Citizen Science and Sudden Oak Death

by Sophia Emmons-Bell

Driving down California’s Highway 101, hugging the coast and cutting through the state’s most famous nature reserves, you will pass by hundreds of diseased tanoaks, bay laurels, and California black oaks. These trees, sick with Sudden Oak Death (SOD), are bruised with red and black splotches and bleed sap from cankers on their trunks. The disease has invaded their trunks, leaves, and sap. However, you don’t need to make the coastal drive to see the disease.

In an effort to anticipate and track the spread of SOD, thousands of trees have been tested and GPS-tagged by teams of scientists and grassroots volunteers. The data is collected during intense periods called “Blitzes” that rely heavily on the manpower of the community. In the Blitzes, scientists at the University of California at Berkeley both disseminate information to those who live in at-risk areas and ask volunteers to sample and tag trees in their neighborhoods.1 The maps the volunteers produce are eerily detailed, showing the reach of the disease tree by tree.

Upwards of 2,000 trees are tagged in the Redwood Forests surrounding Humboldt County, over 3,000 in Marin County, just a few miles from San Francisco, and over 1,500 more in forests just south of the San Francisco bay.1 There are even a few scattered throughout my hometown, Berkeley, where the blitzes are organized. On the interactive map, I can see trees tagged frighteningly close to my old middle school.

These trees are both victims and models. Plagued by the plant pathogen Phytophthora ramorum, these sick trees represent the toll that invasive species and infectious plant diseases take on our ecosystem, often a result from trade and globalization.2 However, the unique response, dependent on volunteer involvement, is a novel idea that allows for the masses (a huge group with unfulfilled potential) to contribute to science. Against an invisible invader, the community promises a substantive defense.


A medical epidemic isn’t complete without the obsession over “patient zero”. Zombie movies and epidemiological journals alike track infections back to a single source, a foreign presence that made a community sick. However, when diseases are limited to plants and epidemiologic spreads are nearly impossible to track, fear and blame do not necessarily go away.

Most modern plant ailments (pathogens, invasive insects/pests, etc.) can be traced back, not to a “patient zero”, but to trade.3 Earlier in evolutionary history, species migrated slowly, constrained by large geographical barriers; oceans and mountains were not navigable until humans learned to travel.4 This allowed isolation and protection for millions of species to blossom and differentiate. However, as humans explored the world, interests in trade and scientific curiosity brought novel plants and animals across the globe. Often by accident, many pests, seeds, and spores were transported as well. Most recently, in our globalized world where trade spans time zones and continents, the rate of invasion has exploded. Agricultural and commercial plants are now circulated almost freely, bringing with them profound ecological implications.4

The United States alone is home to more than 50,000 non-native species.3 Much of the Midwest is plagued by the Emerald Ash Borer, an insect that lethally burrows into trees. Asian Cogongrass is outcompeting native plants in the Southeast, starving native animals due to its low nutrition value.3 The West Coast is plagued by SOD. These invasions are increasingly common and have the potential to destroy entire webs and ecosystems. Additionally, significant changes in climate and habitat have driven plant and animal species alike into migration.4 In this way, humans are responsible for much of the biological invasion they are now fearing.


First noticed in the nature-conscious counties around the Bay, Sudden Oak Death has frightened California’s scientists and citizens for more than a decade. The disease, Phytophthora ramorum, was brought into the West Coast by infected nursery plants, and has exploded along the coastline in the last quarter century. Currently reaching from Oregon to a few miles south of California’s Bay Area, the disease attacks more than 100 types of trees, not only the eponymous Oak. In most regards, this pathogen resembles a fungus. P. ramorum produce sexual and asexual spores, are parasitic to many plants, and feed on decaying plant matter.2 However, the pathogen belongs to the class Oomycetes, biologically distinct due to the presence of diploid cells (whereas fungi have haploid cells).2 P. ramorum distributes large spores that float from host to host into an ecosystem, causing the epidemic spread around coastal zones.

As P. ramorum affects many types of plants—ferns to shrubs to oaks—symptoms can vary widely. Classic afflictions are trunk ailments, such as sores and cankers, and leaf diseases like tissue death and spots.2 However, these symptoms are not unique: pests and other diseases often appear clinically similar. To diagnose with certainty, further molecular tests must be performed.2 Only within the last 8 years have scientists at the University of California at Berkeley begun to adopt responsibility of organizing groups to visually screen and molecularly test plants for SOD.


