks,” Garbelotto said.
He pointed to the 2019 Blitz, during which volunteers detected SOD in Del Norte County for the first time, and to last year’s Blitz, when they helped uncover E1, a new, more aggressive variant of Phytophthora ramorum in the same area. Up until then, E1’s range was considered limited to Europe.
Record rainfall in 2017 coincided with the infection of several manzanita species within Santa Cruz County, Garbelotto said. Although manzanita was previously identified as a potential host of the pathogen, it typically grows in warmer, drier climates considered less hospitable to SOD.
“This is kind of an unforeseeable consequence of the pathogen becoming a very good generalist, meaning that it can infect almost every plant that we know of,” Garbelotto said. “The limiting factor is not so much the plant. It’s really the environment: Is the environment favorable or not?”
Sentinel trees
California bay laurel trees serve as both the most active spreaders of Phytophthora ramorum and the most helpful indicators of its presence. Although SOD doesn’t kill bay laurels, the trees are quick to reveal signs of infection on their leaves, including brown lesions on the tips, irregular margins and yellow halos bordering blackish spots. Inspecting bay laurels for these telltale signs is much easier than detecting SOD in oaks, a process that requires chipping away at tree bark.
SOD Blitz volunteers are asked to designate 20 seconds per each of the four “sides” of bay laurel trees in their vicinity, examining leaves on the lower branches for necrotic tips. Suspect leaves are collected within envelopes, and sampled trees are recorded by GPS coordinates. The trunks are physically marked with colored tape or aluminum tags to alert other volunteers that a survey is complete.
Leaf samples that test positive for SOD are used to populate the Berkeley lab’s ever-growing SODMAP app, a free guide for calculating an area’s risk for the disease. By consulting the app, California homeowners can determine whether they should take action to protect their oaks like fortifying them with fungicides, as Clifford does, or by sacrificing their bays, as Welch has done. (Garbelotto advises it’s time to start weighing options once SOD creeps within a kilometer or so of a property.)
So far, more than 3 million people have consulted the SODMAP app, and Garbelotto considers that a testament to the work of volunteers who donate their time each year to make UC Berkeley’s database richer. In 2020, 267 participants surveyed 21,943 trees and helped determine their average estimated true infection rate, 7.4%.
“I’m very proud of Californians doing this,” Garbelotto said. “It warms my heart to see that people can collaborate on these important issues.”
Free SOD leaf collection kits are available Friday through Tuesday in front of the council chambers at Los Altos Hills Town Hall, 26379 Fremont Road.
To view UC Berkeley SOD Blitz training videos and for more information, visit sodblitz.org.
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