Each spring, before what’s left of California’s rainfall begins to spread SOD around the coast, a group of volunteers gathers in a classroom on UC Berkeley’s own tree-heavy campus. They are caught up on a quarter century’s worth of SOD research and almost a decade’s worth of Blitzes, receiving colorful collection materials and free coffee. Meetings like these occur throughout the greater Bay Area— Marin to Berkeley to Sebastopol—in order to prepare for a quick, concentrated sampling effort.5

“We’re using you as free labor,” Matteo Garbolleto, an adjunct professor at Berkeley and head scientist of the Blitz project, explains to volunteers in this year’s spring meetings.6 He is an animated presenter, receiving laughs throughout his hour-long meeting, but is frank about the importance of volunteers in the Blitz’s success. “We’re trying to engage you,” he says, “to determine the distribution of the disease in your county.”6 Indeed, he cannot understate the importance of the citizens he is talking to; since the spring of 2008, more than 500 volunteers have sampled over 6,000 trees along California’s coastline.5

Blitz meetings begin with a crash-course on Sudden Oak Death. Addressing veterans of the project, fellow UC Berkeley scientists and professors, and new members of the community, Blitz leaders explain why SOD has been so devastating to the ecosystem. Here, the volunteers learn about SOD’s introduction to California in the late 70’s through infected ornamental stock, and its escape in the late 80’s from nurseries to larger parts of the state’s ecosystem. They learn that it is a moisture-dependent disease, requiring consistent rainfall to proliferate. They also learn that, while many trees are susceptible to SOD, only tanoaks and bay laurels are infectious hosts. At this Berkeley meeting, volunteers laugh as Garbolleto describes the giant SOD spores as “elephants”, gliding short relatively distances because of their size.6 However, this background information is not just to entertain—it translates directly into Blitz procedures.

Because the disease is mostly transmitted via infectious tanoaks and bay laurels, Blitzes focus on tagging and testing these trees. This leads to a fairly ironic dearth of oaks tested for Sudden Oak Death. As the name “Blitz” implies, the symptoms are distilled and streamlined to fit on a 4”x8” placard placed inside every Blitz envelope. To these volunteers, the leaves are critical. Bay laurels, for examples, show classic SOD symptoms of blackened, dead leaf tissue where water often pools. These are usually on the end of the leaves, with a dark, uneven line marking the barrier between healthy and dead tissue. Depending on leaf position (and where water has pooled on specific trees), these dead spots can be seen on the bottom side of the blade or in tiny spots along the leaf. Luckily for volunteers, diseased leaves are often found in the lower canopy, where tree climbing and scrambling is not required for retrieval. In tanoak trees, volunteers look to the mid-veins of the leaves instead of the tips. Infected trees will reveal blackened tissue along the middle of their leaves, as well as strange drying patterns. Leaves are browned, dried, and distorted. When they are sent into California forests, volunteers refer back to these descriptions.5

Blitzes happen quickly, often only spanning one weekend. Volunteers are given agency to choose their location and timing. Their instructions are to collect samples, track location through GPS, and clean their clothes and boots upon their return in order to not spread the disease any further. The process is made cheery by the colorful contents of the collection packets: infected samples are put into pink envelopes, healthy trees into white envelopes, and all trees are marked by blue ribbons or aluminum tags. However, before the leaves can be taken back to UC Berkeley for further testing, a careful record has to be made of the locations of tagged trees. This is made easy as the smartphones can act as GPS devices. Ideally, volunteers will “pin” their location on the phone app that has been custom-made for the Blitzes. If they lack smartphones, volunteers can guess their location in relationship to other markers and estimate their GPS coordinates upon return.5 It is this freedom that allows volunteers to sample either their neighborhoods or locations they feel could use more illumination.

When they return, UC Berkeley tests all samples for SOD, piecing together a definite record of the disease across the map. This data can be used to identify new infestations and determine the intensity of known ones. It can illuminate areas at risk for the disease to reach, and show if attempts to fight it have been fruitful. It can even show you how close infected trees have come to your favorite coffee shop.

This year, 504 volunteers surveyed over 10,000 trees in a total of 19 weekend Blitzes. While the infection rate has reached an alltime low (3.7%), Garbolleto’s team attributes this to the severity of California’s drought.1 The data collected shows that the disease continues to spread, breaking out of its coastal confines and inching East. One new diseased tree was found on UC Berkeley’s campus, now closely monitored by those closest—physically and academically—to the disease. However, a better understanding of the topographical spread of SOD has allowed Garbolleto’s team to visualize the disease and release pointed recommendations to limit its spread.

By harnessing the power of the community, Garbolleto’s team at UC Berkeley has mapped out both SOD and a new model for large scientific undertakings. Against an invader as invisible and mysterious as P. ramorum, the community becomes a tangible defender.

Sophia Emmons-Bell ’18 is a sophomore in Eliot House.

Works Cited

  1. UC Berkeley Forest Pathology and Mycology Lab. http://nature.berkeley. edu/garbelottowp/?page_id=148 (accessed Oct. 2, 2015).
  2. Kliejunas, J. T. Sudden oak death and Phytophthora ramorum: a summary of the literature. 2010. Gen. Tech. Rep. PSW-GTR-234. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 181 pages.
  3. Walsh, B. Time. 2014, 184(4), 20-27.
  4. Van Driesche, J. Nature out of Place: Biological Invasions in the Global Age; Island Press; Washington, DC; 2000.
  5. UC Berkeley Forest Pathology and Mycology lab. http://nature.berkeley. edu/garbelottowp/?page_id=1275 (accessed Oct. 2, 2015)
  6. Garballoto, M. “SOD Blitzes 2015”. UC Berkeley Forest Pathology and Mycology Laboratory. May 2015. Lecture.





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